Risks of Methidathion Use to Federally
Threatened California Red-legged Frog
(Rana aurora draytonii)
Pesticide Effects Determination
Environmental Fate and Effects Division
Office of Pesticide Programs
Washington, D.C. 20460
February 18,2009
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Primary Authors:
Colleen Flaherty, Biologist
Keara Moore, Chemist
Pamela Hurley, Toxicologist
Secondary Review:
Brian Anderson, Biologist
James Hetrick, Senior Scientist
Branch Chief, Environmental Risk Assessment Branch 3:
Dana Spatz
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Table of Contents
1. Executive Summary 8
2. Problem Formulation 16
2.1 Purpose 16
2.2 Scope 18
2.3 Previous Assessments 18
2.3.1 Methidathion Interim Registration Eligibility Decision, 2002 18
2.3.2. Organophosphate Cumulative Assessment, and Methidathion
Reregistration Eligibility Decision, 2006 20
2.4 Stressor Source and Distribution 20
2.4.1 Environmental Fate and Transport 20
2.4.2 Mechanism of Action 25
2.4.3 Use Characterization 25
2.5 Assessed Species 29
2.5.1 Distribution 29
2.5.2 Reproduction 32
2.5.3 Diet 32
2.5.4 Habitat 33
2.6 Designated Critical Habitat 34
2.7 Action Area 36
2.8 Assessment Endpoints and Measures of Ecological Effect 39
2.8.1. Assessment Endpoints for the CRLF 39
2.8.2 Assessment Endpoints for Designated Critical Habitat 40
2.9 Conceptual Model 43
2.9.1 Risk Hypotheses 43
2.9.2 Diagram 43
2.10 Analysis Plan 45
2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model 46
2.10.2 Data Gaps 49
3.1 Label Application Rates and Intervals 50
3.2 Aquatic Exposure Assessment 51
3.2.1 Modeling Approach 51
3.2.2 Model Inputs 51
3.2.3 Results 54
3.2.4 Existing Monitoring Data 55
3.3 Terrestrial Animal Exposure Assessment 59
3.3.1 Modeling Approach 59
3.4 Terrestrial Plant Exposure Assessment 62
4. Effects Assessment 62
4.1 Toxicity of Methidathion to Aquatic Organisms 63
4.1.1 Toxicity to Freshwater Fish 64
4.1.2 Toxicity to Freshwater Invertebrates 67
4.1.3 Toxicity to Aquatic Plants 68
4.2 Toxicity of Methidathion to Terrestrial Organisms 68
4.2.1 Toxicity to Birds 69
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4.2.2 Toxicity to Mammals 71
4.2.3 Toxicity to Terrestrial Invertebrates 72
4.2.4 Toxicity to Terrestrial Plants 72
4.3 Use of Probit Slope Response Relationship to Provide Information on the
Endangered Species Levels of Concern 73
4.4 Incident Database Review 73
4.4.1 Terrestrial Incidents 74
4.4.2 Aquatic Incidents 75
5. Risk Characterization 75
5.1 Risk Estimation 75
5.1.1 Exposures in the Aquatic Habitat 76
5.1.2 Exposures in the Terrestrial Habitat 80
5.1.3 Primary Constituent Elements of Designated Critical Habitat 87
5.1.4 Spatial Extent of Potential Effects 89
5.2 Risk Description 93
5.2.1 Direct Effects 97
5.2.2 Indirect Effects (via Reductions in Prey Base) 105
5.2.3 Indirect Effects (via Habitat Effects) 109
5.2.4 Modification to Designated Critical Habitat 110
6. Uncertainties 112
6.1 Exposure Assessment Uncertainties 112
6.1.1 Maximum Use Scenario 112
6.1.2 Aquatic Exposure Modeling of Methidathion 112
6.1.3 Potential Groundwater Contributions to Surface Water Chemical
Concentrations 114
6.1.4 Action Area Uncertainties 114
6.1.5 Usage Uncertainties 115
6.1.6 Terrestrial Exposure Modeling of Methidathion 116
6.1.7 Spray Drift Modeling 117
6.2 Effects Assessment Uncertainties 117
6.2.1 Age Class and Sensitivity of Effects Thresholds 117
6.2.2 Use of Surrogate Species Effects Data 118
6.2.3 Sublethal Effects 118
6.2.4 Location of Wildlife Species 119
7. Risk Conclusions 119
8. References 123
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Appendices
Attachment 1 CRLF Life History
Attachment 2 Final CRLF Baseline Status and Cumulative Effects
Appendix A Bibliography of submitted fate studies
Appendix B Chemical Names and Structures
Appendix C California PUR Data
Appendix D CRLF Spatial Summary
Appendix E Risk Quotient Method and Levels of Concern
Appendix F PRZM/EXAMS Model Outputs
Appendix G T-REX Example Output
Appendix H Bibliography of ECOTOX Papers
Appendix I Accepted ECOTOX Data Table (sorted by effect)
Appendix J Ecological Effects
Appendix K Data Table for Mammalian Studies from Health Effects Division
Appendix L Incident Data
Appendix M T-HERPS Example Output
Appendix N Bibliography of Submitted Ecotoxicity Studies
List of Tables
Table 1.1 Effects Determination Summary for Methidathion Use and the CRLF 12
Table 1.2 Effects Determination Summary for Methidathion Use and CRLF Critical
Habitat Impact Analysis 13
Table 1.3 Methidathion Use-specific Direct Effects Determinations1 for the CRLF 13
Table 1.4 Methidathion Use-specific Indirect Effects Determinations1 Based on Effects to
Prey 14
Table 2.1. Summary of Methidathion Environmental Fate Properties 22
Table 2.2 Methidathion Uses Assessed for the CRLF 28
Table 2.3 Assessment Endpoints and Measures of Ecological Effects 40
Table 2.4 Summary of Assessment Endpoints and Measures of Ecological Effect for
Primary Constituent Elements of Designated Critical Habitata 42
Table 3.1. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to
Methidathion 51
Table 3.2. PRZM/EXAMS Crop-Specific Inputs for Aquatic Exposure to Methidathion 53
Table 3.3 Aquatic EECs ((J-g/L) for Methidathion Uses in California 54
Table 3.4 Input Parameters for Foliar Applications Used to Derive Terrestrial EECs for
Methidathion with T-REX 60
Table 3.5 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based
Exposures of the CRLF and its Prey to Methidathion 61
Table 3.6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase CRLF via Effects to
Terrestrial Invertebrate Prey Items 61
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Table 4.1 Freshwater Aquatic Toxicity Profile for Methidathion 64
Table 4.2 Categories of Acute Toxicity for Aquatic Organisms 64
Table 4.3 Terrestrial Toxicity Profile for Methidathion 68
Table 4.4 Categories of Acute Toxicity for Avian and Mammalian Studies 69
Table 5.1 Summary of Acute Direct Effect RQs for the Aquatic-phase CRLFa 77
Table 5.2 Summary of Chronic Direct Effect RQs for the Aquatic-phase CRLFa 78
Table 5.3 Summary of Acute RQs Used to Estimate Indirect Effects to the CRLF via
Direct Effects on Aquatic Invertebrates as Dietary Food Items 79
Table 5.4 Summary of Chronic RQs Used to Estimate Indirect Effects to the CRLF via
Direct Effects on Aquatic Invertebrates as Dietary Food Items 80
Table 5.5. Summary of Direct Effect Acute Dose-Based RQs for the Terrestrial-phase
CRLF for Spray Applications of Methidathion 81
Table 5.6. Summary of Direct Effect Acute Dietary-Based RQs for the Terrestrial-phase
CRLF for Spray Applications of Methidathion 82
Table 5.7. Summary of Direct Effect Chronic RQs for the Terrestrial-phase CRLF for
Spray Applications of Methidathion 83
Table 5.8 Summary of RQs For Indirect Effects to the Terrestrial-phase CRLF via Direct
Effects on Terrestrial Invertebrates as Dietary Food Items 84
Table 5.9. Summary of Acute RQs For Indirect Effects to the Terrestrial-phase CRLF via
Direct Effects on Small Mammals as Dietary Food Items 85
Table 5.10. Summary of Chronic RQs For Indirect Effects to the Terrestrial-phase CRLF
via Direct Effects on Small Mammals as Dietary Food Items 86
Table 5.11. Input parameters for simulation of methidathion in spray drift using AgDisp
(v. 8.15) with Gaussian far field extension 90
Table 5.12 Summary of maximum predicted distances for potential spray drift effects 91
Table 5.13 Risk Estimation Summary for Methidathion - Direct and Indirect Effects to
CRLF 94
Table 5.14 Risk Estimation Summary for Methidathion - PCEs of Designated Critical
Habitat for the CRLF 95
Table 5.15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk
Quotients 100
Table 5.16. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk
Quotients 107
Table 7.1 Effects Determination Summary for Methidathion Use and the CRLF 121
Table 7.2 Effects Determination Summary for Methidathion Use and CRLF Critical
Habitat Impact Analysis 122
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List of Figures
Figure 2.1. Map of Estimated Annual Agricultural Use of Methidathion in 2002 26
Figure 2.2 Recovery Unit, Core Area, Critical Habitat, and Occurrence
Designations for CRLF 31
Figure 2.3 - CRLF Reproductive Events by Month 32
Figure 2.4 Initial area of concern, or "footprint" of potential use, for methidathion 38
Figure 2.5 Conceptual Model for Methidathion Effects on Terrestrial Phase of the
CRLF 44
Figure 2.6 Conceptual Model for Methidathion Effects on Aquatic Phase of the
CRLF 45
Figure 5.1. Overlap Map: CRLF Habitat and Methidathion Initial Area of Concern 93
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1. Executive Summary
The purpose of this assessment is to evaluate potential direct and indirect effects on the
California red-legged frog (Rana aurora draytonii) (CRLF) arising from FIFRA
regulatory actions regarding use of methidathion on agricultural and non-agricultural
sites. In addition, this assessment evaluates whether these actions can be expected to
result in modification of the species' designated critical habitat. This assessment was
completed in accordance with the U.S. Fish and Wildlife Service (USFWS) and National
Marine Fisheries Service (NMFS) Endangered Species Consultation Handbook
(USFWS/NMFS, 1998 and procedures outlined in the Agency's Overview Document
(U.S. EPA, 2004).
The CRLF was listed as a threatened species by USFWS in 1996. The species is endemic
to California and Baja California (Mexico) and inhabits both coastal and interior
mountain ranges. A total of 243 streams or drainages are believed to be currently
occupied by the species, with the greatest numbers in Monterey, San Luis Obispo, and
Santa Barbara counties (USFWS, 1996) in California.
Methidathion is a non-systemic organophosphate insecticide/acaricide registered for use
to control a wide range of sucking, leaf-eating, and scale insects. Currently labeled uses
of methidathion considered as part of the federal action evaluated in this assessment
include: Alfalfa, almond, apple, apricot, artichoke, calemondin, cherry, citron, citrus,
clover, cotton, grapefruit, kiwi, kumquat, lemon, lime, mango, nectarine, olive, peach,
pear, plum, prune, pummelo, safflower, sunflower, tangelo, tangerine, timothy, walnut,
and nursery stock. Methidathion is formulated as an emulsifiable concentrate or a
wettable powder. Applications can be applied by ground, airblast, or aerial spray with
annual application rates that range from 1 lb active ingredient/A to 10 lb a.i./A. Label
requirements include restrictions on spray heights and buffer widths in order to reduce
risk from spray drift.
Based on laboratory studies, methidathion has generally low persistence in soil, with
aerobic and anaerobic soil metabolism half-lives of less than two weeks. Dissipation
rates appear to slow over time, however, so there is a possibility that in some conditions
low levels may persist. Terrestrial field dissipation studies found half-lives of less than
30 days. In water, methidathion has limited hydrolysis except in alkaline conditions, but
it is subject to aquatic photolysis with a half-life of 10 days. There are no aquatic
metabolism data available. Soil adsorption studies indicate that methidathion is
moderately mobile, but it has not been detected below 18 inches in any terrestrial field
dissipation studies. There is some potential for long range atmospheric transport.
Although the physical properties suggest that volatilization would be limited, monitoring
studies have found methidathion residues at more than 20 km from any use site.
Soil metabolism of methidathion leads primarily to carbon dioxide and bound residues,
but one unidentified degradate was formed at up to 13% and appeared to be persistent.
Other major degradates include S-129561, formed through hydrolysis and through aquatic
1 5-methyl-l-3,4-thiadazol-2 (3H)-one
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and soil photolysis, and des-methyl GS-130072 and PAMMTO3, formed through
hydrolysis in some conditions. Methidathion oxon4 is formed only as a minor degradate,
but it is included as a stressor of concern in this assessment because, based on data for
other organophosphate pesiticides, oxon degradates have the potential to be much more
toxic than the parent. Laboratory studies demonstrate that at least 4.9% of applied
methidathion can be transformed to methidathion oxon through soil photolysis, but no
data are available regarding photo-oxidation in air, which could also be an important
route of transformation.
Since CRLFs exist within aquatic and terrestrial habitats, exposure of the CRLF, its prey
and its habitats to methidathion are assessed separately for the two habitats. Tier-II
aquatic exposure models are used to estimate high-end exposures of methidathion in
aquatic habitats resulting from runoff and spray drift from different uses. Peak model-
estimated aquatic environmental concentrations resulting from different methidathion
uses range from 7.3 to 114.7 |ig/L. These estimates are supplemented with analysis of
available California surface water monitoring data from U. S. Geological Survey's
National Water Quality Assessment (NAWQA) program and the California Department
of Pesticide Regulation. The maximum concentration of methidathion reported by
NAWQA for California surface waters from 15 sites is 0.31 |ig/L. The California
Department of Pesticide Regulation surface water database included more than 4,000
samples from 135 locations in agricultural areas and reported a maximum measured
concentration of 15.1 |ig/L, which is in the range of peak modeled values for the different
uses of methidathion. Ground water per se is not evaluated herein, but it is nonetheless
significant because discharging groundwater is likely to support low-order streams,
wetlands, and intermittent ponds - environments that are favorable to CRLFs. Long-term
chronic concentrations derived from the PRZM-EXAMS model could reflect background
concentrations that might be found in discharged groundwater/stream baseflow.
NAWQA monitoring data did not detect methidathion in ground water sampling at more
than 200 sites, most of which were collected in counties with high methidathion use in
areas with land cover classified as agricultural or mixed use.
To estimate methidathion exposures to the terrestrial-phase CRLF, and its potential prey
resulting from uses involving methidathion applications, the T-REX model is used for
foliar uses. AgDRIFT is used to estimate deposition of methidathion on terrestrial and
aquatic habitats from spray drift. The TerrPlant model is used to estimate methidathion
exposures to terrestrial-phase CRLF habitat, including plants inhabiting semi-aquatic and
dry areas, resulting from uses involving foliar methidathion applications. The T-HERPS
model is used to allow for further characterization of dietary exposures of terrestrial-
phase CRLFs relative to birds.
There are also atmospheric monitoring data which demonstrate long range transport of
both methidathion and methidathion oxon. Both species have been detected in air, fog,
and/or rain at local, intermediate, and long-range locations. In the monitoring study with
2 des-methyl S-[(5-methoxy-2-oxo-l, 3, 4-thiadiazol-3 (2-l)-yl-methyl 0,0-dimethylphosphorothioate]
3 phosphorthioic acid, 4-(mercaptomethyl)-2-meth-oxy-A2-,13 ,4-thiadiazolin-5-one
4 S-[(5-methione-2-oxo-l,3,4-thiodiazol-3(2H)-yl methyl o,o dimethyl phosphorothioate; (GS-13007)
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data at the furthest range, methidathion was detected in air at 22 km from the nearest use
site and methidathion oxon was detected at a site another 10 km away. Methidathion
oxon was also detected on pine needles at the 22 km site.
The effects determination assessment endpoints for the CRLF include direct toxic effects
on the survival, reproduction, and growth of the CRLF itself, as well as indirect effects,
such as reduction of the prey base or modification of its habitat. Direct effects to the
CRLF in the aquatic habitat are based on toxicity information for freshwater fish, which
are generally used as a surrogate for aquatic-phase amphibians. In the terrestrial habitat,
direct effects are based on toxicity information for birds, which are used as a surrogate
for terrestrial-phase amphibians. Given that the CRLF's prey items and designated
critical habitat requirements in the aquatic habitat are dependant on the availability of
freshwater aquatic invertebrates and aquatic plants, toxicity information for these
taxonomic groups is also discussed. In the terrestrial habitat, indirect effects due to
depletion of prey are assessed by considering effects to terrestrial insects, small terrestrial
mammals, and frogs. Indirect effects due to modification of the terrestrial habitat are
characterized by available data for terrestrial monocots and dicots.
This assessment primarily considers effects of exposures to methidathion only but
quantitative characterization is provided for methidathion oxon, the one known degradate
of concern. Although no toxicity data are available for methidathion oxon, it is presumed
to be of concern because toxicity data for oxon transformation products of similar
pesticides in the organophospate class suggest they will be of equal or greater toxicity
than the parent. There are other degradates suggested by the degradation pathway but not
tested for in laboratory studies that may also be of concern. No data are available
regarding the formation and decline or the toxicity of methidathion oxon or of any of the
other potential OP or oxon degradates. Given that acute and chronic risk quotients are
exceeded for the parent compound alone, any contribution in toxicity from any of the
major degradates or from methidathion oxon or other OP degradates would increase the
risk estimates.
Risk quotients (RQs) are derived as quantitative estimates of potential high-end risk.
Acute and chronic RQs are compared to the Agency's levels of concern (LOCs) to
identify instances where methidathion use within the action area has the potential to
adversely affect the CRLF and its designated critical habitat via direct toxicity or
indirectly based on direct effects to its food supply (i.e., freshwater invertebrates, algae,
fish, frogs, terrestrial invertebrates, and mammals) or habitat (i.e., aquatic plants and
terrestrial upland and riparian vegetation). When RQs for each particular type of effect
are below LOCs, the pesticide is determined to have "no effect" on the CRLF. Where
RQs exceed LOCs, a potential to cause adverse effects is identified, leading to a
conclusion of "may affect." If a determination is made that use of methidathion use
within the action area "may affect" the CRLF and its designated critical habitat,
additional information is considered to refine the potential for exposure and effects, and
the best available information is used to distinguish those actions that "may affect, but are
not likely to adversely affect" (NLAA) from those actions that are "likely to adversely
affect" (LAA) the CRLF and its critical habitat.
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Based on the best available information, the Agency makes a Likely to Adversely Affect
determination for the CRLF from the use of methidathion. Additionally, the Agency has
determined that there is the potential for modification of CRLF designated critical habitat
from the use of the chemical. For direct effects, the acute listed species LOCs for
freshwater fish and for birds (surrogate to the aquatic-and terrestrial-phase CRLF,
respectively) are exceeded for all uses. The chronic LOC for both freshwater fish and
birds is also exceeded for all uses. For indirect effects, with the exception of mangos on
an acute dietary basis for non-listed species, the acute dose-based, acute dietary-based
and chronic dietary-based RQs for birds (terrestrial-phase amphibians) exceed both the
acute non-listed species LOC and/or the chronic LOC for all of the assessed uses. For
aquatic and terrestrial invertebrates and mammals, the acute RQs exceed the acute non-
listed LOC for aquatic and/or terrestrial animals for all uses. The chronic LOC for
aquatic and/or terrestrial animals is also exceeded for aquatic invertebrates, mammals
and fish/aquatic-phase amphibians (all uses). No studies are available for aquatic
vascular and non-vascular and terrestrial plants; however, the weight of the evidence
indicates an effect determination of may affect, not likely to adversely affect (NLAA).
A summary of the risk conclusions and effects determinations for the CRLF and its
critical habitat is presented in Tables 1.1 and 1.2. Use-specific determinations for direct
and indirect effects to the CRLF are provided in Tables 1.3 and 1.4. Further
information on the results of the effects determination is included as part of the Risk
Description in Section 5.2. Given the LAA determination for the CRLF and potential
modification of designated critical habitat, a description of the baseline status and
cumulative effects for the CRLF is provided in Attachment 2.
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Table 1.1 Effects Determination Summary for Methidathion Use and the CRLF
Assessment
Endpoint
Effects
Determination 1
Basis for Determination
Survival, growth,
and/or reproduction
of CRLF
individuals
LAA1
Potential for Direct Effects
Aquatic-phase (Eggs, Larvae, and Adults) :
Highly toxic to freshwater fish. Acute listed species LOC exceeded for all uses
for three tested surrogate species. Chronic effects based on survival and growth.
Chronic LOC exceeded for all uses. Probability of an individual effect on an
acute basis is high, both at the acute LOC and at the RQ levels. Incident data
also indicate freshwater fish vulnerability, and monitoring has detected surface
water concentrations above the acute endpoint. Significant overlap expected to
exist between use sites and CRLF habitat, particularly when the spraydrift and
downstream dilution buffers are added to the methidathion use area. Spraydrift
distances range from 23 to over 1000 feet and the downstream dilution buffer is
285 km.
Terrestrial-phase (Juveniles and Adults) :
Moderately toxic to very highly toxic to avian species. Acute dose-based, acute
dietary-based and chronic dietary-based RQs for the terrestrial-phase CRLF,
using avian data as a surrogate, exceed acute listed species LOC and chronic
LOC for all of the assessed uses. Chronic effects based on reduction in number
of normal hatchlings/live 3-week embryos. Limited data indicate that on a dose
basis, the acute LOC is exceeded for all uses, even with the least sensitive
species at the lowest application rate (mangos). On a dietary basis, the same is
true for all uses except mangos. Probability of an individual effect on an acute
basis is high at the RQ levels. Incident data indicate that birds are vulnerable.
Significant overlap exists between use sites and CRLF habitat, particularly when
the spraydrift buffers are applied to the methidathion use area. Spraydrift
distances range from 194 to over 1000 feet.
Potential for Indirect Effects
Aquatic prey items, aquatic habitat, cover and/or primary productivity
Very highly toxic to freshwater invertebrates. Acute and chronic RQs exceed
acute LOC for non-listed species and chronic LOC, respectively for all assessed
uses. The percentage effect to the aquatic invertebrate prey base for all uses is
very high. For fish (aquatic-phase amphibians), acute non-listed species LOC
exceeded for all uses. Chronic LOC exceeded for all uses. Percentage effect to
the freshwater fish/aquatic-phase amphibian prey base is very high. No studies
are available for aquatic vascular and non-vascular plants.
Terrestrial prey items, riparian habitat
RQs for both small and large insects exceed acute LOC for non-listed species for
all methidathion uses, even with only a single use. Insufficient studies to
conduct a sensitivity analysis; however, lowest RQ, using highest terrestrial
invertebrate endpoint still exceeds the acute list species LOC. Percentage effect
to the terrestrial invertebrate prey base for all uses is very high. For terrestrial-
phase amphibians using avian data as a surrogate, with the exception of mangos
on an acute dietary basis, the acute dose-based, acute dietary-based and chronic
dietary-based RQs exceed the acute non-listed species LOC and the chronic LOC
for all of the assessed methidathion uses, respectively. Percentage effect to the
avian/terrestrial-phase amphibian prey base is very high. For mammals, the
acute and chronic RQs exceed the non-listed species acute LOC and the chronic
LOC, respectively for all uses. At the lowest RQ, the percentage effect to the
mammalian prey base is very high. No studies are available for terrestrial plants.
1 No effect (NE); May affect, but not likely to adversely affect (NLAA); May affect, likely to adversely
affect (LAA)
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Table 1.2 Effects Determination Summary for Methidathion Use and CRLF Critical Habitat
Impact Analysis
Assessment
Endpoint
Effects
Determination 1
Basis for Determination
Modification of
aquatic-phase PCE
Habitat
Modification1
No studies available for aquatic vascular and non-vascular plants and
terrestrial plants.
For the aquatic-phase CRLF, acute non-listed species LOC for freshwater
fish (aquatic-phase amphibians) exceeded for all uses. Chronic LOC for
freshwater fish exceeded for all uses. Probability of an individual effect and
percentage effect to the freshwater fisli/aquatic-phase amphibian prey base
on an acute basis at the RQ are high. Incident data support freshwater fish
vulnerability. For freshwater invertebrates, acute and chronic RQs exceed
acute LOC (non-listed species) and chronic LOC, respectively for all
assessed uses. Again the percentage effect to the aquatic invertebrate prey
base is very high. Significant overlap expected between use sites and CRLF
habitat, particularly when the spraydrift and downstream dilution buffers are
added to the use area. Spraydrift distances range from 23 (mangos) to over
1000 feet and the downstream dilution buffer is 285 km.
Modification of
terrestrial-phase
PCE
No studies are available for terrestrial plants.
For the terrestrial-phase CRLF, using avian data as a surrogate, with the
exception of mangos on a dietary basis for non-listed species, the acute dose-
based, acute dietary-based and chronic dietary-based RQs for the terrestrial-
phase CRLF, exceed both the acute listed and non-listed species LOC and
chronic LOC for all of the assessed uses. Probability of an individual effect
on an acute basis at the lowest RQ level and percentage effect to the
avian/terrestrial-phase amphibian prey base are very high. Incident data
indicate that birds are vulnerable. RQs for both small and large insects
exceed acute LOC for non-listed species for all methidathion uses, even with
only a single use. Percentage effect to terrestrial invertebrate prey base for
all uses are very high. For mammals, the acute and chronic RQs exceed the
non-listed species acute LOC and the chronic LOC, respectively for all uses.
The percentage effect to the mammalian prey base is very high for all uses.
Significant overlap exists between use sites and CRLF habitat, particularly
when the spraydrift buffers are applied to the methidathion use area.
Spraydrift distances range from 194 (mangos) to over 1000 feet.
1 Habitat Modification or No effect (NE)
Table 1.3 Methidathion Use-specific Direct Effects Determinations1 for the CRLF
Use(s)
Aquatic Habitat
Terrestrial Habitat
Acute
Chronic
Acute
Chronic
All uses
LAA
LAA
LAA
LAA
1 NE = No effect; NLAA = May affect, but not likely to adversely affect; LAA =
Likely to adversely affect
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Table 1.4 Methidathion Use-specific Indirect Effects Determinations1 Based on Effects to Prey
Use(s)
Algae
Aquatic
Invertebrates
Terrestrial
Invertebrates
(Acute)
Aquatic-phase
frogs and fish
T errestrial-phase
frogs
Small Mammals
Acute
Chronic
Acute
Chronic
Acute
Chronic
Acute
Chronic
All uses
NLAA
LAA
LAA
LAA
LAA
LAA
LAA
LAA
LAA
LAA
1 NE = No effect; NLAA = May affect, not likely to adversely affect; LAA = Likely to adversely affect
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Based on the conclusions of this assessment, a formal consultation with the U. S. Fish
and Wildlife Service under Section 7 of the Endangered Species Act should be initiated.
When evaluating the significance of this risk assessment's direct/indirect and adverse
habitat modification effects determinations, it is important to note that pesticide
exposures and predicted risks to the species and its resources (i.e., food and habitat) are
not expected to be uniform across the action area. In fact, given the assumptions of drift
and downstream transport (i.e., attenuation with distance), pesticide exposure and
associated risks to the species and its resources are expected to decrease with increasing
distance away from the treated field or site of application. Evaluation of the implication
of this non-uniform distribution of risk to the species would require information and
assessment techniques that are not currently available. Examples of such information and
methodology required for this type of analysis would include the following:
• Enhanced information on the density and distribution of CRLF life stages
within specific recovery units and/or designated critical habitat within the
action area. This information would allow for quantitative extrapolation
of the present risk assessment's predictions of individual effects to the
proportion of the population extant within geographical areas where those
effects are predicted. Furthermore, such population information would
allow for a more comprehensive evaluation of the significance of potential
resource impairment to individuals of the species.
• Quantitative information on prey base requirements for individual aquatic-
and terrestrial-phase frogs. While existing information provides a
preliminary picture of the types of food sources utilized by the frog, it
does not establish minimal requirements to sustain healthy individuals at
varying life stages. Such information could be used to establish
biologically relevant thresholds of effects on the prey base, and ultimately
establish geographical limits to those effects. This information could be
used together with the density data discussed above to characterize the
likelihood of adverse effects to individuals.
• Information on population responses of prey base organisms to the
pesticide. Currently, methodologies are limited to predicting exposures
and likely levels of direct mortality, growth or reproductive impairment
immediately following exposure to the pesticide. The degree to which
repeated exposure events and the inherent demographic characteristics of
the prey population play into the extent to which prey resources may
recover is not predictable. An enhanced understanding of long-term prey
responses to pesticide exposure would allow for a more refined
determination of the magnitude and duration of resource impairment, and
together with the information described above, a more complete prediction
of effects to individual frogs and potential modification to critical habitat.
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2. Problem Formulation
Problem formulation provides a strategic framework for the risk assessment. By
identifying the important components of the problem, it focuses the assessment on the
most relevant life history stages, habitat components, chemical properties, exposure
routes, and endpoints. The structure of this risk assessment is based on guidance
contained in U.S. EPA's Guidance for Ecological Risk Assessment (U.S. EPA 1998), the
Services' Endangered Species Consultation Handbook (USFWS/NMFS 1998) and is
consistent with procedures and methodology outlined in the Overview Document (U.S.
EPA 2004) and reviewed by the U.S. Fish and Wildlife Service and National Marine
Fisheries Service (USFWS/NMFS 2004).
2.1 Purpose
The purpose of this endangered species assessment is to evaluate potential direct and
indirect effects on individuals of the federally threatened California red-legged frog
(Rana aurora draytonii) (CRLF) arising from FIFRA regulatory actions regarding use of
methidathion on various agricultural and non-agricultural uses. In addition, this
assessment evaluates whether use on these crops is expected to result in modification of
the species' designated critical habitat. This ecological risk assessment has been
prepared consistent with a settlement agreement in the case Center for Biological
Diversity (CBD) vs. EPA et al. (Case No. 02-1580-JSW(JL)) entered in Federal District
Court for the Northern District of California on October 20, 2006.
In this assessment, direct and indirect effects to the CRLF and potential modification to
its designated critical habitat are evaluated in accordance with the methods described in
the Agency's Overview Document (U.S. EPA 2004). Screening level methods include
use of standard models such as PRZM-EXAMS, T-REX, and AgDRIFT, all of which are
described at length in the Overview Document. Use of such information is consistent
with the methodology described in the Overview Document (U.S. EPA 2004), which
specifies that "the assessment process may, on a case-by-case basis, incorporate
additional methods, models, and lines of evidence that EPA finds technically appropriate
for risk management objectives" (Section V, page 31 of U.S. EPA 2004).
In accordance with the Overview Document, provisions of the ESA, and the Services'
Endangered Species Consultation Handbook, the assessment of effects associated with
registrations of methidathion is based on an action area. The action area is the area
directly or indirectly affected by the federal action, as indicated by the exceedence of the
Agency's Levels of Concern (LOCs). It is acknowledged that the action area for a
national-level FIFRA regulatory decision associated with a use of methidathion may
potentially involve numerous areas throughout the United States and its Territories.
However, for the purposes of this assessment, attention will be focused on relevant
sections of the action area including those geographic areas associated with locations of
the CRLF and its designated critical habitat within the state of California. As part of the
"effects determination," one of the following three conclusions will be reached regarding
the potential use of methidathion in accordance with current labels:
16
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• "No effect";
• "May affect, but not likely to adversely affect"; or
• "May affect and likely to adversely affect".
Designated critical habitat identifies specific areas that have the physical and biological
features, (known as primary constituent elements or PCEs) essential to the conservation
of the listed species. The PCEs for CRLFs are aquatic and upland areas where suitable
breeding and non-breeding aquatic habitat is located, interspersed with upland foraging
and dispersal habitat.
If the results of initial screening-level assessment methods show no direct or indirect
effects (no LOC exceedances) upon individual CRLFs or upon the PCEs of the species'
designated critical habitat, a "no effect" determination is made for use of methidathion as
it relates to this species and its designated critical habitat. If, however, potential direct or
indirect effects to individual CRLFs are anticipated or effects may impact the PCEs of the
CRLF's designated critical habitat, a preliminary "may affect" determination is made for
the FIFRA regulatory action regarding methidathion.
If a determination is made that use of methidathion within the action area(s) associated
with the CRLF "may affect" this species or its designated critical habitat, additional
information is considered to refine the potential for exposure and for effects to the CRLF
and other taxonomic groups upon which these species depend (e.g., aquatic and terrestrial
vertebrates and invertebrates, aquatic plants, riparian vegetation, etc.). Additional
information, including spatial analysis (to determine the geographical proximity of CRLF
habitat and methidathion use sites) and further evaluation of the potential impact of
methidathion on the PCEs is also used to determine whether modification of designated
critical habitat may occur. Based on the refined information, the Agency uses the best
available information to distinguish those actions that "may affect, but are not likely to
adversely affect" from those actions that "may affect and are likely to adversely affect"
the CRLF or the PCEs of its designated critical habitat. This information is presented as
part of the Risk Characterization in Section 5 of this document.
The Agency believes that the analysis of direct and indirect effects to listed species
provides the basis for an analysis of potential effects on the designated critical habitat.
Because methidathion is expected to directly impact living organisms within the action
area (defined in Section 2.7), critical habitat analysis for methidathion is limited in a
practical sense to those PCEs of critical habitat that are biological or that can be
reasonably linked to biologically mediated processes (i.e., the biological resource
requirements for the listed species associated with the critical habitat or important
physical aspects of the habitat that may be reasonably influenced through biological
processes). Activities that may modify critical habitat are those that alter the PCEs and
appreciably diminish the value of the habitat. Evaluation of actions related to use of
methidathion that may alter the PCEs of the CRLF's critical habitat form the basis of the
critical habitat impact analysis. Actions that may affect the CRLF's designated critical
habitat have been identified by the Services and are discussed further in Section 2.6.
17
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2.2 Scope
Methidathion is used as an insecticide on a variety of terrestrial food and feed crops and
terrestrial non-food crops. Based on usage data provided by the Biological and Economic
Analysis Division (BEAD), on average, roughly 110,000 pounds of methidathion are
applied annually to agricultural crops. Methidathion usage is highest on almonds and
oranges, with annual average applications of 20,000 lbs. a.i. applied. The crop with the
highest average percent crop treated with methidathion is artichokes at 60% (10,000 lbs.
a.i. applied).
The end result of the EPA pesticide registration process {i.e., the FIFRA regulatory
action) is an approved product label. The label is a legal document that stipulates how
and where a given pesticide may be used. Product labels (also known as end-use labels)
describe the formulation type {e.g., liquid or granular), acceptable methods of application,
approved use sites, and any restrictions on how applications may be conducted. Thus, the
use or potential use of methidathion in accordance with the approved product labels for
California is "the action" relevant to this ecological risk assessment.
Although current registrations of methidathion allow for use nationwide, this ecological
risk assessment and effects determination addresses currently registered uses of
methidathion in portions of the action area that are reasonably assumed to be biologically
relevant to the CRLF and its designated critical habitat. Further discussion of the action
area for the CRLF and its critical habitat is provided in Section 2.7.
Methidathion degrades into one known degradate of concern, methidathion oxon.
Although no toxicity data are available for methidathion oxon, it is presumed to be of
concern because toxicity data for oxon transformation products of similar pesticides in
the organophospate class suggest they will be of equal or greater toxicity than the parent.
Methidathion oxon is the only degradate considered in this analysis. There are no
toxicity data for the four major degradates, but these compounds no longer contain the
OP moiety and so are assumed to be of lower toxicity than the parent.
Methidathion has no registered products that contain multiple active ingredients.
2.3 Previous Assessments
2.3.1 Methidathion Interim Registration Eligibility Decision, 2002
The Agency completed a screening-level ecological risk assessment (USEPA, 1999) in
support of the Interim Reregi strati on Eligibility Decision (IRED) for methidathion
(USEPA 2002). Completion of the organophosphate (OP) cumulative assessment
(USEPA 2006b) resulted in finalization of the IRED as a Reregi strati on Eligibility
Decision (RED) (USEPA 2006a), which is summarized below.
The Agency's ecological risk assessment was based principally on methidathion's use
on cotton, citrus, stone fruits, nut crops, and artichokes. The assessment was based on
data collected in the laboratory and in the field to characterize the fate and
18
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ecotoxicological effects of methidathion. Data sources used in this assessment
included: 1) registrant submissions in support of reregi strati on, 2) publicly available
literature on ecological effects, and 3) incident reports of adverse effects on aquatic and
terrestrial organisms associated with the use of methidathion.
The RED concluded that methidathion poses a risk to ecosystems in use areas. The levels
of concern were exceeded for both acute and chronic effects to mammals, birds, fish, and
aquatic invertebrates. In addition, risk to terrestrial invertebrates was presumed. The
presumption of risk to non-target aquatic and terrestrial animals was supported by field
studies and adverse ecological incidents. The potential risk to terrestrial plants was not
assessed due to a lack of toxicity data. Toxicity data available for degradates of
methidathion, including methidathion oxon, were not available for consideration in this
assessment.
To address the presumed risk to birds, the Agency required the additional dilution of
methidathion products (a minimum of 500 gallons of water per acre) in an attempt to
reduce the amount of methidathion on food items. In addition, precautionary statements
regarding the risk to birds were required to be added to the label in order to protect avian
species.
To address the presumed risk to fish and aquatic invertebrates, the following risk
mitigation strategies were required:
For all applications applied at rates greater than 3.0s a.i./A:
o Do not apply within 50 feet of lakes, reservoirs, rivers, permanent streams,
natural ponds, marshes or estuaries
- For all applications applied at rates of 3.0s a.i./A or less:
o Do not apply within 25 feet of lakes, reservoirs, rivers, permanent streams,
natural ponds, marshes or estuaries
- For ground applications:
o Shut off sprayer when turning at end rows
o Do not apply when gusts or sustained winds exceed 12 mph
- For air blast application:
o Adjust deflectors and aiming devices so that spray is only directed into the
canopy
o Block off upward pointed nozzles when there is no overhanging canopy
o Use only enough air volume to penetrate the canopy and provide good
coverage
o Do not allow spray to go beyond the edge of the cultivated area. Spray the
outside row only from outside the planting
- For aerial application:
o Do not apply within 150 feet of water
o Do not apply when gusts or sustained winds exceed 8 mph
In addition, to mitigate the risk to aquatic animals, a label amendment was required to
include a surface water advisory statement.
19
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The Agency was also concerned about the risk to beneficial insects from the use of
methidathion. To reduce the likelihood for significant mortality to bees, precautionary
labeling was required.
2.3.2. Organophosphate Cumulative Assessment, and Methidathion
Reregistration Eligibility Decision, 2006
Because the Agency determined that methidathion shares a common mechanism of
toxicity with the structurally-related organophosphate insecticides, a cumulative human
health risk assessment for the organophosphate (OP) pesticides was necessary before the
Agency could make a final determination of reregistration eligibility of methidathion.
This cumulative assessment was finalized in 2006 (USEPA 2006b). The results of the
Agency's ecological assessments for methidathion are discussed in the July 31, 2006,
final Reregistration Eligibility Decision (RED) (USEPA 2006a).
The OP cumulative relied on a combined assessment methodology of modeling and
monitoring data for human health exposure via drinking water. Unlike other assessments,
the cumulative approach focused on regions of high OP use. No ecological risks were
evaluated in the OP cumulative process, but its analysis of methidathion fate properties
and its modeling approach remain relevant to this assessment. Additionally, the Agency
requested toxicity data on methidathion oxon to further refine estimates.
2.3.3 Assessment of Potential Effects to 26 Evolutionarily Significant Units
of Pacific Salmon and Steelhead
In April 2004, an assessment of methidathion uses in the Pacific northwest and California
was conducted to determine potential effects to 26 evolutionarily significant units (ESUs)
of Pacific salmon and steelhead. Based on toxicity and fate data, the usage information,
habitat information and other factors, that assessment determined that uses of
methidathion would have no effect on 7 ESUs, was not likely to adversely affect 9 ESUs
and may affect the remaining 10 ESUs.
2.4 Stressor Source and Distribution
2.4.1 Environmental Fate and Transport
2.4.1.1. Physical and Chemical Properties
Chemical name: 3-Dimethoxyphosphinothioylthiomethyl-5-methoxy-1,3,4-
thiadiazol-2(3H)-one.
Chemical structure:
20
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OCH
PSCH
Molecular formula:
Molecular weight:
Physical state:
Melting point:
Solubility (20°C):
Vapor pressure (20°C):
Henry's Law Constant:
Octanol/W ater Partition
39-40°C.
250 mg/L water;
2.48 x 10"6 mm Hg
3.97 x 10"9 atm m3/mol
295 at pH 6.1
CfiHuNjO^Sa.
302.3 g/mol
Colorless crystals.
Coefficient:
2.4.1.2. Environmental Fate Properties
Methidathion is classified as moderately mobile, and through most routes of
transformation for which data are available, initial degradation is rapid and occurs with
half4ives of less than two weeks, although in some cases, transformation slows and lower
levels can persist for a longer time. Table 2.1 summarizes the environmental fate
properties of methidathion, along with the major and minor degradates detected in the
submitted environmental fate and transport studies. See Appendix A for a bibliography
of these submitted studies.
21
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Table 2.1. Summary of Methidathion Environmental Fate Properties
Study
Value (units)
Major Degradates3
Minor Degradates
mrii) #
Study Status
Hydrolysis
half life
pH 5: 37, 27d
pH 7: 48, 25d
pH 9: 13, 8d
GS-12956
Des-methyl GS-13007
42037701
44554501
Acceptable
Acceptable
Direct Aqueous
Photolysis
10.0 db
GS-12956
42081709
Supplemental
Soil Photolysis
40 dc
GS-12956
Methidathion oxon (4.9%)
GS-28307
42081710
Supplemental
Air Photolysis
No data
-
42215101
Unacceptable
Aerobic Soil
Metabolism
11.3 d (sandy loam)
8.5 d (sandy loam) d
Unidentifiede
Methidathion oxon (0.2%)
GS-12956, GS-28369
GS-283 70, GS-20865
44545101
42262501
Acceptable
Supplemental
Anaerobic Soil
Metabolism
DT90 < 30 d
(System)
GS-28369, GS-12956,
Two unidentified
42262501
Supplemental
Anaerobic Aquatic
Metabolism
No data
-
No study
submitted
Aerobic Aquatic
Metabolism
No data
-
No study
submitted
Mobility
Kd: 2.5 to 14.8 mL/g
Avg = 6.5 mL/g
Koc: 194 to 589 mL/goc
Avg = 364 mL/goc
n/a
00158529
Acceptable
Laboratory Volatility
No data
--
42098801
Unacceptable
Terrestrial Field
Dissipation
CA: 5-30 d (4 plots)
NE: <15 d (1 plot)
GS-12956
methidathion oxon
40094103
41924401
41924402
Supplemental
Supplemental
Supplemental
a Major degradates are those formed at >10% of the applied. Although a minor degradate, methidathion oxon is of toxicological
concern and so the maximum percentage detected is reported here. Other oxon degradates that may be of toxicological concern may
not have been detected because available studies did not include radiolabeling on the phosphate ester side chain of the parent
compound.
b The RED reported half-life of 11.6 days was not corrected for sunlight or for results from the dark control.
c Problems with the study suggest that the measured half-life of 40 days is not representative of actual behavior. An assessment by the
CDPR reports a soil photolysis half-life of 1.5 days.
d Previous assessments reported this value as 3 days. That is in fact the DT50 and so this was updated to represent the linear first
order half-life.
e The degradate was unidentified but was tentatively characterized as a cyclic structure resulting from the reaction of carbazic acid and
cysteine.
Both aerobic and anaerobic soil metabolism show initially rapid degradation. In two
aerobic soil metabolism studies, DT50s were 3 to 7 days and linear first order half-lives
were less than 2 weeks, although one study showed some slowing over time, indicating
that methidathion may persist at lower levels for several months. One anaerobic study
22
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was conducted but sampling was insufficient to determine a half-life. At the time of
flooding, methidathion represented 41% of the initially applied radioactivity and by the
first sampling point at 30 days post-flooding, it had been reduced to 6%. However, by 30
days later metabolism had not progressed, with 5% of the initially applied radioactivity
remaining as methidathion. On the soil surface, photodegradation of methidathion may
occur, but there is some uncertainty regarding the rate. In the submitted laboratory study,
photolysis was slow with a half-life of 40.6 days, but problems in the study suggest this
may underestimate the actual rate. An assessment by the California Department of
Pesticide Regulation (CDPR) reports a much more rapid soil photolysis half-life of 1.5
days (Washburne, 2003).
Methidathion may reach water bodies through spray drift or runoff, in the dissolved phase
or adsorbed to eroding soil. In water, methidathion is only moderately susceptible to
abiotic hydrolysis in acidic and neutral conditions with half-lives in two studies ranging
from 25 to 48 days. Dissipation rates increase in alkaline conditions with half-lives of 8
to 13 days at pH 9. In clear, shallow water, transformation may be more rapid due to
photodegradation. A laboratory aquatic photolysis study measured a half life of 10.0 d,
corrected for the dark control and natural sunlight. There are no data available for
metabolism rates in aquatic environments.
In supplemental terrestrial field dissipation studies on four plots in California,
methidathion had reported DT50s in the top 6 inches of 5 to 30 days, and in one plot in
Nebraska, the DT50 was <15 days. Methidathion was not detected at depths below 18
inches in the field studies. There were several detections of methidathion in samples
collected from the 12-18 inch cores, but these detections occurred in samples collected
before any irrigation or rainfall event occurred and so appear to have been due to
contamination during sample collection.
Dissipation of dislodgeable residues on foliar surfaces was found to occur with half-lives
of <3 days based on the results of six studies, and two studies found half-lives for
dissipation of total residues in foliage to be 3.5 and 5 days. Additionally, methidathion is
not expected to bioaccumulate.
2.4.1.3. Transformation Products
Soil metabolism of methidathion leads primarily to carbon dioxide and bound residues.
Other major degradates include GS-12956, formed through hydrolysis and through
aquatic and soil photolysis (maxima from 18% to 66% of the applied radioactivity); des-
methyl GS-13007 and PAMMTO, formed through hydrolysis at up to 21% and 18%,
respectively; and an unidentified compound formed through aerobic soil metabolism and
tentatively characterized as a cyclic structure resulting from the reaction of carbazic acid
and cysteine. This compound appears to be persistent, as it reaches its maximum level of
13%) of the applied radioactivity on day 11 but remained at 10% by the end of the study
(day 263). Multiple minor degradates were formed, not all of them identified. See
Appendix B for chemical names and structures of all identified degradates. None of the
23
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major degradates contain the OP moiety and so they are likely to be less toxic than the
parent.
The only detected degradate of toxicological concern is minor degradate methidathion
oxon. Many of the OP pesticides can undergo oxidative desulfonation (cleavage of P=S
bond to form P=0 bond) to form oxons, either through photo-oxidation or chemical
oxidation in the presence of oxidizing agents, and oxons have the potential to be
considerably more toxic than the parent compounds. In submitted laboratory studies,
methidathion oxon has been documented to form via chlorination, soil photolysis (up to
4.9% of the applied), and aerobic soil metabolism (0.2%), and the oxon was also detected
in studies of terrestrial field dissipation and of dislodgeable foliar residues. The results of
the soil photolysis study may underestimate oxon formation, because the maximum level
was measured in the final sampling period and so oxon levels may continue to increase,
and also because the study review concluded that due to a study deficiency, the rate of
transformation observed may have been too low, supported by the more rapid
transformation cited by CDPR (Washburn, 2003). In the dislodgeable residue study,
methidathion was detected in citrus at up to 15 ng/cm2 on leaves, with detectable residues
through 7 days after application (MRID 00131096). No data are available regarding
photo-oxidation in air of volatilized methidathion to its oxon forms, which could be an
important route of transformation, as demonstrated by air monitoring data that find both
methidathion and methidathion oxon at more that 20 km away from any use location.
There are no data to characterize the fate of methidathion oxon.
It should be noted that due to an issue in the laboratory study design, there could be
additional degradates of toxicological concern that were not observed. In laboratory
transformation studies, only the thiadiazole ring of the compound had radioactive
labeling and so degradates containing the phosphate ester portion of the parent compound
may not have been observed. In particular, major degradate GS-12956 is formed through
cleavage of the C-N bond. Therefore, GS-12956 does not contain the OP moiety but the
cleavage would result in a corresponding phosphate ester (phophorodithioic acid, o-o-s-
trimethyl ester) which retains the OP moiety and so can be assumed to be toxic. Given
that GS-12956 is formed in amounts up to 66% of the applied radioactivity, it is likely
that the corresponding phosphate ester is formed as well. Additionally, this phosphate
ester degradate could also undergo oxidative desulfonation to lead to additional oxon
transformation products that would not have been detected in laboratory studies and may
be of toxicological concern as well.
2.3.1.4. Environmental Transport Properties
The Koc model appears to be appropriate to describe methidathion adsorption. Based on
measured Koc values of 113 to 342 mL/g0C, methidathion is classified as moderately
mobile according to the FAO classification scheme.
A number of studies have documented atmospheric transport and re-deposition of
pesticides from the Central Valley to the Sierra Nevada Mountains (Fellers et al., 2004,
Sparling et al., 2001, LeNoir et al., 1999, and McConnell et al., 1998). Prevailing winds
24
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blow across the Central Valley eastward to the Sierra Nevada Mountains, transporting
airborne industrial and agricultural pollutants into the Sierra Nevada ecosystems (Fellers
et al., 2004, LeNoir et al., 1999, and McConnell et al., 1998). Several sections of critical
habitat for the CLRF are located east of the Central Valley. The magnitude of transport
via secondary drift depends on methidathion's ability to be mobilized into air and its
eventual removal through wet and dry deposition of gases/particles and photochemical
reactions in the atmosphere. The vapor pressure of methidathion (2.48 x 10"6 mm Hg)
and its Henry's Law constant (3.97 x 10"9 atm m3/mol) would suggest that it has a low
potential to volatize from dry or moist soils or from water, but monitoring studies have
detected methidathion and its oxon in air at distances intermediate and up to 20 km away
from any use sites (Royce et al, 1993; Aston and Seiber, 1997; Majewski, 2006). Given
the potential toxicity of these residues, this route of transport could be significant for
assessing the extent of ecological effects.
2.4.2 Mechanism of Action
Methidathion is an insecticide belonging to the organophosphate class of pesticides. The
pesticide acts through inhibition of acetylcholinesterase and is used to kill a broad range
of insects and mites. Organophosphate toxicity is based on the inhibition of the enzyme
acetylcholinesterase which cleaves the neurotransmitter acetylcholine. Inhibition of
acetylcholinesterase by organophosphate insecticides, such as methidathion, interferes
with proper neurotransmission in cholinergic synapses and neuromuscular junctions
(USEPA 2000).
Target pests include the peach twig borer, scale insects, artichoke plume moth,
leafminers, spider mites, boll weevils, bollworms, lygus bug, whitefly, aphid, pear psylla,
mealybugs, thrips, sunflower stern weevil, sunflower moth, sunflower seed weevil,
sunflower midge, Banks grass mite, flea beetle, hornworm, tobacco budworm, codling
moth and hickory shuckworm.
2.4.3 Use Characterization
2.4.3.1 Use Statistics
Methidathion is used as an insecticide on a variety of terrestrial food and feed crops and
terrestrial non-food crops. As shown in Figure 2.1, U.S. Geological Survey (USGS)
National Water Quality Assessment Program (NAWQA) data indicate that in 2002,
methidathion was used on agricultural crops predominantly in California, Washington,
southern Texas, Pennsylvania, and New Jersey. At that time, the use of methidathion on
citrus fruit represented about 30% of the national use. Based on national usage data
compiled by the Biological and Economic Analysis Division (BEAD) primarily from
2001 to 2006, on average, roughly 110,000 pounds of methidathion are applied annually
to agricultural crops, 95% of it in California. These data show that usage is highest on
almonds and oranges, with annual average applications to each of 20,000 lbs. a.i. The
crop with the highest average percent crop treated with methidathion is artichokes (60%).
25
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METHIDATHION - insecticide
2002 estimated annual agricultural use
Crops
Total
Percent
pounds applied
national use
citrus fruit
25093
31.55
artichokes
11869
14.92
almonds
10971
13.80
apples
6995
8.80
peaches
5670
7.13
plums and prunes
3632
4.57
cherries
3130
3.94
nectarines
3089
3.88
olives
2961
3.72
walnuts
2827
3.56
Average annual use of
active ingredient
(pounds per square mile of agricultural
land in county)
EH no estimated use
~ 0.001 to 0.003
~ 0.004 to 0.009
~ 0.01 to 0.04
~ 0.041 to 0.539
¦ >=0.54
Figure 2.1. Map of Estimated Annual Agricultural Use of Methidathion in 2002.
Source: http://water.usgs.gov/nawaa/pnsp/usage/maps/show map.php?vear=02&map=m6037
Use data specific to California are available from the California Department of Pesticide
Regulation's (CDPR) Pesticide Use Reporting (PUR) database, which includes every
pesticide application made by professional applicators. The Biological and Economic
Analysis Division (BEAD) summarized these data, from 1999 to 2006, to the county
level by site, pesticide, and unit treated. Based on this analysis, an average of 82,031 lbs
of methidathion was applied in California to an average of 52,823 acres per year. Use
was at a maximum of 177,105 lbs in 1999 and then dropped by nearly half the following
year, to 98,129 lbs, and remained relatively stable between 2003 and 2006 with average
applications ranging from around 50,000 lb/year to 60,000 lb/year. From 1999-2006,
methidathion was used in a total of 34 counties involving 41 different uses. Four
counties accounted for 70% of the total lbs applied on average per county [Kern (25%),
Tulare (20%), Monterey (14%), Fresno (11%)] (see Figure 2.2). Fruit orchards,
including apple, apricot, cherry, nectarine, peach, pear, plum, and prune, accounted for
approximately 30% of the total lbs applied per year in CA on average. Other major crops
include almonds (23%), oranges (17%), and artichokes (14%). This analysis may not be
entirely representative of current use patterns because labeled uses may have changed
since these data were collected, and because it may also include misreporting. Complete
data from the BEAD analysis of the CDPR PUR database are presented in Appendix C.
26
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I Lassen
Tehama
:jienn
-'Iscer
alswena
Merced
Fresno
San Luis Obi
Sarta Barbara
ventura
H: I've re de
Imperial
Methidathion Use
Pounds per County
| | 1 - 500
500- 1,000
] 1,000-5,000
| 5,000-10,000
| 10,000- 15,000
¦ 15,000-21,000
County (Founds)
Butte (1480)
Calaveras (1.5)
Colusa (129) '
Contra Costa (110)
Fresno (9005)
Glenn (470)
Imperial (353)
Kem (20,177)
Kings (1350)
Lassen (101)
Madera (2116)
Merced (2220)
Monterey (11,624)
Placer (10)
Riverside (574)
Sacramento (18)
San Joaquin (5901)
San Luis Obispo (5)
San Mateo (3)
Santa Barbara (129)
Santa Clara (6)
Santa Cms (32)
Shasta (592)
Solano (18)
Stanislaus (3143)
Sutter (2978)
Tehama (540)
Tulare (16,998)
Ventura (47)
Yolo (56)
Yuba (1800)
Figure 2.2 Average Pounds of Methidathion Applied Per County from 1996 - 2005
27
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2.4.3.2 Application Rates and Methods
Analysis of labeled use information is the critical first step in evaluating the federal
action. The current label for methidathion represents the FIFRA regulatory action;
therefore, labeled use and application rates specified on the label form the basis of this
assessment. The assessment of use information is critical to the development of the action
area and selection of appropriate modeling scenarios and inputs. Table 2.2 presents the
uses and corresponding application rates considered in this assessment, based on a
verification memorandum from SRRD (Dirk Helder, September 2, 2008).
Methidathion is formulated as a wettable powder in water-soluble bags (25% active
ingredient) or as an emulsifiable concentrate (22% - 24% active ingredient). Application
rates range from 0.25 to 5.0 lbs active ingredient/acre. Application is via foliar treatment
and application equipment includes fixed wing aircraft, ground boom, air blast, low-
pressure handwand or backpack sprayer, although based on application instructions for
citrus, olives, and nursery stock, these uses are assumed to be applied through ground
methods only. Labels have additional requirements in order to reduce spray drift. These
include the buffers and other mitigation actions listed in Section 2.3.1 as well as a
requirement for a spray with medium to coarse droplet size applied at a maximum boom
height of 4 ft about the ground or canopy for ground spray and of 10 ft for aerial spray.
Table 2.2 Methidathion Uses Assessed for the CRLF
Use
Max. Single
Appl. Rate (lb
ai/A)
Max. Number of
Applications
Number of
Crop Cycles Per
Year
Minimum
Application
Interval (Days)
NON-FOOD/NON-FEED USES
alfalfa
1
NS
2-9 a
NS
nursery stock b
(includes ornamental
herbaceous plants and
ornamental woody
shrubs and vines)
10 c
1/cc d
r
NA
FOOD/FEED USES
alfalfa
1
1/cc
2-9 a
NA
almond
3
1/cc
r
EC: 14
WP: NS
apple
3
1/cc
i a
NA
apricot
3
1/cc
i e
NA
artichoke
1
8/cc
i e
14
calamondin b
4
1/cc
ie
NA
cherry
3
1/cc
ie
NA
citron (citrus)b
4
1/cc
ie
NA
citrus b
5
2/cc
i e
45
citrus hybrids other
than tangelo
4
1/cc
i e
NA
clover
1
2/cc
NS
cotton (unspecified)
1
4/cc
i a
5
28
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Table 2.2 Methidathion Uses Assessed for the CRLF
Max. Single
Max. Number of
Applications
Number of
Minimum
Use
Appl. Rate (lb
ai/A)
Crop Cycles Per
Year
Application
Interval (Days)
grapefruit
5
2/cc
la
45
kiwi fruit
2
1/1 yr
NA
NA
kumquatb
4
1/cc
1e
NA
lemon b
5
2/cc
1a
45
lime b
4
1/cc
1e
NA
mango b
0.25
5/cc
1e
21
nectarine
3
1/cc
1a
NA
olive b
3
1/cc
le
NA
orange
5
2/cc
1a
45
peach
3
1/cc
1a
NA
pear
3
1/cc
1a
NA
plum
3
1/cc
1e
NA
prune
3
1/cc
1e
NA
pummelo (shaddock)
4
1/cc
1e
NA
safflower (unspecified)
0.5
3/1 yr
NA
7
sunflower
0.5
3/cc
1e
7
tangelo b
4
1/cc
1a
NA
tangerines b
4
1/cc
1a
NA
timothy
1
NS
1e
NA
walnut (english/black)
EC: 2
WP: 2
EC: 2/cc
WP: 3/cc
le
EC: 2
WP: 2
NA = Not Applicable NS = Not Specified on the label
a U.S. EPA. 2007. Memo from Monisha Kaul (BEAD) to Melissa Panger (EFED). Subject: Maximum Number of Crop Cycles Per
Year in California for Methomyl Use Sites. Dated February 28.
Ground spray only
c The nursery stock application rate is reported on labels as 0.5 lb a.i./100 gal spray with no instructions for application on areal basis.
Therefore, this rate is based on the general labels statement that methidathion should not be applied at greater than 10 lb a.i./A.
cc = crop cycle
e Number of crop cycles per year as assumed by EFED.
2.5 Assessed Species
The CRLF was federally listed as a threatened species by USFWS effective June 24,
1996 (USFWS 1996). It is one of two subspecies of the red-legged frog and is the largest
native frog in the western United States (USFWS 2002). A brief summary of information
regarding CRLF distribution, reproduction, diet, and habitat requirements is provided in
Sections 2.5.1 through 2.5.4, respectively. Further information on the status, distribution,
and life history of and specific threats to the CRLF is provided in Attachment 1.
Final critical habitat for the CRLF was designated by USFWS on April 13, 2006
(USFWS 2006; 71 FR 19244-19346). Further information on designated critical habitat
for the CRLF is provided in Section 2.6.
2.5.1 Distribution
29
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The CRLF is endemic to California and Baja California (Mexico) and historically
inhabited 46 counties in California including the Central Valley and both coastal and
interior mountain ranges (USFWS 1996). Its range has been reduced by about 70%, and
the species currently resides in 22 counties in California (USFWS 1996). The species has
an elevational range of near sea level to 1,500 meters (5,200 feet) (Jennings and Hayes
1994); however, nearly all of the known CRLF populations have been documented below
1,050 meters (3,500 feet) (USFWS 2002).
Populations currently exist along the northern California coast, northern Transverse
Ranges (USFWS 2002), foothills of the Sierra Nevada (5-6 populations), and in southern
California south of Santa Barbara (two populations) (Fellers 2005a). Relatively larger
numbers of CRLFs are located between Marin and Santa Barbara Counties (Jennings and
Hayes 1994). A total of 243 streams or drainages are believed to be currently occupied
by the species, with the greatest numbers in Monterey, San Luis Obispo, and Santa
Barbara counties (USFWS 1996). Occupied drainages or watersheds include all bodies
of water that support CRLFs (i.e., streams, creeks, tributaries, associated natural and
artificial ponds, and adjacent drainages), and habitats through which CRLFs can move
(i.e., riparian vegetation, uplands) (USFWS 2002).
The distribution of CRLFs within California is addressed in this assessment using four
categories of location including recovery units, core areas, designated critical habitat, and
known occurrences of the CRLF reported in the California Natural Diversity Database
(CNDDB) that are not included within core areas and/or designated critical habitat (see
Figure 2.2). Recovery units, core areas, and other known occurrences of the CRLF from
the CNDDB are described in further detail in this section, and designated critical habitat
is addressed in Section 2.6. Recovery units are large areas defined at the watershed level
that have similar conservation needs and management strategies. The recovery unit is
primarily an administrative designation, and land area within the recovery unit boundary
is not exclusively CRLF habitat. Core areas are smaller areas within the recovery units
that comprise portions of the species' historic and current range and have been
determined by USFWS to be important in the preservation of the species. Designated
critical habitat is generally contained within the core areas, although a number of critical
habitat units are outside the boundaries of core areas, but within the boundaries of the
recovery units. Additional information on CRLF occurrences from the CNDDB is used
to cover the current range of the species not included in core areas and/or designated
critical habitat, but within the recovery units.
Other Known Occurrences from the CNDBB
The CNDDB provides location and natural history information on species found in
California. The CNDDB serves as a repository for historical and current species location
sightings. Information regarding known occurrences of CRLFs outside of the currently
occupied core areas and designated critical habitat is considered in defining the current
range of the CRLF. See: http://www.dfg.ca.gov/bdb/html/cnddb info.html for additional
information on the CNDDB.
30
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Legend
~ Recovery Unit Boundaries ^
|] Currently Occupied Core Areas
| Critical Habitat
| CNDDB Occurence Sections
County Boundaries o
Recovery Units
1. Sierra Nevada Foothills and Central Valley
2. North Coast Range Foothills and Western
Sacramento River Valley
3. North Coast and North San Francisco Bay
4. South and East San Francisco Bay
5. Central Coast
6. Diablo Range and Salinas Valley
7. Northern Transverse Ranges and Tehachapi
Mountains
8. Southern Transverse and Peninsular Ranges
180 Miles
_1
Core Areas
1.
Feather River
19.
Watsonville Slough-Elkhorn Slough
2.
Yuba River- S. Fork Feather River
20.
Carmel River - Santa Lucia
3.
Traverse Creek/ Middle Fork/ American R. Rubicon
21.
Gablan Range
4.
Cosumnes River
22.
Estero Bay
5.
South Fork Calaveras River*
23.
Arroyo Grange River
6.
Tuolumne River*
24.
Santa Maria River — Santa Ynez River
7.
Piney Creek*
25.
Sisquoc River
8.
Cottonwood Creek
26.
Ventura River - Santa Clara River
9.
Putah Creek - Cache Creek*
27.
Santa Monica Bay — Venura Coastal Streams
10.
Lake Berryessa Tributaries
28.
Estrella River
11.
Upper Sonoma Creek
29.
San Gabriel Mountain*
12.
Petaluma Creek — Sonoma Creek
30.
Forks of the Mojave*
13.
Pt. Reyes Peninsula
31.
Santa Ana Mountain*
14.
Belvedere Lagoon
32.
Santa Rosa Plateau
15.
Jameson Canyon - Lower Napa River
33.
San Luis Ray*
16.
East San Francisco Bay
34.
Sweetwater*
17.
Santa Clara Valley
35.
Laguna Mountain*
18.
South San Francisco Bay
* Core areas that were historically occupied by the California red-legged frog are not included in the map
figure 2.2 Recovery Unit, Core Area, Critical Habitat, and Occurrence
Designations for CRLF
31
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2.5.2 Reproduction
CRLFs breed primarily in ponds; however, they may also breed in quiescent streams,
marshes, and lagoons (Fellers 2005a). According to the Recovery Plan (USFWS 2002),
CRLFs breed from November through late April. Peaks in spawning activity vary
geographically; Fellers (2005b) reports peak spawning as early as January in parts of
coastal central California. Eggs are fertilized as they are being laid. Egg masses are
typically attached to emergent vegetation, such as bulrushes (Scirpus spp.) and cattails
(Typha spp.) or roots and twigs, and float on or near the surface of the water (Hayes and
Miyamoto 1984). Egg masses contain approximately 2000 to 6000 eggs ranging in size
between 2 and 2.8 mm (Jennings and Hayes 1994). Embryos hatch 10 to 14 days after
fertilization (Fellers 2005a) depending on water temperature. Egg predation is reported
to be infrequent and most mortality is associated with the larval stage (particularly
through predation by fish); however, predation on eggs by newts has also been reported
(Rathburn 1998). Tadpoles require 11 to 28 weeks to metamorphose into juveniles
(terrestrial-phase), typically between May and September (Jennings and Hayes 1994,
USFWS 2002); tadpoles have been observed to over-winter (delay metamorphosis until
the following year) (Fellers 2005b, USFWS 2002). Males reach sexual maturity at 2
years, and females reach sexual maturity at 3 years of age; adults have been reported to
live 8 to 10 years (USFWS 2002). Figure 2.3 depicts CRLF annual reproductive timing
Figure 2.3 - CRLF J
teproductive Events by Mont
h
J
F
M
A
M
J
J
A
S
o
N
D
Light Blue = Breeding/Egg Masses
Green = Tadpoles (except those that over-winter)
Orange =
Adults and juveniles can be present all year
2.5.3 Diet
Although the diet of CRLF aquatic-phase larvae (tadpoles) has not been studied
specifically, it is assumed that their diet is similar to that of other frog species, with the
aquatic phase feeding exclusively in water and consuming diatoms, algae, and detritus
(USFWS 2002). Tadpoles filter and entrap suspended algae (Seale and Beckvar, 1980)
via mouthparts designed for effective grazing of periphyton (Wassersug, 1984,
Kupferberg et al.\ 1994; Kupferberg, 1997; Altig and McDiarmid, 1999).
Juvenile and adult CRLFs forage in aquatic and terrestrial habitats, and their diet differs
greatly from that of larvae. The main food source for juvenile aquatic- and terrestrial-
phase CRLFs is thought to be aquatic and terrestrial invertebrates found along the
shoreline and on the water surface. Hayes and Tennant (1985) report, based on a study
examining the gut content of 35 juvenile and adult CRLFs, that the species feeds on as
many as 42 different invertebrate taxa, including Arachnida, Amphipoda, Isopoda,
32
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Insecta, and Mollusca. The most commonly observed prey species were larval alderflies
(Sialis cf. californica), pillbugs (Armadilliadrium vulgare), and water striders (Gerris sp).
The preferred prey species, however, was the sowbug (Hayes and Tennant, 1985). This
study suggests that CRLFs forage primarily above water, although the authors note other
data reporting that adults also feed under water, are cannibalistic, and consume fish. For
larger CRLFs, over 50% of the prey mass may consists of vertebrates such as mice, frogs,
and fish, although aquatic and terrestrial invertebrates were the most numerous food
items (Hayes and Tennant 1985). For adults, feeding activity takes place primarily at
night; for juveniles feeding occurs during the day and at night (Hayes and Tennant 1985).
2.5.4 Habitat
CRLFs require aquatic habitat for breeding, but also use other habitat types including
riparian and upland areas throughout their life cycle. CRLF use of their environment
varies; they may complete their entire life cycle in a particular habitat or they may utilize
multiple habitat types. Overall, populations are most likely to exist where multiple
breeding areas are embedded within varying habitats used for dispersal (USFWS 2002).
Generally, CRLFs utilize habitat with perennial or near-perennial water (Jennings et al.
1997). Dense vegetation close to water, shading, and water of moderate depth are habitat
features that appear especially important for CRLF (Hayes and Jennings 1988).
Breeding sites include streams, deep pools, backwaters within streams and creeks, ponds,
marshes, sag ponds (land depressions between fault zones that have filled with water),
dune ponds, and lagoons. Breeding adults have been found near deep (0.7 m) still or slow
moving water surrounded by dense vegetation (USFWS 2002); however, the largest
number of tadpoles have been found in shallower pools (0.26 - 0.5 m) (Reis, 1999). Data
indicate that CRLFs do not frequently inhabit vernal pools, as conditions in these habitats
generally are not suitable (Hayes and Jennings 1988).
CRLFs also frequently breed in artificial impoundments such as stock ponds, although
additional research is needed to identify habitat requirements within artificial ponds
(USFWS 2002). Adult CRLFs use dense, shrubby, or emergent vegetation closely
associated with deep-water pools bordered with cattails and dense stands of overhanging
vegetation (http://www.fws.gov/endangered/features/rl frog/rlfrog.html#where).
In general, dispersal and habitat use depends on climatic conditions, habitat suitability,
and life stage. Adults rely on riparian vegetation for resting, feeding, and dispersal. The
foraging quality of the riparian habitat depends on moisture, composition of the plant
community, and presence of pools and backwater aquatic areas for breeding. CRLFs can
be found living within streams at distances up to 3 km (2 miles) from their breeding site
and have been found up to 30 m (100 feet) from water in dense riparian vegetation for up
to 77 days (USFWS 2002).
During dry periods, the CRLF is rarely found far from water, although it will sometimes
disperse from its breeding habitat to forage and seek other suitable habitat under downed
trees or logs, industrial debris, and agricultural features (UWFWS 2002). According to
Jennings and Hayes (1994), CRLFs also use small mammal burrows and moist leaf litter
33
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as habitat. In addition, CRLFs may also use large cracks in the bottom of dried ponds as
refugia; these cracks may provide moisture for individuals avoiding predation and solar
exposure (Alvarez 2000).
2.6 Designated Critical Habitat
In a final rule published on April 13, 2006, 34 separate units of critical habitat were
designated for the CRLF by USFWS (USFWS 2006; FR 51 19244-19346). A summary
of the 34 critical habitat units relative to USFWS-designated recovery units and core
areas (previously discussed in Section 2.5.1) is provided in Attachment 1.
'Critical habitat' is defined in the ESA as the geographic area occupied by the species at
the time of the listing where the physical and biological features necessary for the
conservation of the species exist, and there is a need for special management to protect
the listed species. It may also include areas outside the occupied area at the time of
listing if such areas are 'essential to the conservation of the species.' All designated
critical habitat for the CRLF was occupied at the time of listing. Critical habitat receives
protection under Section 7 of the ESA through prohibition against destruction or
modification with regard to actions carried out, funded, or authorized by a federal
Agency. Section 7 requires consultation on federal actions that are likely to result in the
destruction or modification of critical habitat.
To be included in a critical habitat designation, the habitat must be 'essential to the
conservation of the species.' Critical habitat designations identify, to the extent known
using the best scientific and commercial data available, habitat areas that provide
essential life cycle needs of the species or areas that contain certain primary constituent
elements (PCEs) (as defined in 50 CFR 414.12(b)). PCEs include, but are not limited to,
space for individual and population growth and for normal behavior; food, water, air,
light, minerals, or other nutritional or physiological requirements; cover or shelter; sites
for breeding, reproduction, rearing (or development) of offspring; and habitats that are
protected from disturbance or are representative of the historic geographical and
ecological distributions of a species. The designated critical habitat areas for the CRLF
are considered to have the following PCEs that justify critical habitat designation:
• Breeding aquatic habitat;
• Non-breeding aquatic habitat;
• Upland habitat; and
• Dispersal habitat.
Further description of these habitat types is provided in Attachment 1.
Occupied habitat may be included in the critical habitat only if essential features within
the habitat may require special management or protection. Therefore, USFWS does not
include areas where existing management is sufficient to conserve the species. Critical
habitat is designated outside the geographic area presently occupied by the species only
when a designation limited to its present range would be inadequate to ensure the
34
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conservation of the species. For the CRLF, all designated critical habitat units contain all
four of the PCEs, and were occupied by the CRLF at the time of FR listing notice in
April 2006. The FR notice designating critical habitat for the CRLF includes a special
rule exempting routine ranching activities associated with livestock ranching from
incidental take prohibitions. The purpose of this exemption is to promote the
conservation of rangelands, which could be beneficial to the CRLF, and to reduce the rate
of conversion to other land uses that are incompatible with CRLF conservation. Please
see Attachment 1 for a full explanation on this special rule.
USFWS has established modification standards for designated critical habitat (USFWS
2006). Activities that may destroy or modify critical habitat are those that alter the PCEs
and jeopardize the continued existence of the species. Evaluation of actions related to
use of methidathion that may alter the PCEs of the CRLF's critical habitat form the basis
of the critical habitat impact analysis. According to USFWS (2006), activities that may
affect critical habitat and therefore result in adverse effects to the CRLF include, but are
not limited to the following:
(1) Significant alteration of water chemistry or temperature to levels beyond the
tolerances of the CRLF that result in direct or cumulative adverse effects to
individuals and their life-cycles.
(2) Alteration of chemical characteristics necessary for normal growth and viability
of juvenile and adult CRLFs.
(3) Significant increase in sediment deposition within the stream channel or pond or
disturbance of upland foraging and dispersal habitat that could result in
elimination or reduction of habitat necessary for the growth and reproduction of
the CRLF by increasing the sediment deposition to levels that would adversely
affect their ability to complete their life cycles.
(4) Significant alteration of channel/pond morphology or geometry that may lead to
changes to the hydrologic functioning of the stream or pond and alter the timing,
duration, water flows, and levels that would degrade or eliminate the CRLF
and/or its habitat. Such an effect could also lead to increased sedimentation and
degradation in water quality to levels that are beyond the CRLF's tolerances.
(5) Elimination of upland foraging and/or aestivating habitat or dispersal habitat.
(6) Introduction, spread, or augmentation of non-native aquatic species in stream
segments or ponds used by the CRLF.
(7) Alteration or elimination of the CRLF's food sources or prey base (also
evaluated as indirect effects to the CRLF).
As previously noted in Section 2.1, the Agency believes that the analysis of direct and
indirect effects to listed species provides the basis for an analysis of potential effects on
the designated critical habitat. Because methidathion is expected to directly impact living
organisms within the action area, critical habitat analysis for methidathion is limited in a
practical sense to those PCEs of critical habitat that are biological or that can be
reasonably linked to biologically mediated processes.
35
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2.7 Action Area
For listed species assessment purposes, the action area is considered to be the area
affected directly or indirectly by the federal action and not merely the immediate area
involved in the action (50 CFR 402.02). It is recognized that the overall action area for
the national registration of methidathion is likely to encompass considerable portions of
the United States based on the large array of agricultural uses. However, the scope of this
assessment limits consideration of the overall action area to those portions that may be
applicable to the protection of the CRLF and its designated critical habitat within the state
of California. The Agency's approach to defining the action area under the provisions of
the Overview Document (USEPA 2004) considers the results of the risk assessment
process to establish boundaries for that action area with the understanding that exposures
below the Agency's defined Levels of Concern (LOCs) constitute a no-effect threshold.
For the purposes of this assessment, attention will be focused on the footprint of the
action (i.e., the area where pesticide application occurs), plus all areas where offsite
transport (i.e., spray drift, downstream dilution, etc.) may result in potential exposure
within the state of California that exceeds the Agency's LOCs.
Deriving the geographical extent of this portion of the action area is based on
consideration of the types of effects that methidathion may be expected to have on the
environment, the exposure levels to methidathion that are associated with those effects,
and the best available information concerning the use of methidathion and its fate and
transport within the state of California. Specific measures of ecological effect for the
CRLF that define the action area include any direct and indirect toxic effect to the CRLF
and any potential modification of its critical habitat, including reduction in survival,
growth, and fecundity as well as the full suite of sublethal effects available in the effects
literature. Therefore, the action area extends to a point where environmental exposures
are below any measured lethal or sublethal effect threshold for any biological entity at the
whole organism, organ, tissue, and cellular level of organization. In situations where it is
not possible to determine the threshold for an observed effect, the action area is not
spatially limited and is assumed to be the entire state of California.
The definition of action area requires a stepwise approach that begins with an
understanding of the federal action. The federal action is defined by the currently labeled
uses for methidathion. An analysis of labeled uses and review of available product labels
was completed. Several of the currently labeled uses are special local needs (SLN) uses
or are restricted to specific states and are excluded from this assessment. In addition, a
distinction has been made between food use crops and those that are non-food/non-
agricultural uses. For those uses relevant to the CRLF, the analysis indicates that, for
methidathion, the following agricultural uses are considered as part of the federal action
evaluated in this assessment:
• Alfalfa, almond, apple, apricot, artichoke, calemondin, cherry, citron, citrus,
clover, cotton, grapefruit, kiwi, kumquat, lemon, lime, mango, nectarine, olive,
oranges, peach, pear, plum, prune, pummelo, safflower, sunflower, tangelo,
tangerine, timothy, and walnut.
36
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In addition, the following non-food and non-agricultural uses are considered:
• Nursery stock, ornamental and herbaceous plants, woody shrubs and vines
Following a determination of the assessed uses, an evaluation of the potential "footprint"
of methidathion use patterns (i.e., the area where pesticide application occurs) is
determined. This "footprint" represents the initial area of concern, based on an analysis
of available land cover data for the state of California. The initial area of concern is
defined as all land cover types and the stream reaches within the land cover areas that
represent the labeled uses described above. A map representing all the land cover types
that make up the initial area of concern for methidathion is presented in Figure 2.4.
The following land cover types were used for methidathion: cultivated crops, developed
(low, medium and high intensity and open space), forest, open water, orchards and
vineyards, pasture/hay, wetlands, turf and rights-of-way. More information regarding
which specific uses are represented for each land cover types can be found in Appendix
D.
37
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Methidathion Initial Area of Concern
f
Pasture/hay use
IHI
Orchard vineyard use
¦¦
Cultivated crop use
I I
County boundaries
0 20 40 80 120
i Kilometers
160
Compiled from California County boundaries (ESRI, 2002),
IJSQA. Gap Analysis Program Orchard/ Vineyard Landcover (GAP)
National Land Cewer Database (NLCD) (MRLC, 2001)
Map created by US Environmental Protection Agency, Office
of Pesticides Programs, Environmental Fate and Effects Division.
Projection: Albers Equal Area Conic USGS, North American
Datum of 1933 (NAD 1 083).
Produced 11/13/2008
Figure 2.4 Initial area of concern, or "footprint" of potential use, for methidathion
Once the initial area of concern is defined, the next step is to define the potenti al
boundaries of the action area by determining the extent of offsite transport via spray drift
38
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and runoff where exposure of one or more taxonomic groups to the pesticide exceeds the
listed species LOCs.
As previously discussed, the action area is defined by the most sensitive measure of
direct and indirect ecological toxic effects including reduction in survival, growth,
reproduction, and the entire suite of sublethal effects from valid, peer-reviewed studies.
Due to the lack of a defined no effect concentration for a mammalian toxicity study
(Yavuz et al., 2005; see Section 4.2.2 for study details), the spatial extent of the action
area (i.e., the boundary where exposures and potential effects are less than the Agency's
LOC) for methidathion cannot be determined. Therefore, it is assumed that the action
area encompasses the entire state of California, regardless of the spatial extent (i.e., initial
area of concern or footprint) of the pesticide use(s).
2.8 Assessment Endpoints and Measures of Ecological Effect
Assessment endpoints are defined as "explicit expressions of the actual environmental
value that is to be protected."5 Selection of the assessment endpoints is based on valued
entities (e.g., CRLF, organisms important in the life cycle of the CRLF, and the PCEs of
its designated critical habitat), the ecosystems potentially at risk (e.g., waterbodies,
riparian vegetation, and upland and dispersal habitats), the migration pathways of
methidathion (e.g., runoff, spray drift, etc.), and the routes by which ecological receptors
are exposed to methidathion (e.g., direct contact, etc.).
2.8.1. Assessment Endpoints for the CRLF
Assessment endpoints for the CRLF include direct toxic effects on the survival,
reproduction, and growth of the CRLF, as well as indirect effects, such as reduction of
the prey base or modification of its habitat. In addition, potential modification of critical
habitat is assessed by evaluating potential effects to PCEs, which are components of the
habitat areas that provide essential life cycle needs of the CRLF. Each assessment
endpoint requires one or more "measures of ecological effect," defined as changes in the
attributes of an assessment endpoint or changes in a surrogate entity or attribute in
response to exposure to a pesticide. Specific measures of ecological effect are generally
evaluated based on acute and chronic toxicity information from registrant-submitted
guideline tests that are performed on a limited number of organisms. Additional
ecological effects data from the open literature are also considered. It should be noted
that assessment endpoints are limited to direct and indirect effects associated with
survival, growth, and fecundity, and do not include the full suite of sublethal effects used
to define the action area. According the Overview Document (USEPA 2004), the
Agency relies on acute and chronic effects endpoints that are either direct measures of
impairment of survival, growth, or fecundity or endpoints for which there is a
scientifically robust, peer reviewed relationship that can quantify the impact of the
measured effect endpoint on the assessment endpoints of survival, growth, and fecundity.
5 From U.S. EPA (1992). Framework for Ecological Risk Assessment. EPA/630/R-92/001.
39
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A complete discussion of all the toxicity data available for this risk assessment, including
resulting measures of ecological effect selected for each taxonomic group of concern, is
included in Section 4 of this document. A summary of the assessment endpoints and
measures of ecological effect selected to characterize potential assessed direct and
indirect CRLF risks associated with exposure to methidathion is provided in Table 2.3.
Table 2.3 Assessment Endpoints and Measures of Ecological Effects
Assessment Endpoint
Measures of Ecological Effects6
Aquatic-Phase CRLF
(Eggs, larvae, juveniles, and adultsf
Direct Effects
1. Survival, growth, and reproduction of CRLF
la. Freshwater fish 96-hour acute LC50
lb. Freshwater fish chronic NOAEC
Indirect Effects and Critical Habitat Effects
2. Survival, growth, and reproduction of CRLF
individuals via indirect effects on aquatic prey food
supply (i.e., fish, freshwater invertebrates, non-
vascular plants)
la. Freshwater fish 96-hour acute LC50; freshwater
invertebrate 48-hr EC50; aquatic non-vascular plant
acute EC50
2b. Freshwater fish chronic NOAEC; freshwater
invertebrate chronic NOAEC
3. Survival, growth, and reproduction of CRLF
individuals via indirect effects on habitat, cover,
food supply, and/or primary productivity (i.e.,
aquatic plant community)
3a. Vascular plant acute EC50
3b. Non-vascular plant acute EC50
4. Survival, growth, and reproduction of CRLF
individuals via effects to riparian vegetation
4a. Distribution of EC25's for monocots
4b. Distribution of EC25's for dicots
Terrestrial-Phase CRLF
(Juveniles and adults)
Direct Effects
5. Survival, growth, and reproduction of CRLF
individuals via direct effects on terrestrial phase
adults and juveniles
5a. Avian acute LD50
5b. Avian chronic NOAEC
Indirect Effects and Critical Habitat Effects
6. Survival, growth, and reproduction of CRLF
individuals via effects on terrestrial prey (i.e.,
terrestrial invertebrates, small mammals , and frogs)
6a. Terrestrial invertebrate acute LD50; mammalian
acute LD50
6b. Mammalian chronic NOAEC
7. Survival, growth, and reproduction of CRLF
individuals via indirect effects on habitat (i.e.,
riparian and upland vegetation)
7a. Distribution of ECNs's for monocots
7b. Distribution of EC25's for dicots
3 Adult frogs are no longer in the "aquatic phase" of the amphibian life cycle; however, submerged adult frogs are considered
"aquatic" for the purposes of this assessment because exposure pathways in the water are considerably different that exposure
pathways on land.
b According to Mayer, D. & C. Johansen. 1990. Pollinator Protection: A Bee & Pesticide Handbook. Wicwas Press. Cheshire, Conn,
p. 161
2.8.2 Assessment Endpoints for Designated Critical Habitat
As previously discussed, designated critical habitat is assessed to evaluate actions related
to the use of methidathion that may alter the PCEs of the CRLF's critical habitat. PCEs
for the CRLF were previously described in Section 2.6. Actions that may modify critical
habitat are those that alter the PCEs and jeopardize the continued existence of the CRLF.
6 All registrant-submitted and open literature toxicity data reviewed for this assessment are included in
Appendix A.
40
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Therefore, these actions are identified as assessment endpoints. It should be noted that
evaluation of PCEs as assessment endpoints is limited to those of a biological nature (i.e.,
the biological resource requirements for the listed species associated with the critical
habitat) and those for which methidathion effects data are available.
Modification to the critical habitat of the CRLF includes, but is not limited to, the
following, as specified by USFWS (2006):
1. Alteration of water chemistry/quality including temperature, turbidity, and
oxygen content necessary for normal growth and viability of juvenile and
adult CRLFs.
2. Alteration of chemical characteristics necessary for normal growth and
viability of juvenile and adult CRLFs.
3. Significant increase in sediment deposition within the stream channel or pond
or disturbance of upland foraging and dispersal habitat.
4. Significant alteration of channel/pond morphology or geometry.
5. Elimination of upland foraging and/or aestivating habitat, as well as dispersal
habitat.
6. Introduction, spread, or augmentation of non-native aquatic species in stream
segments or ponds used by the CRLF.
7. Alteration or elimination of the CRLF's food sources or prey base.
Measures of such possible effects by labeled use of methidathion on critical habitat of the
CRLF are described in Table 2.4. Some components of these PCEs are associated with
physical abiotic features (e.g., presence and/or depth of a water body, or distance between
two sites), which are not expected to be measurably altered by use of pesticides.
Assessment endpoints used for the analysis of designated critical habitat are based on the
modification standard established by USFWS (2006).
41
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Table 2.4 Summary of Assessment Endpoints and Measures of Ecological Effect for
Primary Constituent Elements of Designated Critical Habitat3
Assessment Endpoint
Measures of Ecological Effect
Aquatic-Phase CRLFPCEs
(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat)
Alteration of channel/pond morphology or geometry
and/or increase in sediment deposition within the
stream channel or pond: aquatic habitat (including
riparian vegetation) provides for shelter, foraging,
predator avoidance, and aquatic dispersal for juvenile
and adult CRLFs.
a. Most sensitive aquatic plant EC50
b. Distribution of EC25 values for terrestrial monocots
c. Distribution of EC25 values for terrestrial dicots
Alteration in water chemistry/quality including
temperature, turbidity, and oxygen content necessary
for normal growth and viability of juvenile and adult
CRLFs and their food source.
a. Most sensitive EC50 values for aquatic plants
b. Distribution of EC25 values for terrestrial monocots
c. Distribution of EC25 values for terrestrial dicots
Alteration of other chemical characteristics necessary
for normal growth and viability of CRLFs and their
food source.
a. Freshwater fish 96-hour acute LC\,,: freshwater
invertebrate 48-hr EC50; aquatic plant acute EC50
b. Freshwater fish chronic NOAEC; freshwater
invertebrate chronic NOAEC
Reduction and/or modification of aquatic-based food
sources for pre-metamorphs (e.g., algae)
a. Most sensitive aquatic plant EC50
Terrestrial-Phase CRLF PCEs
(Upland Habitat and Dispersal Habitat)
Elimination and/or disturbance of upland habitat;
ability of habitat to support food source of CRLFs:
Upland areas within 200 ft of the edge of the riparian
vegetation or dripline surrounding aquatic and riparian
habitat that are comprised of grasslands, woodlands,
and/or wetland/riparian plant species that provides the
CRLF shelter, forage, and predator avoidance
a. Distribution of EC25 values for monocots
b. Distribution of EC2S values for dicots
c. Mammalian acute LD50; mammalian chronic NOAEC
d. Freshwater fish 96-hour acute LC50; freshwater fish
chronic NOAEC
e. Avian acute LD™; avian acute LC™; avian chronic
NOAEC
f. Honey Bee acute LD50
Elimination and/or disturbance of dispersal habitat:
Upland or riparian dispersal habitat within designated
units and between occupied locations within 0.7 mi of
each other that allow for movement between sites
including both natural and altered sites which do not
contain barriers to dispersal
Reduction and/or modification of food sources for
terrestrial phase juveniles and adults
Alteration of chemical characteristics necessary for
normal growth and viability of juvenile and adult
CRLFs and their food source.
a Physico-chemical water quality parameters such as salinity, pH, and hardness are not evaluated because these processes are not
biologically mediated and, therefore, are not relevant to the endpoints included in this assessment.
b According to Mayer, D. & C. Johansen. 1990. Pollinator Protection: A Bee & Pesticide Handbook. Wicwas Press. Cheshire, Conn,
p. 161
42
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2.9 Conceptual Model
2.9.1 Risk Hypotheses
Risk hypotheses are specific assumptions about potential adverse effects {i.e.,changes in
assessment endpoints) and may be based on theory and logic, empirical data,
mathematical models, or probability models (U.S. EPA, 1998). For this assessment, the
risk is stressor-linked, where the stressor is the release of methidathion to the
environment. The following risk hypotheses are presumed for this endangered species
assessment:
The labeled use of methidathion within the action area may:
• directly affect the CRLF by causing mortality or by adversely affecting growth or
fecundity;
• indirectly affect the CRLF by reducing or changing the composition of food
supply;
• indirectly affect the CRLF or modify designated critical habitat by reducing or
changing the composition of the aquatic plant community in the ponds and streams
comprising the species' current range and designated critical habitat, thus affecting
primary productivity and/or cover;
• indirectly affect the CRLF or modify designated critical habitat by reducing or
changing the composition of the terrestrial plant community (i.e., riparian habitat)
required to maintain acceptable water quality and habitat in the ponds and streams
comprising the species' current range and designated critical habitat;
• modify the designated critical habitat of the CRLF by reducing or changing
breeding and non-breeding aquatic habitat (via modification of water quality parameters,
habitat morphology, and/or sedimentation);
• modify the designated critical habitat of the CRLF by reducing the food supply
required for normal growth and viability of juvenile and adult CRLFs;
• modify the designated critical habitat of the CRLF by reducing or changing
upland habitat within 200 ft of the edge of the riparian vegetation necessary for shelter,
foraging, and predator avoidance.
• modify the designated critical habitat of the CRLF by reducing or changing
dispersal habitat within designated units and between occupied locations within 0.7 mi of
each other that allow for movement between sites including both natural and altered sites
which do not contain barriers to dispersal.
• modify the designated critical habitat of the CRLF by altering chemical
characteristics necessary for normal growth and viability of juvenile and adult CRLFs.
2.9.2 Diagram
The conceptual model is a graphic representation of the structure of the risk assessment.
It specifies the methidathion and methidathion oxon release mechanisms, biological
receptor types, and effects endpoints of potential concern. The conceptual models for
terrestrial and aquatic exposures are shown in Figures 2.5 and 2.6, respectively, which
43
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include the conceptual models for the aquatic and terrestrial PCE components of critical
habitat. Exposure routes shown in dashed lines are not quantitatively considered because
the contribution of those potential exposure routes to potential risks to the CRLF and
modification to designated critical habitat is expected to be negligible.
Stressor
Source
Exposure
Media
Methidathion applied to use site and its degradate, methidathion oxon,
formed after application
Direct
application
| Spray drift |
Dermal uptake/lnqestio
II Rimoff I
Soil
Long range
atmospheric
transport
Terrestrial
n insects
Ingestion
_L
Terrestrial-phase
amphibians
Terrestrial/riparian plants
grasses/forbs, fruit, seeds
(trees, shrubs)
Root uptake^! ^
Wet/drv deposition"*
-~Ingestion
Ingestion
Receptors
i
-~ Inqestion
Ingestion,
I
Attribute
Change
Birds/terrestrial-
phase amphibians/
reptiles/mammals
1
r
Individual
organisms
Reduced survival
Reduced growth
Reduced reproduction
1
Mammals/
birds
Food chain
Reduction in prey
Modification of PCEs
related to prey availability
Habitat integrity
Reduction in primary productivity
Reduced cover
Community change
Modification of PCEs related to
habitat
Figure 2.5 Conceptual Model for Methidathion Effects on Terrestrial Phase of the
CRLF
44
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Stressor
— — ~! Groundwater |
Source
Runoff
Exposure
Media
Wet/dry deposition ^
Uptake/cell,
roots, leaves
Uptake/gills
or integument
Receptors
Inqe^tion
Inqestion
S 1
Attribute
Change
Long range
atmospheric
transport
Uptake/gills
or integument
Aquatic Animals
Invertebrates
Vertebrates
Aquatic Plants
Non-vascular
Vascular
Surface water/
Sediment
Riparian plants
terrestrial
exposure
pathways see
Figure 2.5
Fish/aquatic-phase
amphibians
**Piscivorous mammals
and birds
Individual
organisms
Reduced survival
Reduced growth
Reduced reproduction
Food chain
Reduction in algae
Reduction in prey
Modification of PCEs
related to prey availability
Methidathion applied to use site and its degradate, methidathion oxon
formed after application
Habitat integrity
Reduction in primary
productivity
Reduced cover
Community change
Modification of PCEs related to
habitat
Figure 2.6 Conceptual Model for Methidathion Effects on Aquatic Phase of the
CRLF
2.10 Analysis Plan
In order to address the risk hypothesis, the potential for direct and indirect effects to the
CRLF, its prey, and its habitat is estimated. In the following sections, the use,
environmental fate, and ecological effects of methidathion are characterized and
integrated to assess the risks. This is accomplished using a risk quotient (ratio of
exposure concentration to effects concentration) approach. Although risk is often defined
as the likelihood and magnitude of adverse ecological effects, the risk quotient-based
approach does not provide a quantitative estimate of likelihood and/or magnitude of an
adverse effect. However, as outlined in the Overview Document (U.S. EPA, 2004), the
likelihood of effects to individual organisms from particular uses of methidathion is
estimated using the probit dose-response slope and either the level of concern (discussed
below) or actual calculated risk quotient value.
45
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2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model
2.10.1.1 Measures of Exposure
The environmental fate properties of methidathion along with available monitoring data
indicate that runoff and spray drift are the principle potential transport mechanisms of
methidathion to the aquatic and terrestrial habitats of the CRLF, and monitoring data
indicate that long range atmospheric transport is a concern as well. In this assessment,
transport of methidathion through runoff and spray drift is considered in deriving
quantitative estimates of methidathion exposure to CRLF, its prey and its habitats. Long
range atmospheric transport is considered qualitatively.
Measures of exposure are based on aquatic and terrestrial models that predict estimated
environmental concentrations (EECs) of methidathion using maximum labeled
application rates and methods of application. The models used to predict aquatic EECs
are the Pesticide Root Zone Model coupled with the Exposure Analysis Model System
(PRZM/EXAMS). The model used to predict terrestrial EECs on food items is T-REX.
The model used to derive EECs relevant to terrestrial and wetland plants is TerrPlant.
These models are parameterized using relevant reviewed registrant-submitted
environmental fate data.
PRZM (v3.12.2, May 2005) and EXAMS (v2.98.4.6, April 2005) are screening
simulation models coupled with the input shell pe5.pl (Aug 2007) to generate daily
exposures and l-in-10 year EECs of methidathion that may occur in surface water bodies
adjacent to application sites receiving methidathion through runoff and spray drift.
PRZM simulates pesticide application, movement and transformation on an agricultural
field and the resultant pesticide loadings to a receiving water body via runoff, erosion and
spray drift. EXAMS simulates the fate of the pesticide and resulting concentrations in
the water body. The standard scenario used for ecological pesticide assessments assumes
application to a 10-hectare agricultural field that drains into an adjacent 1-hectare water
body, 2-meters deep (20,000 m3 volume) with no outlet. PRZM/EXAMS was used to
estimate screening-level exposure of aquatic organisms to methidathion. The measure of
exposure for aquatic species is the l-in-10 year return peak or rolling mean concentration.
The l-in-10 year peak is used for estimating acute exposures of direct effects to the
CRLF, as well as indirect effects to the CRLF through effects to potential prey items,
including: algae, aquatic invertebrates, fish and frogs. The l-in-10-year 60-day mean is
used for assessing chronic exposure to the CRLF and fish and frogs serving as prey
items; the 1-in-10-year 21-day mean is used for assessing chronic exposure for aquatic
invertebrates, which are also potential prey items.
Exposure estimates for the terrestrial-phase CRLF and terrestrial invertebrates and
mammals (serving as potential prey) assumed to be in the target area or in an area
exposed to spray drift are derived using the T-REX model (version 1.3.1, 12/07/2006).
This model incorporates the Kenega nomograph, as modified by Fletcher et al. (1994),
which is based on a large set of actual field residue data. The upper limit values from the
nomograph represented the 95th percentile of residue values from actual field
46
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measurements (Hoerger and Kenega, 1972). For modeling purposes, direct exposures of
the CRLF to methidathion through contaminated food are estimated using the EECs for
the small bird (20 g) which consumes small insects. Dietary-based and dose-based
exposures of potential prey (small mammals) are assessed using the small mammal (15 g)
which consumes short grass. The small bird (20g) consuming small insects and the small
mammal (15g) consuming short grass are used because these categories represent the
largest RQs of the size and dietary categories in T-REX that are appropriate surrogates
for the CRLF and one of its prey items. Estimated exposures of terrestrial insects to
methidathion are bound by using the dietary based EECs for small insects and large
insects.
Birds are currently used as surrogates for terrestrial-phase CRLF. However, amphibians
are poikilotherms (body temperature varies with environmental temperature) while birds
are homeotherms (temperature is regulated, constant, and largely independent of
environmental temperatures). Therefore, amphibians tend to have much lower metabolic
rates and lower caloric intake requirements than birds or mammals. As a consequence,
birds are likely to consume more food than amphibians on a daily dietary intake basis,
assuming similar caloric content of the food items. Therefore, the use of avian food
intake allometric equation as a surrogate to amphibians is likely to result in an over-
estimation of exposure and risk for reptiles and terrestrial-phase amphibians. Therefore,
T-REX (version 1.3.1) has been refined to the T-HERPS model (v. 1.0), which allows for
an estimation of food intake for poikilotherms using the same basic procedure as T-REX
to estimate avian food intake.
EECs for terrestrial plants inhabiting dry and wetland areas are derived using TerrPlant
(version 1.2.2, 12/26/2006). This model uses estimates of pesticides in runoff and in
spray drift to calculate EECs. EECs are based upon solubility, application rate and
minimum incorporation depth.
The spray drift model AgDrift (v. 2.01) was used to assess exposures of terrestrial phase
CRLF and its prey to methidathion deposited on terrestrial habitats by spray drift from
ground and aerial applications. In addition to the buffered area from the spray drift
analysis, the downstream extent of methidathion that exceeds the LOC for the effects
determination is also considered.
2.10.1.2 Measures of Effect
Data identified in Section 2.8 are used as measures of effect for direct and indirect effects
to the CRLF. Data were obtained from registrant submitted studies or from literature
studies identified by ECOTOX. The ECOTOXicology database (ECOTOX) was searched
in order to provide more ecological effects data and in an attempt to bridge existing data
gaps. ECOTOX is a source for locating single chemical toxicity data for aquatic life,
terrestrial plants, and wildlife. ECOTOX was created and is maintained by the USEPA,
Office of Research and Development, and the National Health and Environmental Effects
Research Laboratory's Mid-Continent Ecology Division.
47
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The assessment of risk for direct effects to the terrestrial-phase CRLF makes the
assumption that toxicity of methidathion to birds is similar to or less than the toxicity to
the terrestrial-phase CRLF. The same assumption is made for fish and aquatic-phase
CRLF. Algae, aquatic invertebrates, fish, and amphibians represent potential prey of the
CRLF in the aquatic habitat. Terrestrial invertebrates, small mammals, and terrestrial-
phase amphibians represent potential prey of the CRLF in the terrestrial habitat. Aquatic,
semi-aquatic, and terrestrial plants represent habitat of CRLF.
The acute measures of effect used for animals in this screening level assessment are the
LD5o, LC50 and EC50. LD stands for "Lethal Dose", and LD50 is the amount of a material,
given all at once, that is estimated to cause the death of 50% of the test organisms. LC
stands for "Lethal Concentration" and LC50 is the concentration of a chemical that is
estimated to kill 50% of the test organisms. EC stands for "Effective Concentration" and
the EC50 is the concentration of a chemical that is estimated to produce a specific effect in
50% of the test organisms. Endpoints for chronic measures of exposure for listed and
non-listed animals are the NOAEL/NOAEC and NOEC. NOAEL stands for "No
Ob served-Adverse-Effect-Level" and refers to the highest tested dose of a substance that
has been reported to have no harmful (adverse) effects on test organisms. The NOAEC
(i.e., "No-Observed-Adverse-Effect-Concentration") is the highest test concentration at
which none of the observed effects were statistically different from the control. The
NOEC is the No-Observed-Effects-Concentration. For non-listed plants, only acute
exposures are assessed (i.e., EC25 for terrestrial plants and EC50 for aquatic plants).
It is important to note that the measures of effect for direct and indirect effects to the
CRLF and its designated critical habitat are associated with impacts to survival, growth,
and fecundity, and do not include the full suite of sublethal effects used to define the
action area. According the Overview Document (USEPA 2004), the Agency relies on
effects endpoints that are either direct measures of impairment of survival, growth, or
fecundity or endpoints for which there is a scientifically robust, peer reviewed
relationship that can quantify the impact of the measured effect endpoint on the
assessment endpoints of survival, growth, and fecundity.
2.10.1.3 Integration of Exposure and Effects
Risk characterization is the integration of exposure and ecological effects characterization
to determine the potential ecological risk from agricultural and non-agricultural uses of
methidathion, and the likelihood of direct and indirect effects to CRLF in aquatic and
terrestrial habitats. The exposure and toxicity effects data are integrated in order to
evaluate the risks of adverse ecological effects on non-target species. For the assessment
of methidathion risks, the risk quotient (RQ) method is used to compare exposure and
measured toxicity values. EECs are divided by acute and chronic toxicity values. The
resulting RQs are then compared to the Agency's levels of concern (LOCs) (USEPA,
2004) (see Appendix E).
For this endangered species assessment, listed species LOCs are used for comparing RQ
values for acute and chronic exposures of methidathion directly to the CRLF. If
48
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estimated exposures directly to the CRLF of methidathion resulting from a particular use
are sufficient to exceed the listed species LOC, then the effects determination for that use
is "may affect". When considering indirect effects to the CRLF due to effects to animal
prey (aquatic and terrestrial invertebrates, fish, frogs, and mice), the listed species LOCs
are also used. If estimated exposures to CRLF prey of methidathion resulting from a
particular use are sufficient to exceed the listed species LOC, then the effects
determination for that use is a "may affect." If the RQ being considered also exceeds the
non-listed species acute risk LOC, then the effects determination is a LAA. If the acute
RQ is between the listed species LOC and the non-listed acute risk species LOC, then
further lines of evidence {i.e. probability of individual effects, species sensitivity
distributions) are considered in distinguishing between a determination of NLAA and a
LAA. When considering indirect effects to the CRLF due to effects to algae as dietary
items or plants as habitat, the non-listed species LOC for plants is used because the CRLF
does not have an obligate relationship with any particular aquatic and/or terrestrial plant.
If the RQ being considered for a particular use exceeds the non-listed species LOC for
plants, the effects determination is "may affect". Further information on LOCs is
provided in Appendix E.
2.10.2 Data Gaps
For environmental fate, there are no data gaps which will affect conclusions regarding
potential environmental exposure to the parent compound, methidathion. Guideline
aerobic and anaerobic aquatic metabolism studies have not been submitted, and the
anaerobic soil metabolism study is not sufficient to determine a degradation half-life.
These data would be useful to refine characterization of the environmental fate of
methidathion, but they would not change quantitative exposure estimates. Submitted
laboratory volatility and photodegradation in air were determined to be unacceptable.
These studies would be necessary to quantitatively assess the potential for atmospheric
transport of methidathion and methidathion oxon.
Several fate studies do not provide adequate identification of potential degradates of
methidathion. None of the available laboratory transformation studies include labeling of
the phosphate ester portion of the methidathion compound and so potentially toxic
degradates would not have been observed. This may lead to increased risk but cannot be
characterized based on existing data and so is addressed as an uncertainty.
For environmental effects, there are no aquatic or terrestrial plant data. Limited data
from ECOTOX are discussed in the risk description section along with the risk
conclusions for aquatic and terrestrial plants from assessments with other
organophosphate pesticides.
There are no toxicity data for methidathion oxon. Toxicity information for other
organophosphate insecticides {e.g., diazinon, chlorpyrifos) indicate that oxon degradates
may be considerably more toxic than the parent OP. Methidathion oxon has been
detected in monitoring data and has been observed to form in field and laboratory studies.
49
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Without toxicity data, risk from this degradate of toxicological concern is calculated
based on assumptions about formation and toxicity.
Although not specifically a data gap, it is noted that the chronic NOAEC of 6.3 |ig/L for
the fathead minnow is similar to the acute 96-hour LC50 of several other freshwater fish
species: rainbow trout (6.6 to 14 |ig/L), bluegill sunfish (2.2 to 9 |ig/L), and goldfish (6.8
|ig/L), A chronic NOAEC for any of these other species would likely be lower than the
available fathead minnow NOAEC. Therefore, the estimation of risk to freshwater fish
and aquatic-phase amphibians is based on the most conservative acute to chronic ratio
from other organophosphates that have both an acute and chronic study for rainbow trout.
3. Exposure Assessment
Methidathion is formulated as a wettable powder in water-soluble bags (25% active
ingredient) and emulsifiable concentrate (22% - 24% active ingredient). Application
equipment includes fixed wing aircraft, ground boom, air blast, low-pressure
handwand or backpack sprayer. Application is via foliar treatment, and annual
application rates range from 1.0 to 10.0 lbs a.i./A. Risks from ground boom and aerial
applications are expected to result in the highest off-target levels of methidathion due
to generally higher spray drift levels. Ground boom and aerial modes of application
tend to use lower volumes of application applied in finer sprays than applications
coincident with sprayers and spreaders and thus have a higher potential for off-target
movement via spray drift.
3.1 Label Application Rates and Intervals
Methidathion labels may be categorized into two types: labels for manufacturing uses
(including technical grade methidathion and its formulated products) and end-use
products. While technical products, which contain methidathion of high purity, are
not used directly in the environment, they are used to make formulated products,
which can be applied in specific areas to control insects. The formulated product
labels legally limit methidathion's potential use to only those sites that are specified
on the labels.
Currently registered agricultural and non-agricultural uses of methidathion within
California are alfalfa, almond, apple, apricot, artichoke, cherry, citrus (calemondin,
citron, grapefruit, kumquat, lemon, lime, orange, pummelo, tangelo, and tangerine),
clover, cotton, kiwi, mango, nectarine, nursery stock (including ornamental herbaceous
plans ant ornamental woody shrubs and vines), olive, peach, pear, plum, pruneo,
safflower, sunflower, timothy, and walnut. Application rates and information for these
uses were reported in Table 2.2.
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3.2 Aquatic Exposure Assessment
3.2.1 Modeling Approach
Surface water aquatic exposures for all assessed uses are quantitatively estimated using
the Pesticide Root Zone Model coupled with the Exposure Analysis Model System
(PRZM/EXAMS). These screening level models are operated based on scenarios that
represent high exposure sites for methidathion use and represent a 10 hectare field that
drains into a 1-hectare pond that is 2 meters deep and has no outlet. PRZM/EXAMS
modeling does not account for transport to groundwater followed by discharge to surface
water as a possible route of aquatic exposure. In this assessment, long-term chronic
concentrations derived from the PRZM-EXAMS model are assumed to reflect
background concentrations that might be found in discharged groundwater/stream
baseflow, but this factor remains an uncertainty, and will be discussed as such in Section
6.1.3.
3.2.2 Model Inputs
3.2.2.1 Physical Properties and Environmental Fate Inputs
Methidathion environmental fate data were discussed previously and are listed in Table
2.1. Chemical-specific model input parameters for PRZM and EXAMS are based on
these data and are listed in Table 3.1.
Table 3.1. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to
Methidathion
Fate Property
Value
Comment1
Source
Molecular Weight
302.3
Henry's constant
3.97 x 10"9 atm m3/mol
USEPA, 1999
Vapor Pressure
2.5 x 10 6 mm Hg
USEPA, 1999
Solubility in Water
2500 mg/L
Measured value x 10
USEPA 1999
Photolysis in Water
10 days
MRID 42081709
Aerobic Soil Metabolism
Half-lives
17.5 days
Upper 90% confidence
bound on the mean of two
values
MRID 44545101,
MRID 42262501
Hydrolysis at pH 7
48 days
Maximum of two values
MRID 42037701
Aerobic Aquatic
Metabolism
(water column)
35 days
Twice the aerobic soil
metabolsim half-life.
Anaerobic Aquatic
Metabolism (benthic)
Stable
No data available
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Table 3.1. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to
Methidathion
Fate Property
Value
Comment1
Source
Koc
364 mL/goc
Mean of four values
MRID 00158529
1 - Inputs determined in accordance with EFED "Guidance for Chemistry and Management Practice Input Parameters
for Use in Modeling the Environmental Fate and Transport of Pesticides " dated February 28, 2002
3.2.2.2 Use-specific Management Practices Inputs
Crop-specific management practices for all of the assessed uses of methidathion were
accounted for in aquatic exposure modeling. Application rates and intervals, application
dates, and crop scenarios used as modeling inputs are listed in Table 3.2 and the
justification for these inputs are described below.
Use-specific parameters are input into modeling scenarios which have been developed to
represent locally specific soil and climatic conditions in vulnerable use sites. Scenarios
also include crop specific agronomic data and management practices such as planting and
harvest date. The scenarios used in modeling were selected from the group of standard
and CRLF-specific scenarios representing crops grown in California. For most
methidathion uses, scenarios are available which were developed specifically for that use.
In cases where a scenario does not exist for a specific use, it is necessary to assign a
surrogate scenario. For sunflower, the California Corn scenario was selected because it
represents a non-irrigated row crop. For safflower, the California Wheat scenario was
used because it is an annual, broadleaf oilseed crop that can be grown in the small-grain
production areas of California. For kiwi, the California Grape scenario was used because
the kiwi is a woody vine grown primarily in the same counties as grapes. Some scenarios
represent multiple uses, as listed in Table 3.2. For these uses, the application rate
modeled is the highest from all uses within the group. Unless otherwise specified, the
application method is aerial spray.
Application dates are not specified on product labels. For modeling, an estimated
application window was developed based on label instructions, crop profiles maintained
by the USD A (http://www.ipmcenters.org/cropprofiles/GetCropProfiles.cfm; accessed
12/08), historical use data from the California PUR dataset, and planting dates and
precipitation data specific to each scenario. From within this window, multiple
application dates were modeled using a multi-run tool in pe5 and the date leading to the
most conservative EEC was used. For example, the distribution of applications seen in
the CDPR PUR data indicates that methidathion is typically applied to olives between
mid-October and mid-December, while the scenario precipitation data show that within
this time period, late November has the highest precipitation and so the greatest
likelihood of runoff. Using the pe5 multi-run tool for this window, an application date of
December 1 was shown to lead to the most conservative aquatic exposure estimates.
52
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Table 3.2. PRZM/EXAMS Crop-Specific Inputs for Aquatic Exposure to
Methidathion
Scenario
Uses Represented
by Scenario
Date of first
application
Application
Rate
(lbs a.i./A)
Number of
Applications
Per Crop Cycle
Application
Interval
(Days)
CA Alfalfa
Alfalfa, Clover
Timothy
8/15
1
2
5a
CA Almond
Almond, Walnut
1/7
2
3
2
CA Citrus b
Calemondin,
Citron, Citrus,
Grapefruit,
Kumquat, Lemon,
Lime, Orange,
Pummelo, Tangelo,
Tangerine
8/15
5
2
45
CA Citrus c
Mango
8/15
0.25
1
NA
CA Corn
Sunflower
1/7
0.5
3
7
CA Cotton
Cotton
7/10
1
4
5
CA Grapes
Kiwi
2/15
2
1
NA
CA Tree Fruit
Apple, Apricot,
Cherry, Nectarine,
Peach Pear, Plum,
Prune
12/18
3
1
NA
CA Olive b
Olive
12/1
3
1
NA
CA Row
Crop
Artichoke
7/25
1
8
14
CA Wheat
Safflower
5/25
0.5
3
7
CA Nursery d
Nursery
1/22
10 c
1
NA
a The application interval is not specified on the label. A 5 day interval was assumed to be conservative because the
target pests are similar to those for cotton.
b Application through ground spray only.
c Mango was modeled separately from the other citrus uses because it is the lowest application rate of all labeled
uses and so EECs were needed to provide characterization of the full range of exposures.
d The nursery stock application rate is reported on labels as 0.5 lb a.i./lOO gal spray with no instructions for application
on areal basis. Therefore, this rate is based on the general labels statement that methidathion should not be applied at
greater than 10 lb a.i./A.
For all uses, application was modeled as foliar applied (CAM = 2) with no removal of
foliar pesticide after harvest (IPSCND = 3). Application efficiency inputs were set to the
default values corresponding to each application method: 99% efficiency for ground
applications (citrus, olives, and nursery stock) and 95% for all other aerial spray
applications. Spray drift inputs were estimated using the AgDRIFT model in order to
account for the effect of the labels' mitigation requirements, which include buffer zones
of 25 to 150 ft between cropped areas and water bodies (see Section 2.3.1) and a medium
to coarse droplet size. Using these inputs, AgDRIFT estimated spray drift of 0.6% of the
applied active ingredient for ground applications and of 2% for aerial uses.
53
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3.2.3 Results
PRZM/EXAMS EECs representing l-in-10 year peak, 21-day, and 60-day concentrations
of total toxic residues of methidathion in the aquatic environment are located in Table
3.5. All model outputs are included in Appendix F. Estimated aquatic exposures from
terrestrial food and feed uses are highest for the CA Almond scenario, representing
methidathion use on almonds and walnuts, with a peak EEC of 49.3 ug/L. Peak EECs for
other agricultural crops ranged from 4.6 ug/L for alfalfa to 20.7 ug/L for sunflowers.
The non-food use in ornamental nurseries resulted in a peak EEC of 115.6 ug/L. This
result is based on an application rate of 10 lb a.i./A, the maximum application rate
reported in the general instructions, because labeled application rates specific to the
nursery use are reported in terms of product per foot of plant, rather than per unit area, as
modeling requires. Aquatic EECs for the non-food uses on alfalfa, clover, and timothy
have a peak EEC of 4.6 ug/L, but these results may underestimate exposure because the
modeled use only includes a single crop cycle, while these crops can be grown at up to
nine crop cycles per year.
Table 3.3 Aquatic EECs (jig/L) for Methidathion Uses in California
Scenario
Crops Represented
Peak EEC
21-day average
EEC
60-day average
EEC
CA Alfalfa
Alfalfa, Clover
Timothy
4.6
3.4
2.5
CA Almond
Almond, Walnut
49.3
40.2
26.8
CA Citrus
Calemondin, Citron,
Citrus, Grapefruit,
Kumquat, Lemon, Lime,
Orange, Pummelo,
Tangelo, Tangerine
18.3
13.6
9.0
CA Citrus
Mango
0.45
0.34
0.22
CA Corn
Sunflower
20.7
16.2
10.9
CA Cotton
Cotton
13.0
9.3
6.4
CA Grapes
Kiwi
6.6
5.0
3.2
CA Tree Fruit
Apple, Apricot, Cherry,
Nectarine, Peach, Pear,
Plum, Prune
15.6
9.2
5.0
CA Olive
Olive
15.2
11.7
7.6
CA Row Crop
Artichoke
16.0
12.1
8.7
CA Wheat
Safflower
13.6
10.4
6.6
CA Nursery
Nursery
115.6
84.8
54.3
54
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3.2.4 Existing Monitoring Data
3.2.4.1 USGS NAWQA Surface Water Data
The USGS NAWQA surface water dataset in California includes 322 samples collected
between 2001 and 2007 at 15 sites, all but one in the San Joaquin and Sacramento study
areas. Six of the sites are located on Mustang Creek in Merced County in an area with
landcover classified as "other, " and the remaining sites represent a mix of landcover
classes. Methidathion was detected 13 times at 7 sites, but more than half of the
detections (8 detections, 0.01 to 0.04 ppb) were measured in one 24 hour period at two
sites on Mustang Creek. At these and other Mustang Creek sites, sampling was
conducted in winter months with multiple sampling per day, with the detections
occurring in February, 2004. The other nine sites were generally sampled between 30
and 60 times, biweekly or bimonthly for one to several years. Methidathion was detected
at four of these sites at levels of 0.1 to 0.31 ppb. Limits of quantitation (LOQ) were
0.006 ug/L to 0.009 ug/L. The sampling did not include analysis for any of the
degradates of methidathion.
3.2.4.2 USGS NAWQA Groundwater Data
The NAWQA groundwater California dataset included 271 samples from 223 wells
analyzed for methidathion between 2001 and 2006, with no detections. Detection
limits were 0.0058 to 0.0087 ug/L. Most of the samples (77%) were in the San Joaquin
study area, primarily in Fresno, Merced, and Stanislaus Counties. The remaining sites
were in the Sacramento study area with a few in the Santa Ana study area as well. 30
sites in Sacramento county were classified as urban landcover, while the remaining
samples were about half classified as agricultural landcover and half classified as
mixed or other. No samples were analyzed for methidathion degradates.
3.2.4.3 California Department of Pesticide Regulation (CDPR)
Surface Water Data
CDPR's monitoring dataset for methidathion includes 4,044 samples from 135 sites, most
targeted to agricultural use areas, although specific land cover information is not
available. The sampling was conducted for 19 different studies, primarily in the
Sacramento and San Joaquin River drainages, although there was no sampling in the
southern San Joaquin Valley (Fresno, Kern, and Tulare Counties), which is the area with
the greatest methidathion use (see Figure 2.2). More than half of the samples (58%) were
collected at five sites. They are primarily from two studies which were targeted to OP
use areas and that had daily to weekly sampling over several years. These studies have
implications more specific to methidathion and so the results will be discussed separately
in more detail below.
55
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The remainder of the sampling included 1,694 samples from 130 sites, with
approximately one-third of the samples (28%) from the Sacramento Valley and another
44% in the northern San Joaquin Valley. LOQs were between 0.0028 ug/L to 0.10 ug/L.
In the Sacramento Valley, methidathion was detected in 46 out of 472 samples, with a
maximum detection of 15.1 ug/L in Sutter County, and an average across all detections of
1.2 ug/L. In the San Joaquin Valley, there were 30 detections out of 738 samples, with
the the maximum detection of 2.4 ug/L found in San Joaquin County and the average of
0.3 ug/L. Additionally, there were 5 detections at two sites in Orange County at
concentrations of 0.05 ug/L to 0.17 ug/L. Acute toxicity endpoints for fish and aquatic
invertebrates are 2.2 ug/L and 3.0 ug/L, respectively, so monitoring data demonstrate the
potential for surface water concentrations in excess of these endpoints. Although
Monterey is one of the highest use counties, methidathion was not detected in any of the
144 samples taken there, which were mostly from sites on waterways dominated by
summer agricultural runoff. Overall, although only 35% of the total samples were
collected in January or February, 90% of the detections (72 samples) were measured in
those months. This reflects the fact that the typical use period of methidathion on fruit
and nut orchards is in the winter dormant season.
Two studies monitored for methidathion oxon in surface water as well as for the parent.
There were no detections of the oxon in either study, even though in some cases
methidathion had been detected in samples collected at the same time. In most cases, the
limit of quantitation was 0.05 ug/L. One study included 450 samples, collected weekly to
monthly at 7 sites, with no detections (Ganapathy, 1997). At one Merced County site in
this study, methidathion had been detected in each of 4 samples collected weekly in
January and February. The second methidathion monitoring study included 290 samples
at 26 sites, primarily in Stanislaus and Merced County (Ross et al, 2000). Methidathion
had been detected at 7 sites in this study area, at concentrations up to 12.4 ug/L. There
were no detections of methidathion oxon sampled at the same times.
Studies targeted to organophosphate pesticide use included extensive monitoring at
several sites on Orestimba Creek, a tributary of the San Joaquin River located in
Stanislaus County in an area expected to be vulnerable to runoff due to local soil and
hydrology conditions. Agriculture in the area includes almond and walnut orchards as
well as alfalfa. The sampling sites were selected primarily due to expected use of
diazinon and chlorpyrifos, but methidathion is used in this area as well. In one study
(Poletika & Robb, 1998), daily samples were taken at three sites on Orestimba Creek
between June, 1996 to April, 1997. Methidathion was detected in 58 of 967 samples at
levels from 0.001 ug/L to 0.331 ug/L, with an average across detected samples of 0.035,
and based on an LOQ of 0.002 ug/L. There were 9 detections at the upstream site, with a
maximum of 0.023 ug/L and mostly occurring in the first week of November, 1996. The
midstream location had 22 detections, up to 0.078 ug/L, and the downstream site near the
confluence with the San Joaquin River had 27 detections of up to 0.331 ug/L. Both sites
had multiple detections in late July and early August. The highest peak occurred at the
downstream site on March 20 and had decreased to undetectable levels by April 12. The
study report indicates that, according to data from the county agricultural commisioner,
five applications of methidathion were made in fields in the drainage during the sampling
56
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period. However, the timing of the detections did not correspond with any of the
reported pesticide applications, so there were either unreported applications or alternative
sources of runoff. Additional monitoring at the downstream site on Orestimba Creek was
conducted by CDPR, with 136 samples collected, primarily during winter months in 1997
to 2002. There were four detections of 0.06 ug/L to 2.14 ug/L (CDPR, metadata
reference 10).
Another study conducted by the USGS included long term monitoring at two sites
draining to the San Francisco Bay (Maccoy, et al, 1995). Unlike the previously discussed
study, which was in a small water body local to use areas, the sites sampled here are near
outlets of large agricultural watersheds, one site on the Sacramento River and
downstream of most other riverine inputs in the Sacramento Valley, and one on the San
Joaquin River and downstream of most other rivierine inputs in the San Joaquin Valley.
The San Joaquin Valley is the area of highest methidathion use and the Sacramento
Valley has substantial use as well, so these locations represent high potential for
methidathion exposure. On the San Joaquin River near Vernalis, sampling went from
January, 1991 to April, 1994, and methidathion was detected 49 times out of 515
samples. All detections occurred in January or February at concentrations of 0.028 ug/L
to 0.802 ug/L. On the Sacramento River, sampling began in May, 1991 and methidathion
was detected in 23 of the 562 samples taken, all in February. In 1993, methidathion was
detected in 16 of 23 samples collected in February at levels from 0.03 ug/L to 0.21 ug/L,
and in 1994, 7 of the 17 samples collected in February had methidathion, concentrations
of 0.04 ug/L to 0.7 ug/L.
3.2.4.4 Atmospheric Monitoring Data
There are several studies available which monitor for methidathion and methidathion
oxon in air, fog, and/or rain onsite and at intermediate and long range distances from
treated fields. A literature review by Majewski (1996) found three studies which tested
air and fog and two studies which tested fog only for a total of 11 sites tested in
California, both in the San Joaquin Valley and on the coast near Monterey. At eight of
the sites, monitoring was also conducted for methidathion oxon. Methidathion was
detected in both matrices at all 11 locations in all samples tested, while methidathion
oxon was detected at 7 locations (all but one of those monitored) and in most of the
samples tested. Detected concentrations of methidathion in air ranged from 0.01 ng/m3 to
23.8 ng/m3 and in fog from 4 x 10"5 ug/L (0.04 ng/L) to 15.5 ug/L. This maximum
concentration in fog exceeds the effects levels for fish and aquatic invertebrates.
Concentrations for methidathion oxon were not reported. Majewski attributes the high
detection frequency to the fact that most sampling was done in or near orchards, and the
fact that most samples were collected in January, during the typical use period. It is
likely, then, that at least some of these detections were the result of spray drift rather than
atmospheric transport. The study near Monterey, however, was conducted in September
and states that at least one of the three sites was in a non-agricultural area, so there is the
possibility that longer range transport occurred. The frequency of detection of the oxon
indicates that oxidation was occurring, either in the air directly or in soil or on plant
surfaces followed by volatilization.
57
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Another air monitoring study was conducted for the California DPR in support of their
Toxic Air Contaminant Program (Ross, 1991). In Tulare County, an area with high
methidathion use on oranges, air sampling was conducted at five sites, four within a
quarter mile of orange groves. The fifth site was intended to be used as background and
was located in a downtown location with no orange groves in the immediate area. At
each site, 17 samples per site were taken from June 27, 1991 to July 25, 1991, the time of
peak use locally. Samplers were placed on the roof of a building or in an open area and
collection occurred over a 23 to 25 hour period. All sites, including the intended
background site, had samples with measured concentrations above the level of detection
(LOD) for both methidathion (16 samples; LOD = 10 ng/m3) and methidathion oxon (27
samples; LOD = 30 ng/m3). Methidathion was detected above the LOQ in 7 samples (32
to 78 ng/m3) at two sites and methidathion oxon was detected in 7 samples (11 to 120
ng/m3) at 4 sites, with the lowest detection found at the background site. Methidathion
oxon was detected more frequently and at higher concentrations than the parent
compound, including detections at a site with no use in the immediate area, indicating
that transformation occurs and that there is a potential for off-site atmospheric transport.
Vogel et al. (2008) monitored for methidathion in rain at a local and intermediate scale at
a site in the Merced River basin where a variety of crops, including grapes, almonds, and
corn, were grown. Between February and April of 2003 and December 2003 and April
2004, methidathion was measured for in 23 single event rain samples at lower, middle,
and upper positions in the watershed. Detections above the limit of quantitation of 0.006
ug/L were found in 11 samples, with the highest detection of 0.69 ug/L found on
December 16, 2003. Seven samples had detections between 0.01 ug/L and 0.1 ug/L and
the other three were <0.01 ug/L. The detections were presumed to be the result of
atmospheric transport because while the highest detection occurred before spraying began
in local orchards, during that time methidathion was used on crops at the intermediate
and broad scales. No sampling for methidathion oxon was conducted.
Another study by Aston and Seiber (1997) showed the most clear evidence of long range
transport of both methidathion and its oxon degradate with detections in air at up to 30
km from any agricultural site and on pine needles 22 km away from use areas. In May
through October of 1994, Aston and Seiber (1997) monitored for methidathion and
methidathion oxon at three sites near Sequoia National Park. Biweekly samples of air
and of pine needles were taken at three sites; the first was located in Tulare County next
to a citrus grove treated with methidathion and within an area of high citrus production,
the second was at a site within Sequoia National Park that was 22 km east of the nearest
agricultural spraying, and the third site at an elevation on 1920 m was another 10 km
away to the northeast. Each air sample represented air collected over a 24 h period, and
pine needles were collected from potted pines placed at each site. The flow of air from
the agricultural areas of the Central Valley to the two long range sites was reported to be
unobstructed.
Methidathion was detected in air in every sample at the citrus grove location and
concentrations and detection frequency decreased with distance. Methidathion oxon was
also detected in all samples at the citrus grove and it was more persistent with distance
58
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than the parent. One week following spraying in the onsite grove, methidathion was
detected in air at the highest concentration of 17.0 ng/m3. Concentrations at the four
sampling events prior to spraying ranged from 1.1 ng/m3 to 15.0 ng/m3 and in the four
following sampling events air concentrations ranged from 0.4 ng/m3 to 2.7 ng/m3. Over
the sampling period, methidathion oxon was detected onsite at levels from 0.28 ng/m3 to
10.0 ng/m3, with the highest concentration at the initial sampling period, followed by
steady decline interrupted by a peak after spraying. At the intermediate site,
methidathion was detected only in the initial sampling, June 7, at 0.23 ng/m3. The oxon
degradate was detected on the first date as well, at 0.66 ng/ng/m3, and declined over the
next month when it was detected a third time, at 0.21 ng/m3. At all but one of the 5 final
sampling events, methidathion oxon was detected but it was either below the limit of
quantitation (0.17 ng/m3) or it was not quantifiable due to inconsistencies in duplicate
samples. At the far site, methidathion was reported as being unquantifiable in all samples
and methidathion oxon was detected at 0.21 ng/m3 on June 20 and was undetected or
unquantifiable in all other samples.
In pine needles, both methidathion and methidathion oxon were detected in all samples at
the citrus grove site but only the oxon degradate was detected at the first site in Sequoia
National Park and neither was detected at the further site. Next to the citrus grove,
methidathion concentrations ranged from approximately 50 ng/g to 65 ng/g and
methidathion oxon from approximately 15 ng/g to 150 ng/g (values estimated based on
charts presented in the study report). The highest methidathion concentration was
measured a week after spraying. Local spray drift is unlikely to be the cause because the
pots had been removed before spraying and were not returned for 48 hours, although
spray drift from other fields may be a possibility if they were treated at the same time. At
this site, methidathion oxon was highest on September 18. At the intermediate site, there
were no detections of methidathion, but methidathion oxon was detected at the three
sampling events between July 11 and August 8 at levels from approximately 8 ng/g to 50
ng/g. There were no detections of either compound on pine needles at the furthest site.
3.3 Terrestrial Animal Exposure Assessment
3.3.1 Modeling Approach
T-REX (Version 1.3.1) is used to calculate dietary and dose-based EECs of methidathion
for the CRLF and its potential prey (e.g. small mammals and terrestrial insects)
inhabiting terrestrial areas. EECs used to represent the CRLF are also used to represent
exposure values for frogs serving as potential prey of CRLF adults. T-REX simulates a 1-
year time period. For this assessment, spray applications of methidathion are considered,
as discussed in below.
Terrestrial EECs for foliar formulations of methidathion were derived for the uses
summarized in Table 3.4. In an effort to be consistent with the aquatic exposure
assessment, the same uses and rates were modeled for terrestrial exposure estimation (see
Table 3.2). For terrestrial exposure estimation purposes, a half-life of 6.6 days was used
as an input parameter based the 90% upper bound of the total foliar dissipation half-life
59
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values, 3.5 and 5.0 days, on alfalfa (Willis and McDowell, 1987). Use specific input
values, including number of applications, application rate and application interval are
provided in Table 3.4. An example output from T-REX is available in Appendix G.
Table 3.4 Input Parameters for Foliar Applications Used to Derive Terrestrial EECs
for Methidathion with T-REX
Uses
Application Rate
(lbs a.i./A)
Number of Applications Per
Crop Cycle
Application Interval
(Days)
Artichoke
1
8
14
Citrus
5
2
45
Clover
1
2
7 (assumed)
Cotton
1
4
5
Kiwi
2
1
NA
Nursery
10
1
NA
Safflower, Sunflower
0.5
3
7
Tree Fruit, Olives,
3
1
NA
Almonds
Walnut
2
3
2
T-REX is also used to calculate EECs for terrestrial insects exposed to methidathion.
Dietary-based EECs calculated by T-REX for small and large insects (units of a.i./g) are
used to bound an estimate of exposure to terrestrial invertebrates. Available acute contact
toxicity data for bees exposed to methidathion (in units of |ig a.i./bee), are converted to
|ig a.i./g (of bee) by multiplying by 1 bee/0.128 g. The EECs are later compared to the
adjusted acute contact toxicity data for bees in order to derive RQs.
For modeling purposes, exposures of the CRLF to methidathion through contaminated
food are estimated using the EECs for the small bird (20 g) which consumes small
insects. Dietary-based and dose-based exposures of potential mammalian prey are
assessed using the small mammal (15 g) which consumes short grass. Upper-bound
Kenega nomogram values reported by T-REX for these two organism types are used for
derivation of EECs for the CRLF and its potential prey (Table 3.5). Dietary-based EECs
for small and large insects reported by T-REX as well as the resulting adjusted EECs are
available in Table 3.6. An example output from T-REX v. 1.3.1 is available in Appendix
G
60
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Table 3.5 Upper-bound Kenega Nomogram EECs fo
Exposures of the CRLF and its Prey to
r Dietary- and Dose-based
Methidathion
Use
EECs for CRLF
EECs for Prey
(small mammals)
Dietary-based
EEC (ppm)
Dose-based EEC
(mg/kg-bw)
Dietary-based
EEC (ppm)
Dose-based EEC
(mg/kg-bw)
Artichoke
175
200
312
297
Citrus
681
776
1211
1154
Clover
200
227
355
339
Cotton
290
330
516
492
Kiwi
270
308
480
458
Nursery
68
77
120
114
Safflower, Sunflower
115
131
205
196
Tree Fruit, Olive, Almonds
405
461
720
686
Walnut
666
759
1184
1129
Table 3.6 EECs (ppm) for Indirect Effects to the Terrestrial-Phase
CRLF via Effects to Terrestrial Invertebrate Prey Items
Use
Small Insect
Large Insect
Artichoke
175
19
Citrus
681
76
Clover
200
22
Cotton
290
32
Kiwi
270
30
Nursery
68
8
Safflower, Sunflower
115
13
Tree Fruit, Olive, Almonds
405
45
Walnut
666
74
3.3.2 Field Studies
In a field study on California citrus (Brewer et al., 1998; MRID 44806601), methidathion
was applied to foliage by air blast spray equipment for one application at 10.0 lb ai/A.
Immediately after application the residues on hulled millet seed ranged from 248 to 305
(mean 276) ppm, and residues on crickets ranged from 22 to 59 (mean 40) ppm. T-REX
model estimates - 150 ppm for seeds, 1350 ppm for small insects, and 150 ppm for large
insects - are similar to actual field residues, and in the case of residues on seeds, the
model may underestimate exposure. It is noted, however, that although it may
underestimate potential risks to seed eaters, this dietary food item is not used in the
assessment, and the foliage data did not suggest an underestimation of exposures.
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3.4 Terrestrial Plant Exposure Assessment
Since there are no terrestrial plant toxicity data available, exposures were not
quantitatively estimated. See Section 5.2 for a qualitative discussion regarding the
potential effects of methidathion on CRLF via effects to terrestrial plants.
4. Effects Assessment
This assessment evaluates the potential for methidathion to directly or indirectly affect
the CRLF or modify its designated critical habitat. As previously discussed in Section
2.7, assessment endpoints for the CRLF effects determination include direct toxic effects
on the survival, reproduction, and growth of CRLF, as well as indirect effects, such as
reduction of the prey base or modification of its habitat. In addition, potential
modification of critical habitat is assessed by evaluating effects to the PCEs, which are
components of the critical habitat areas that provide essential life cycle needs of the
CRLF. Direct effects to the aquatic-phase of the CRLF are based on toxicity information
for freshwater fish, while terrestrial-phase effects are based on avian toxicity data, given
that birds are generally used as a surrogate for terrestrial-phase amphibians. Because the
frog's prey items and habitat requirements are dependent on the availability of freshwater
fish and invertebrates, small mammals, terrestrial invertebrates, and aquatic and
terrestrial plants, toxicity information for these taxa are also discussed. Acute (short-
term) and chronic (long-term) toxicity information is characterized based on registrant-
submitted studies and a comprehensive review of the open literature on methidathion.
As described in the Agency's Overview Document (U.S. EPA, 2004), the most sensitive
endpoint for each taxon is used for risk estimation. For this assessment, evaluated taxa
include aquatic-phase amphibians, freshwater fish, freshwater invertebrates, aquatic
plants, birds (surrogate for terrestrial-phase amphibians), mammals, terrestrial
invertebrates, and terrestrial plants.
Toxicity endpoints are established based on data generated from guideline studies
submitted by the registrant, and from open literature studies that meet the criteria for
inclusion into the ECOTOX database maintained by EPA/Office of Research and
Development (ORD) (U.S. EPA, 2004). Open literature data presented in this assessment
were obtained from ECOTOX information obtained on 24 August 2008. In order to be
included in the ECOTOX database, papers must meet the following minimum criteria:
(1) the toxic effects are related to single chemical exposure;
(2) the toxic effects are on an aquatic or terrestrial plant or animal species;
(3) there is a biological effect on live, whole organisms;
(4) a concurrent environmental chemical concentration/dose or application
rate is reported; and
(5) there is an explicit duration of exposure.
Data that pass the ECOTOX screen are evaluated along with the registrant-submitted
data, and may be incorporated qualitatively or quantitatively into this endangered species
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assessment. In general, effects data in the open literature that are more conservative than
the registrant-submitted data are considered. The degree to which open literature data are
quantitatively or qualitatively characterized for the effects determination is dependent on
whether the information is relevant to the assessment endpoints (i.e., maintenance of
CRLF survival, reproduction, and growth) identified in Section 2.8. For example,
endpoints such as behavior modifications are likely to be qualitatively evaluated, because
quantitative relationships between modifications and reduction in species survival,
reproduction, and/or growth are not available. Although the effects determination relies
on endpoints that are relevant to the assessment endpoints of survival, growth, or
reproduction, it is important to note that the full suite of sublethal endpoints potentially
available in the effects literature (regardless of their significance to the assessment
endpoints) are considered to define the action area for methidathion.
Citations of all open literature not considered as part of this assessment because they
were either rejected by the ECOTOX screen or accepted by ECOTOX but not used (e.g.,
the endpoint is less sensitive) are included in Appendix H. Appendix H also includes a
rationale for rejection of those studies that did not pass the ECOTOX screen and those
that were not evaluated as part of this endangered species risk assessment.
A detailed spreadsheet of the available ECOTOX open literature data, including the full
suite of lethal and sublethal endpoints is presented in Appendix I. Appendix I also
includes a summary of the human health effects data for methidathion.
In addition to registrant-submitted and open literature toxicity information, other sources
of information, including use of the acute probit dose response relationship to establish
the probability of an individual effect and reviews of the Ecological Incident Information
System (EIIS), are conducted to further refine the characterization of potential ecological
effects associated with exposure to methidathion. A summary of the available aquatic
and terrestrial ecotoxicity information, use of the probit dose response relationship, and
the incident information for methidathion are provided in Sections 4.1 through 4.4,
respectively.
There are no toxicity data available for methidathion oxon, a degradate of the parent
methidathion; however, based on toxicity information for other organophosphate
insecticides (e.g., diazinon, chlorpyrifos), methidathion oxon may be considerably more
toxic than methidathion. The potential for additional risk to the CRLF from exposure to
methidathion oxon as a transformation product of applied methidathion will be
characterized in the risk description (Section 5.2).
A detailed summary of the available ecotoxicity information for all methidathion
degradates and formulated products is presented in Appendix A.
4.1 Toxicity of Methidathion to Aquatic Organisms
Table 4.1 summarizes the most sensitive aquatic toxicity endpoints for the CRLF, based
on an evaluation of both the submitted studies and the open literature, as previously
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discussed. A brief summary of submitted and open literature data considered relevant to
this ecological risk assessment for the CRLF is presented below. Additional information
is provided in Appendix J.
Table 4.1
freshwater Aquatic Toxicity Profile for Methidathion
Assessment Endpoint
Species
Toxicity Value Used
in Risk Assessment
mrii)
Study
Classification
Acute Direct Toxicity to
Aquatic-Phase CRLF
Bluegill Sunfish
Lepomis macrochinis
LC50 = 2.2 ng/L
00011841
Supplemental
Chronic Direct Toxicity
to Aquatic-Phase CRLF
Fathead minnow
Pimephales prom e las
NOAEC = 6.3 ng/L
LOAEC = 12.0 ng/L
post-hatch survival;
growth
00015735
Acceptable
Indirect Toxicity to
Aquatic-Phase CRLF via
Acute Toxicity to
Freshwater Invertebrates
(i.e. prey items)
Water Flea
Daphnia magna
LC50= 3.0 ng/L
42081704
Acceptable
Indirect Toxicity to
Aquatic-Phase CRLF via
Chronic Toxicity to
Freshwater Invertebrates
(i.e. prey items)
Water Flea
Daphnia magna
NOAEC = 0.66 ng/L
LOAEC = 1.13 ng/L
survival; number of
young per female per
reproductive day
42081707
Acceptable
Indirect Toxicity to
Aquatic-Phase CRLF via
Toxicity to Non-vascular
Aquatic Plants
No data available
Indirect Toxicity to
Aquatic-Phase CRLF via
Toxicity to Vascular
Aquatic Plants
No data available
Toxicity to aquatic fish and invertebrates is categorized using the system shown in Table
4.2 (U.S. EPA, 2004). Toxicity categories for aquatic plants have not been defined.
Table 4.2 Categories of Acute Toxicity for Aquatic Organisms
LCso (ppm)
Toxicity Category
<0.1
Very highly toxic
0.1 - 1
Highly toxic
1-10
Moderately toxic
10 - 100
Slightly toxic
> 100
Practically nontoxic
4.1.1 Toxicity to Freshwater Fish
Given that no methidathion toxicity data are available for aquatic-phase amphibians,
freshwater fish data were used as a surrogate to estimate direct acute and chronic risks to
the CRLF. Freshwater fish toxicity data were also used to assess potential indirect effects
of methidathion to the CRLF. Effects to freshwater fish resulting from exposure to
methidathion could indirectly affect the CRLF via reduction in available food. As
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discussed in Section 2.5.3, over 50% of the prey mass of the CRLF may consist of
vertebrates such as mice, frogs, and fish (Hayes and Tennant, 1985).
A summary of acute and chronic freshwater fish data, including data from the open
literature, is provided below in Sections 4.1.1.1 through 4.1.1.3.
4.1.1.1 Freshwater Fish: Acute Exposure (Mortality) Studies
Available data indicate that methidathion is very highly toxic on an acute basis to three
surrogate freshwater fish species (Appendix J). The acute 96-hour LC50s for bluegill
sunfish, rainbow trout, and goldfish range from 2.2 to 14 |ig/L. The most sensitive
endpoint, the bluegill sunfish 96-hour LC50 of 2.2 (0.9 - 5.1) |ig a.i./L (MRID 00011841),
will be used to calculate RQs for direct effects to the aquatic-phase CRLF. The probit
dose-response slope is 2.9 (1.9 - 4.0) for this study.
Two acute toxicity studies with the methidathion formulation 2E (25.2% a.i.) are also
available for consideration in this risk assessment. These studies suggest that the tested
formulation and technical grade methidathion are similarly toxic to bluegill sunfish and
rainbow trout on an acute basis.
4.1.1.2 Freshwater Fish: Chronic Exposure
(Growth/Reproduction) Studies
Chronic fish toxicity data for methidathion are limited to one early life stage study with
fathead minnow as the test organism (MRID 00015730 or 45822701) (Appendix J). The
35-day flow-through study investigated the chronic effects following mean-measured
concentration levels up to 12 |ig ai/L. For survival, total length, and wet weight, the
NOAEC, LOAEC, and EC50 were 6.3, 12, and >12 |ig ai/L respectively. It is important
to note that the chronic NOAEC of 6.3 |ig/L for the fathead minnow is similar to the
acute 96-hour LC50 of several other test species: rainbow trout (6.6 to 14 |ig/L), bluegill
sunfish (2.2 to 9 |ig/L), and goldfish (6.8 |ig/L). A chronic NOAEC for any of these other
species would likely be lower than the available fathead minnow NOAEC. Therefore,
instead of using the chronic NOAEC for fathead minnows, this assessment estimates a
chronic NOAEC for rainbow trout using an acute to chronic ratio (ACR) calculation.
Methidathion is an organophosphate insecticide. The EFED toxicity database, which
contains the results from submitted studies that have been previously reviewed for
scientific soundness, was accessed to derive an acute to chronic ratio of all
organophosphate insecticides that have an acute LC50 and an early life stage fish study for
rainbow trout (the species which would most likely have chronic data). Nineteen
organophosphates were found that have both an acute and chronic study for rainbow
trout. The ACRs ranged from 5.4 for Terbufos to 140 for Dichlorvos. Using this range of
ACRs, the estimated chronic NOAECs for rainbow trout tested with methidathion would
range from 0.07 to 1.9 ppb. In order to provide the most conservative estimate for the
chronic freshwater fish NOAEC for methidathion, the ACR of 140 is utilized. Using that
value and an LC50 of 10 ppb for rainbow trout tested with methidathion (MRID
00011841), the estimated chronic NOAEC for rainbow trout is 0.07 ppb.
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The following section presents a modification (for methidathion) of the methodology
used in deriving a freshwater fish ACR for the acephate CRLF assessment
(http://www.epa.gov/espp/litstatus/effects/redleg-frog/acephate/analysis_acephate.pdf).
Of the 19 organophosphates found to have both acute and chronic rainbow trout data, 12
were evaluated for the ACR extrapolation Table 4.3.
The estimated fish (aquatic phase amphibians) chronic NOAEC for methidathion is
derived as follows. The (methidathion) rainbow trout LC50 used for the ACR calculation
is 10 ppb ai. The largest acute-to-chronic ratio from the organophosphates is 140 for
Dichlorvos. This ratio is used to calculate the final NOAEC for methidathion.
ACR ratio for Dichlorvos: 0.75 ppm (acute LCso)/0.0052 ppm (chronic NOAEC) = 140
Estimated NOAEC for methidathion = LCsn = 10 ppb = 140
NOAEC est. NOAEC
Estimated Trout NOAEC for methidathion = 10/140 = 0.07 ppb ai
The table below (4.3) shows the inputs for the organophosphates that were considered for
the methidathion ACR.
Acute to Chronic Table for Organophosphates
Table 4.3. Methidathion Acute to Chronic Rat
io for Rainbow Trout N<
OAEC
Chemical
96-hr LC
50
(PPm ai)a
MRIDs
NOAEC
(PPm ai)a
MRIDs
ACRa
Methidathion
NOAEC
(ppb ai)a
Azinphos methyl
0.0088
03125193
0.00029
00145592
30
0.33
Coumaphos
0.89
40098001
0.012
43066301
76
0.13
Dichlorvos
0.75
43284702
0.0052
43788001
140
0.07
Dimethoate
7.5
TN 1069b
0.43
43106303
17
0.59
Disulfoton
1.9
40098001
0.22
41935801
8.4
1.2
Fenamiphos
0.07
40799701
0.0038
41064301
18
0.56
Fenitrothion
2.0
40098001
0.046
40891201
43
0.23
Fenthion
0.83
40214201
0.0075
40564102
110
0.09
Fonofos
0.05
00090820
0.0047
40375001
11
0.91
Isofenphos
1.8
00096659
0.15
00126777
12
0.83
Phosmet
0.11
40098001
0.0032
40938701
33
0.30
Terbufos
0.0076
40098001
0.0014
41475801
5.4
1.9
a Rounded to two significant figures
b TN 1069 is test number for EPA's Animal Biology Lab, McCann, 1977
4.1.1.3 Freshwater Fish: Sublethal Effects and Additional Open
Literature Information
In the bluegill sunfish acute toxicity study (MRID 00011841) used to calculate RQs, it
was reported that test organisms in the highest exposure group (1000 |ig/L) exhibited
slight spastic motions and swam on their sides.
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In the chronic fish early life stage study for methidathion (MRID 00015730 or
45822701), clinical signs of toxicity during the definitive toxicity test were not provided
in the raw data. Total length and wet weight were the most sensitive endpoints. There
were statistically significant reductions detected for both endpoints at the highest
measured treatment level (12 |ig ai/L) relative to the pooled control. The NOAEC and
LOAEC for both endpoints were 6.3 and 12 |ig ai/L, respectively.
There is one additional fish toxicity study available in the open literature for
methidathion. The test organism, the common eel (Anguilla anguilla), appears to be
about 3 orders of magnitude less sensitive than the bluegill sunfish, the surrogate
organism used here to assess direct effects to the aquatic-phase CRLF. See Appendix I
for more details.
4.1.2 Toxicity to Freshwater Invertebrates
Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects
of methidathion to the CRLF. Effects to freshwater invertebrates resulting from exposure
to methidathion could indirectly affect the CRLF via reduction in available food items.
As discussed in Section 2.5.3, the main food source for juvenile aquatic- and terrestrial-
phase CRLFs is thought to be aquatic invertebrates found along the shoreline and on the
water surface, including aquatic sowbugs, larval alderflies and water striders.
A summary of acute and chronic freshwater invertebrate data, including data published in
the open literature, is provided below in Sections 4.1.2.1 through 4.1.2.3.
4.1.2.1 Freshwater Invertebrates: Acute Exposure Studies
Available data indicate that methidathion is very highly toxic on an acute basis to a
surrogate freshwater invertebrate species, Daphnia magna (Appendix J). A daphnid
acute toxicity study is available for methidathion technical grade active ingredient
(TGAI) and for the methidathion formulation 2E (25.2% a.i.). The 48-hour LC50 for
methidathion (TGAI) is 7.2 |ig a.i./L, and the LC50 for methidathion formulation 2E
(25.2% a.i.) is 11.9 |ig product/L or 3.0 |ig a.i./L. The most sensitive endpoint, 48-hour
LC50 of 3.0 |ig a.i./L (MRID 42081704), will be used to calculate RQs for indirect effects
to the aquatic-phase CRLF. The probit dose-response slope is the default of 4.5 (2-9) for
this study because there are less than two concentrations at which the percent dead is
between 0 and 100.
4.1.2.2 Freshwater Invertebrates: Chronic Exposure Studies
Chronic toxicity data are available for a common freshwater zooplankton, Daphnia
magna (Appendix J). In a 21-day lifecycle study (MRID 42081707), survivorship was
reduced by 97% at 1.1 |ig a.i./L; the NOAEC (mortality) was 0.66 |ig a.i./L. This
endpoint will be used to calculate RQs in this assessment. There were no significant
effects of methidathion on reproduction or growth in the study.
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4.1.2.3 Freshwater Invertebrates: Open Literature Data
There are no freshwater invertebrate toxicity studies identified in the open literature for
methidathion (Appendix H).
4.1.3 Toxicity to Aquatic Plants
There are no aquatic plant toxicity data for methidathion with which to assess the
potential for indirect effects to the CRLF via effects on habitat, cover, and/or primary
productivity or effects to the primary constituent elements (PCEs) relevant to the aquatic-
phase CRLF.
4.2 Toxicity of Methidathion to Terrestrial Organisms
Table 4.4 summarizes the most sensitive terrestrial toxicity endpoints for the CRLF,
based on an evaluation of both the submitted studies and the open literature. A brief
summary of submitted and open literature data considered relevant to this ecological risk
assessment for the CRLF is presented below.
Table 4.4 Terrestrial Toxicity Profile for Methidathion
Endpoint
Species
Toxicity Value Used in
Risk Assessment
mrii)
Study
Classification
Acute Direct Toxicity
to Terrestrial-Phase
CRLF (LD5„)
Mallard duck
Anas platyrhynchos
LD50 = 6.7 mg/kg
00159201
Supplemental
Acute Direct Toxicity
to Terrestrial-Phase
CRLF (LC50)
Bobwhite quail
Colinus virginianus
LC50 = 224 ppm
42081701
Acceptable
Chronic Direct Toxicity
to Terrestrial-Phase
CRLF
Mallard Duck
Anas platyrhynchos
NOAEC = 10 ppm
LOAEC = 30 ppm
Number of normal
hatchlings/live 3-week
embryos
44381602
Acceptable
Indirect Toxicity to
Terrestrial-Phase CRLF
(via acute toxicity to
mammalian prey items)
Rat
Rattus nor\>egicus
LD50 = 12 mg/kg
00012714
Acceptable
Indirect Toxicity to
Terrestrial-Phase CRLF
(via chronic toxicity to
mammalian prey items)
Rat
Rattus nor\>egicus
NOAEL = 5 ppm
LOAEL = 25 ppm
Decreased mating index,
decreased pup weight,
tremors, decreased food
consumption during
lactation, transient
decrease in body weight
40079812
40079813
Acceptable
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Table 4.4 Terrestrial Toxicity Profile for Methidathion
Endpoint
Species
Toxicity Value Used in
Risk Assessment
MRID
Study
Classification
(males and females)
Indirect Toxicity to
Terrestrial-Phase CRLF
(via acute toxicity to
terrestrial invertebrate
prey items)
Honey Bee
Apis mellifera
LD50 = 0.236 ng a.i./bee
0036935
Acceptable
Indirect Toxicity to
Terrestrial- and
Aquatic-Phase CRLF
(via toxicity to
terrestrial plants)
Seedlins Emergence
Monocots
No data available
Seedlins Emersence
Dicots
Vesetative Visor
Monocots
Vesetative Visor
Dicots
Acute toxicity to terrestrial animals is categorized using the classification system shown
in Table 4.5 (U.S. EPA, 2004). Toxicity categories for terrestrial plants have not been
defined.
Table 4.5 Categories of Acute Toxicity for Avian and Mammalian Studies
Toxicity Category
Oral LDjo
Dietary LCS0
Very highly toxic
<10 mg/kg
<50 ppm
Highly toxic
10-50 mg/kg
50 - 500 ppm
Moderately toxic
51 -500 mg/kg
501 - 1000 ppm
Slightly toxic
501 - 2000 mg/kg
1001 - 5000 ppm
Practically non-toxic
> 2000 mg/kg
> 5000 ppm
4.2.1 Toxicity to Birds
As specified in the Overview Document, the Agency uses birds as a surrogate for
terrestrial-phase amphibians when amphibian toxicity data are not available (U.S. EPA,
2004). No terrestrial-phase amphibian data are available for methidathion; therefore,
acute and chronic avian toxicity data are used to assess the potential direct effects of
methidathion to terrestrial-phase CRLFs.
4.2.1.1 Birds: Acute Exposure (Mortality) Studies
Based on acute oral toxicity studies for several bird species, methidathion is categorized
as moderately to very highly toxic to birds (Appendix J). The mallard duck was the most
sensitive of the species tested, with an 8-day LD50 of 6.7 (5.4 - 8.4) mg/kg methidathion
(MRID 00159201). This endpoint will be used for risk estimation in this assessment.
These data do not fit the probit dose-response model; thus, a default slope of 4.5 (2-9)
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will be used to calculate the probability of individual effects to the terrestrial-phase
CRLF.
Avian subacute dietary studies are available for methidathion (TGAI) and a formulation
(40% a.i.) (Appendix J). Based on the available information, methidathion and the tested
formulation appear to be similarly toxic to birds on a subacute dietary basis. The
bobwhite quail (Colinus virginianus) was the most sensitive of the species tested, with an
8-day LC50 of 224 (177 - 281) ppm a.i. (MRID 42081701). This endpoint will be used
for risk estimation in this assessment. The probit dose-response slope was 8.7 (3.5 -
13.8).
4.2.1.2 Birds: Chronic Exposure (Growth, Reproduction) Studies
Several avian chronic toxicity studies are available for methidathion. The mallard duck
study (MRID 44381602) reported the most sensitive chronic toxicity endpoint, which will
be used for risk estimation. In this study, the one-generation reproductive toxicity of
methidathion TGAI to 6-month-old mallard ducks was assessed over 140 days.
Methidathion TGAI was administered to the birds in the diet at 0, 1, 10, and 30 ppm diet.
The number of normal hatchlings/live 3-week embryos was significantly reduced (a ratio
of 0.5 versus 0.61 or 18% less than the control ratio) at the highest treatment, 30 ppm
(LOAEC); the NOAEC for this effect is 10 ppm. A significant increase in the number of
eggs cracked was observed when compared to the control group at the 10 ppm dietary
level. In addition, a decrease in the number of eggs not cracked/eggs laid was observed
at the same dietary level (i.e., NOAEC = 1 ppm); however, this is not a relevant
assessment endpoint for the CRLF because endpoints having to do with egg cracking are
not relevant to frogs, whose eggs do not have shells. There were no apparent behavioral
abnormalities or other treatment-related signs of toxicity on the parental generation.
4.2.1.3 Birds: Sublethal Effects and Additional Open Literature
Information
In the mallard duck acute oral toxicity study that was used to calculate RQs (MRID
00159201), toxic symptoms included depression, reduced reaction to external stimuli,
wing droop, convulsions, and salivation. In the bobwhite quail subacute dietary study
used for risk estimation, toxic symptoms including depression, reduced reaction to
external stimuli, wing droop, loss of coordination, lower limb weakness, ruffled
appearance, prostrate posture, and loss of righting reflex were observed at and above 178
ppm a.i., and body weight gain and food consumption were reduced at levels above 316
ppm a.i.
In the mallard duck chronic toxicity study (MRID 44381602) used for risk estimation,
methidathion technical was administered to the birds in the diet at 0 (control), 1,10, and
30 ppm diet. None of the ducks showed symptoms of toxicity or behavioral abnormalities
during the experiment. The number of eggs cracked was significantly increased in the 10
ppm (150%>) and 30 ppm (86%>) exposure concentrations. The number of eggs not
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cracked/eggs laid was also significantly reduced in the 10 ppm and 30 ppm treatment
groups. The number of normal hatchlings/live 3-week embryos was significantly reduced
(5%) at the highest treatment, 30 ppm. Body weight of 14-day-old survivors was
reduced (6%) at 10 ppm, but not at the higher treatment levels. Brain cholinesterase
activity was significantly reduced in drakes at the 10 ppm treatment level during weeks 2,
10, and 20, and at the 30 ppm level during weeks 10 and 20. During week 20, hens
showed a trend toward reduced brain cholinesterase activity at the 10 ppm and 30 ppm
treatment levels.
Additional avian acute toxicity studies for methidathion were identified in the open
literature (Appendices H and I). However, none of the open literature studies resulted in
a more sensitive acute toxicity threshold than the 8-day mallard duck LD50 of 6.7 mg/kg
(MRID 00159201).
4.2.2 Toxicity to Mammals
Mammalian toxicity data are used to assess potential indirect effects of methidathion to
the terrestrial-phase CRLF. Effects to small mammals resulting from exposure to
methidathion could also indirectly affect the CRLF via reduction in available food. As
discussed in Section 2.5.3, over 50% of the prey mass of the CRLF may consist of
vertebrates such as mice, frogs, and fish (Hayes and Tennant, 1985).
The study used to expand the initial area of concern to define the action area examined
the effects of subchronic methidathion administration on vascular wall damage (Yavuz et
al., 2005; ECOTOX ref. 80451). Methidathion was administered by gavage for 5 days a
week for 4 weeks at a dose level of 5 mg/kg/day. The levels of malondialdehyde (MDA),
a biomarker for oxidative stress, were determined in the vascular tissue.
Histopathological examination was performed in the thoracic aortic tissue. The level of
MDA was significantly higher, and cholinesterase activity was significantly lower in the
methidathion group (p < 0.01). Subchronic methidathion administration led to irregular,
prominent breaks and fragmentation of the elastic fibers in the aortic wall. Aortic lesions
such as these are similar to the precursor lesion of a human aortic aneurysm. Since only
one treatment level was tested, the LOAEC is 5 mg/kg/day, and a NOAEC was not
defined. Therefore, a threshold for this effect could not be defined, and thus, this study
was used to define the action area as the entire state of California for this assessment.
4.2.2.1 Mammals: Acute Exposure (Mortality) Studies
Available acute toxicity information suggests that methidathion is very highly toxic
(Category I) to small mammals on an acute oral basis (Appendices J and K). The most
sensitive endpoint, the acute rat (weanling) LD50 of 12 mg/kg, will be used to estimate
risk to the CRLF via indirect effects to mammals. The probit dose-response slope is
assumed to be 4.5 (2-9) for this study because individual animal data are not readily
available for this study.
4.2.2.2 Mammals: Chronic Exposure (Growth, Reproduction) Studies
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Chronic mammalian reproduction toxicity studies are available for methidathion (Table
9). In a 2-generation reproduction study in rats (MRID 40079812, -13), rats were treated
with 0 (control), 5, 25, or 50 ppm methidathion in the diet. The parental systemic
NOAEC was 5 ppm and the LOAEC was 25 ppm, based on tremors and decreased food
consumption during lactation, and decreased ovarian weight. In addition, there was also a
slight decrease in body weight early in the F1 growth phase at 50 ppm. The reproductive
NOAEC was 5 ppm and the LOAEC was 25 ppm based on a decreased mating index and
a generalized indication of pup unthriftyness while nursing (e.g., decreased pup weight
and an increased incidence of hypothermia with an appearance of starvation). In addition,
there was an increase in stillbirths and decreased pup survival at birth and during
lactation at the 50 ppm treatment level. An NOAEL of 5 ppm will be used for risk
estimation in this assessment.
4.2.3 Toxicity to Terrestrial Invertebrates
Terrestrial invertebrate toxicity data are used to assess potential indirect effects of
methidathion to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting
from exposure to methidathion could also indirectly affect the CRLF via reduction in
available food.
4.2.3.1 Terrestrial Invertebrates: Acute Exposure (Mortality) Studies
Methidathion is classified as very highly toxic to bees, with an acute contact LD50 of
0.236 |ig/bee or 1.84 ppm (MRID 00036935). This endpoint will be used to
quantitatively assess the risk to the CRLF via indirect effects to terrestrial invertebrates.
The probit dose-response slope from this study was 9.06.
In addition, a residual toxicity study indicates that the RT25 (i.e., the residual time to kill
25% of the tested population) is greater than 3 days when methidathion (Supracide 2E,
25.2% a.i.) is applied at a rate of 5 lbs a.i./A (MRID 42081708). That is, in addition to
very high acute contact toxicity, field weathered spray residues of methidathion can result
in significant honey bee mortality for more than several days.
4.2.3.2 Terrestrial Invertebrates: Open Literature Studies
There are several terrestrial invertebrate toxicity studies available in the open literature
(Appendices H and I). However, none of these studies identified a more sensitive
endpoint than the submitted honey bee toxicity study that determined an acute contact
LD50 of 0.236 |ig/bee or 1.84 ppm (MRID 00036935).
4.2.4 Toxicity to Terrestrial Plants
Terrestrial plant toxicity data are used to evaluate the potential for methidathion to affect
riparian zone and upland vegetation within the action area for the CRLF. Impacts to
riparian and upland (i.e., grassland, woodland) vegetation could result in indirect effects
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to both aquatic- and terrestrial-phase CRLFs, as well as modification to designated
critical habitat PCEs via increased sedimentation, alteration in water quality, and
reduction in of upland and riparian habitat that provides shelter, foraging, predator
avoidance and dispersal for juvenile and adult CRLFs.
There are no registrant-submitted terrestrial plant toxicity data for methidathion with
which to assess the potential for indirect effects to the aquatic- and terrestrial-phase
CRLF via effects to riparian vegetation or effects to the primary constituent elements
(PCEs) relevant to the aquatic- and terrestrial-phase CRLF. However, there is limited
evidence in the open literature that methidathion has the potential to elicit phototoxic
effects: Godfrey and Holtzer (1992; Ecotox ref. 64451) reported that methidathion
significantly affected corn photosynthesis when applied at 0.5 lbs a.i./A, a rate that is
lower than many of the currently registered rates. In addition, potential phytotoxic effects
have been highlighted on methidathion labels; e.g., EPA SLN No. TX-050003 warns that
methidathion application may result in phytotoxic effects, such as spotting, reddening, or
chlorosis of the leaves, in certain sorghum varieties. In the absence of vegetative vigor
and seedling emergence toxicity data, the potential risk to the CRLF via indirect effects
to terrestrial plants is described in a qualitative manner (Section 5.2).
4.3 Use of Probit Slope Response Relationship to Provide Information on the
Endangered Species Levels of Concern
The Agency uses the probit dose response relationship as a tool for providing additional
information on the potential for acute direct effects to individual listed species and
aquatic animals that may indirectly affect the listed species of concern (U.S. EPA, 2004).
As part of the risk characterization, an interpretation of acute RQ for listed species is
discussed. This interpretation is presented in terms of the chance of an individual event
(i.e., mortality or immobilization) should exposure at the EEC actually occur for a species
with sensitivity to methidathion on par with the acute toxicity endpoint selected for RQ
calculation. To accomplish this interpretation, the Agency uses the slope of the dose
response relationship available from the toxicity study used to establish the acute toxicity
measures of effect for each taxonomic group that is relevant to this assessment. The
individual effects probability associated with the acute RQ is based on the mean estimate
of the slope and an assumption of a probit dose response relationship. In addition to a
single effects probability estimate based on the mean, upper and lower estimates of the
effects probability are also provided to account for variance in the slope, if available.
Individual effect probabilities are calculated based on an Excel spreadsheet tool IECV1.1
(Individual Effect Chance Model Version 1.1) developed by the U.S. EPA, OPP,
Environmental Fate and Effects Division (June 22, 2004). The model allows for such
calculations by entering the mean slope estimate (and the 95% confidence bounds of that
estimate) as the slope parameter for the spreadsheet. In addition, the acute RQ is entered
as the desired threshold.
4.4 Incident Database Review
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A review of the EIIS database for ecological incidents involving methidathion was
completed on 08 September 2008. A total of two aquatic and six terrestrial incidents are
reported, all of which occurred in California. The reported aquatic incidents occurred in
1976 and 2002; all of the reported terrestrial incidents occurred in 1994 and 1995. A
brief description of each of the reported incidents is provided below. A complete list of
the incidents involving methidathion is included as Appendix L.
4.4.1 Terrestrial Incidents
On January 18, 1994, one red-tailed hawk was found dead in Colusa County, California
following application of methidathion to an unknown treatment site. No residue analysis
was completed. The certainty index for this incident (1003351-003) is probable.
Following application of methidathion to an unknown treatment site, a red-tailed hawk
was found dead on January 27, 1994, in Stanislaus County, California. Blood and
footwash sample were collected and sent to the PIU (Department of Fish and Game
Pesticides Investigation Unit (California)) for examination. Blood plasma cholinesterase
and acetlycholinesterase levels were within the normal range. The results of the footwash
showed detections of methidathion at 2.7 ppb. In addition, organophosphates including
chlorpyrifos and diazinon were also detected in the footwash sample at respective
concentrations of 0.2 ppb and 4.1 ppb. It is likely that the hawk was exposed by perching
on substrates which had recently been coated with the detected pesticides. The certainty
index for this incident (1005042-012) is probable.
On February 3, 1994, a red-tailed hawk was found entangled in a fence near an almond
orchard in Merced County. The hawk was handed over to the Stanislaus Wildlife Care
Center. Upon admission to the care center, blood and footwash samples were collected.
The hawk was released following recovery from the injuries. Samples were sent to the
PIU for examination. Plasma cholinesterase and acetylcholinesterase levels were found
significantly below the normal range. Footwash analysis results show that methidathion
was detected at a concentration of 2.7 ppb. Chlorpyrifos and diazinon were also detected
in the footwash sample at respective concentrations of 0.7 ppb and 0.4 ppb. It appears
that the hawk was affected due to exposure via perching on substrates, such as tree
branches, which had recently been treated with these organophosphate pesticides. The
certainty index for this incident (105042-010) is highly probable.
A red-tailed hawk was recovered from an orchard on December 23, 1994 and taken to the
Stanislaus Wildlife Care Center. The bird died one day later. A blood sample and the
carcass were sent to the PIU for examination. The plasma cholinesterase level was low
and the brain cholinesterase level was severely depressed. Feathers and footskin samples
were analyzed. Methidathion was detected at a concentration of 0.7 ppm in the footskin.
In addition, chloropyrifos and diazinon were detected in both the feathers and footskin at
concentrations ranging from 0.02 to 0.08 ppb. It is likely that all three organophosphates
contributed to the death of this bird. The certainty index for this incident (1005042-015)
is possible.
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Following spraying of plum trees in Madera County, California with methidathion on
Feb. 4, 1995 and spraying of a nearby location on the 5th and 6th, pollination hives were
placed in the orchard on those same dates. Heavy fog was present the 4th and 5th, which
limited foraging activity of the bees. Dead bees (number not specified) were found
beginning Feb. 8 and continuing until Feb. 13. Residue analysis of the dead bees was
completed, showing the presence of 2.6 ppm methidathion and 0.02 ppm diazinon. The
Madera County authorities did not indicate any fault or liability, and attributed the bee
kill to poor weather and too early an introduction of the hives. The certainty index for
this incident (1001920-001) is possible.
The most recent terrestrial incident involving methidathion was reported on March 15,
1995. Following application of methidathion to an agricultural area, an unknown number
of bees were killed in an unknown county in California. The incident was reported as
accidental under the misuse category. No further information was provided in the
incident report. No residue analysis was completed, and the certainty index for this
incident (1005895-512) is probable.
4.4.2 Aquatic Incidents
On May 25, 1976, approximately 3,000 fish of unknown species were killed following an
aerial application of methidathion to an agricultural area in Sacramento County,
California. Methidathion had been aerially sprayed on a nearby seed clover field, and
irrigation water from the field had entered Laguna Creek in the area directly upstream of
the fish kill. The incident was attributed to accidental misuse of methidathion. Samples
of the water collected from the field drain six days after the material was applied to the
field showed methidathion concentrations of 60 ppb. It is presumed that the
concentration of methidathion would have been much higher if the samples had been
collected immediately following aerial application of the field. No fish tissue analysis
was completed. The certainty index for this incident (B0000-218-10) is highly probable.
5. Risk Characterization
Risk characterization is the integration of the exposure and effects characterizations.
Risk characterization is used to determine the potential for direct and/or indirect effects to
the CRLF or for modification to its designated critical habitat from the use of
methidathion in CA. The risk characterization provides an estimation (Section 5.1) and a
description (Section 5.2) of the likelihood of adverse effects; articulates risk assessment
assumptions, limitations, and uncertainties; and synthesizes an overall conclusion
regarding the likelihood of adverse effects to the CRLF or its designated critical habitat
(i.e., "no effect," "likely to adversely affect," or "may affect, but not likely to adversely
affect").
5.1 Risk Estimation
Risk is estimated by calculating the ratio of exposure to toxicity. This ratio is the risk
quotient (RQ), which is then compared to pre-established acute and chronic levels of
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concern (LOCs) for each category evaluated (Appendix E). For acute exposures to the
CRLF and its animal prey in aquatic habitats, as well as terrestrial invertebrates, the LOC
is 0.05. For acute exposures to the CRLF and mammals, the LOC is 0.1. The LOC for
chronic exposures to CRLF and its prey, as well as acute exposures to plants is 1.0.
Risk to the aquatic-phase CRLF is estimated by calculating the ratio of exposure to
toxicity using l-in-10 year EECs based on the label-recommended methidathion usage
scenarios summarized in Table 3.2 and the appropriate aquatic toxicity endpoint from
Table 4.1. Risks to the terrestrial-phase CRLF and its prey (e.g. terrestrial insects, small
mammals and terrestrial-phase frogs) are estimated based on exposures resulting from
applications of methidathion (Tables 3.5 through 3.6) and the appropriate toxicity
endpoint from Table 4.3. Due to lack of toxicity data, exposures are not derived for
terrestrial plants.
The minor degradate methidathion oxon is of toxicological concern, but because there are
no toxicity data, this degradate cannot be included directly in calculations of RQs.
Potential risk from methidathion oxon is characterized in the risk description (Section
5.2).
5.1.1 Exposures in the Aquatic Habitat
5.1.1.1 Direct Effects to Aquatic-Phase CRLF
Direct effects acute RQs for the aquatic-phase CRLF are presented in Table 5.1. Based
on l-in-10 year peak aquatic EECs from the PRZM/EXAMS model and the lowest acute
96-hour LC50 for freshwater fish (surrogate for the aquatic-phase CRLF), acute RQs for
the aquatic-phase CRLF exceed the listed species LOC (0.05) for all of the assessed
methidathion uses. Acute RQs for methidathion range from 2.1 for the alfalfa scenario to
52.5 for the nursery scenario. As a note, for the spatial analyses sections, the lowest and
highest EECs were estimated following a single application. The lowest acute RQ for a
single application is calculated to be 0.20 for the lowest use scenario, on mangoes.
Therefore, even after a single application, the acute listed species LOC would be
exceeded for all of the methidathion use scenarios.
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Table 5.1 Summary of Acute Direct E
'feet RQs for the Aquatic-phase CRLF3
Scenario
Crops Represented
Peak l-in-10
Year EEC
(Hg/L)b
Acute
RQC
Probability of
Individual Effect at
LOCd
Probability of
Individual Effect
at RQd
Alfalfa
Alfalfa, Clover,
Timothy
4.6
2.1
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1.2
Almond
Almond, Walnut
49.3
22.4
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Citrus
Calemondin, Citron,
Citrus, Grapefruit,
Kumquat, Lemon,
Lime, Mango,
Pummelo, Tangelo,
Tangerine
18.3
8.3
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Corn
Sunflower
20.7
9.4
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Cotton
Cotton
13.0
5.9
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Grapes
Kiwi
6.6
3.0
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1.1
Tree Fruit
Apple, Apricot,
Cherry, Nectarine,
Peach, Pear, Plum,
Prune
15.6
7.1
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Olive
Olive
15.2
6.9
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Row Crop
Artichoke
16.0
7.3
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Wheat
Safflower
13.6
6.2
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
Nursery
Nursery
115.6
52.5
~1 in 12,400
(~1 in 10,300,000 to
~1 in 149)
~1 in 1
" Based on bluegill sunfish (Lepomis macrochirus) acute 96-hour LC50= 2.2 ug I.
b See Table 3.3.
c Acute RQs that exceed the acute endangered species LOC of 0.05 are in bold.
11 The effect probability was calculated based on a probit dose-response slope of 2.9 (1.9 - 4.0) for the bluegill sunfish acute toxicity
study.
Direct chronic risk estimates for the aquatic-phase CRLF are based on the 60-day average
EECs from the PRZM/EXAMS model and the lowest chronic NOAEC for freshwater
fish (surrogate for the aquatic-phase CRLF). Using the estimated chronic NOAEC of
0.07 ppb based on an ACR of 140 and the acute LC50 value of 10 ppb, the chronic RQs
exceed the LOC (1.0) for all of the assessed methidathion uses (Table 5.2). Based on
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acute and chronic LOC exceedances, methidathion may affect the aquatic-phase of the
CRLF.
Table 5.2 Summary of Chronic Direct Effect RQs for the Aquatic-phase CRLF3
Scenario
Crops Represented
60-day EEC (jtg/L)b
Chronic RQ°
Alfalfa
Alfalfa, Clover, Timothy
2.5
35.7
Almond
Almond, Walnut
26.8
383
Citrus
Calemondin, Citron, Citrus, Grapefruit,
Kumquat, Lemon Lime, Mango, Pummelo,
Tangelo, Tangerine
9.0
129
Corn
Sunflower
10.9
156
Cotton
Cotton
6.4
91.4
Grapes
Kiwi
3.2
45.7
Tree Fruit
Apple, Apricot, Cherry, Nectarine, Peach, Pear,
Plum, Prune
5.0
71.4
Olive
Olive
7.6
109
Row Crop
Artichoke
8.7
124
Wheat
Safflower
6.6
94.3
Nursery
Nursery
54.3
776
" Based on estimated chronic NOAEC of 0.07 |xg/L from ACR ratio (see Section 4.1.1.2).
b See Table 3.3.
c Chronic RQs that exceed the chronic LOC of 1.0 are in bold.
5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey
(non-vascular aquatic plants, aquatic invertebrates, fish, and frogs)
Non-vascular Aquatic Plants
Indirect effects of methidathion to the aquatic-phase CRLF (tadpoles) via reduction in
non-vascular aquatic plants in its diet cannot be quantitatively estimated because there are
no aquatic plant toxicity data available for methidathion. For a qualitative risk
description, see Section 5.2.2.1.
Aquatic Invertebrates
Indirect acute effects to the aquatic-phase CRLF via effects to prey (invertebrates) in
aquatic habitats are based on peak l-in-10 year EECs from the PRZM/EXAMS model
and the lowest 48-hour LC50 for freshwater invertebrates. These acute RQs for aquatic
invertebrates range from 1.5 to 38.5, and thus exceed the acute risk LOC for non-listed
species (0.5) (Table 5.3).
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Table 5.3 Summary of Acute RQs Used to Estimate Indirect Effects to the CRLF via
Direct Effects on Aquatic Invertebrates as Dietary Food Items
Scenario
Crops Represented
Peak l-in-10 Year
EEC (jtg/L)b
Indirect Effects
Acute RQ°
% Expected Effect on
Prey Population at RQd
Alfalfa
Alfalfa, Clover,
Timothy
4.6
1.5
79
Almond
Almond, Walnut
49.3
16.4
100
Citrus
Calemondin Citron,
Citrus, Grapefruit,
Kumquat, Lemon,
Lime, Mango,
Pummelo, Tangelo,
Tangerine
18.3
6.1
100
Corn
Sunflower
20.7
6.9
100
Cotton
Cotton
13.0
4.3
100
Grapes
Kiwi
6.6
2.2
94
Tree Fruit
Apple, Apricot, Cherry,
Nectarine, Peach, Pear,
Plum, Prune
15.6
5.2
100
Olive
Olive
15.2
5.1
100
Row Crop
Artichoke
16.0
5.3
100
Wheat
Safflower
13.6
4.5
100
Nursery
Nursery
115.6
38.5
100
" Based on water flea (Daphnia magna) acute 48-hour LC50 =3.0 ug I.
b See Table 3.3.
c Acute RQs that exceed the acute LOC for nonlisted species of 0.5 are in bold.
d The acute daphnid toxicity study data do not fit the probit model; therefore, the effect probability was calculated based on a default slope
assumption of 4.5 with upper and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986).
Risk estimates for indirect effects to the CRLF via chronic effects to aquatic invertebrates
are based on 21-day average EECs and the lowest chronic NOAEC for freshwater
invertebrates. Chronic RQs for freshwater invertebrates range from 3.4 to 128.4, and thus
exceed the LOC (1.0) for all of the assessed methidathion uses (Table 5.4). Based on
acute and chronic LOC exceedences for all of the assessed uses, methidathion may
indirectly affect the CRLF via reduction in freshwater invertebrates prey items.
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Table 5.4 Summary of Chronic RQs Used to Estimate Indirect Effects to the CRLF via
Direct Effects on Aquatic Invertebrates as Dietary Food Items
Scenario
Crops Represented
21-day EEC (jig/L)b
Indirect Effects
Chronic RQC
Alfalfa
Alfalfa, Clover, Timothy
3.4
5.2
Almond
Almond, Walnut
40.2
60.9
Citrus
Calemondin, Citron, Citrus, Grapefruit,
Kumquat, Lemon Lime, Mango, Pummelo,
Tangelo, Tangerine
13.6
20.6
Corn
Sunflower
16.2
24.5
Cotton
Cotton
9.3
14.1
Grapes
Kiwi
5.0
7.6
Tree Fruit
Apple, Apricot, Cherry, Nectarine, Peach, Pear,
Plum, Prune
9.2
13.9
Olive
Olive
11.7
17.7
Row Crop
Artichoke
12.1
18.3
Wheat
Safflower
10.4
15.8
Nursery
Nursery
84.8
128.4
" Based on water flea (Daphnia magna) chronic NOAEC = 0.66 ug I.
b See Table 3.3.
c Chronic RQs that exceed the chronic LOC of 1.0 are in bold.
Fish and Frogs
Fish and frogs also represent potential prey items of adult aquatic-phase CRLFs. RQs
associated with acute and chronic direct toxicity to the CRLF (Tables 5.1 - 5.2) are used
to assess potential indirect effects to the CRLF based on a reduction in freshwater fish
and frogs as food items. Since the acute and chronic RQs exceed the LOC, methidathion
may indirectly affect the CRLF via reduction in freshwater fish and frogs as food items.
5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and/or
Primary Productivity (Freshwater Aquatic Plants)
Indirect effects to the CRLF via direct toxicity to aquatic plants cannot be quantitatively
estimated because there are no aquatic plant toxicity data available for methidathion. For
a qualitative risk description, see Section 5.2.3.1.
5.1.2 Exposures in the Terrestrial Habitat
5.1.2.1 Direct Effects to Terrestrial-phase CRLF
As previously discussed in Section 3.3, potential direct effects to terrestrial-phase CRLFs
are based on foliar applications of methidathion. Potential direct acute effects to the
terrestrial-phase CRLF are derived by considering dose- and dietary-based EECs
modeled in T-REX for a small bird (20 g) consuming small invertebrates (Table 3.5) and
acute oral and subacute dietary toxicity endpoints for avian species. Potential direct
chronic effects of methidathion to the terrestrial-phase CRLF are derived by considering
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dietary-based exposures modeled in T-REX for a small bird (20g) consuming small
invertebrates. Chronic effects are estimated using the lowest available toxicity data for
birds. EECs are divided by toxicity values to estimate chronic dietary-based RQs. Acute
dose-based, acute dietary-based, and chronic RQs exceed the LOCs for all of the assessed
methidathion uses (Table 5.5-5.7), As a result, methidathion may affect the terrestrial-
phase of the CRLF.
Table 5.5. Summary of Direct Effect Acute Dose-Based RQs for the Terrestrial-
phase CRLF for Spray Applications of Methidathion
Use
App. Rate (lb ai/A)
Number of Apps.
App. Interval (days)
Broadleaf Plants/
Small Insects
Acute Dose-
Based RQa
Probability of
Individual
Effect at LOCb
Probability of
Individual
Effect at RQb
Artichoke
1 lb a.i./acre
8 apps
14 days
57.39
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Citrus
5 lb a.i./acre
2 apps
45 days
222.94
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Clover
1 lb a.i./acre
2 apps.
7 days
65.39
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Cotton
1 lb a.i./acre
4 apps
5 days
94.95
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Kiwi
2 lb a.i./acre
1 app
NA
88.39
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Mango
0.25 lb a.i./acre
5 apps.
21 days
12.42
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Nursery
10 lb a.i./A
1 app
NA
441.97
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Safflower, Sunflower
0.5 lb a.i./acre
3 apps
7 days
37.77
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Tree Fruit, Olives
3 lb a.i./acre
1 app
NA
132.59
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
Walnut
2 lb a.i./acre
3 apps.
2 days
218.11
~1 in 294,000
(~1 in 8.86E+18
to ~1 in 44)
~1 in 1
a Based on mallard duck acute oral LD50 = 6.7 mg/kg
b The acute data do not fit the probit model; therefore, the effect probability was calculated based on a default
slope assumption of 4.5 with upper and lower 95% confidence intervals of 2 and 9 (Urban and Cook, 1986).
cAcute RQs that exceed the acute endangered species LOC of 0.1 are in bold.
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Table 5.6. Summary of Direct Effect Acute Dietary-Based RQs for the
Terrestrial-phase CRLF for Spray Applications of Methidathion
Broadleaf Plants/
App. Rate (lb ai/A)
Small Insects
Use
Number of Apps.
App. Interval (days)
Acute
Dietary-Based
Probability of
Individual
Probability of
Individual
RQa
Effect at LOCb
Effect at RQb
1 lb a.i./acre
~1 in 6.03E+17
~1 in 5
Artichoke
8 apps
0.78
(~1 in 7.85E+42
(~1 in 14
14 days
to ~1 in 4,300)
to ~1 in 2)
5 lb a.i./acre
~1 in 6.03E+17
Citrus
2 apps
45 days
3.04
(~1 in 7.85E+42
to ~1 in 4,300)
~1 in 1
1 lb a.i./acre
~1 in 6.03E+17
~1 in 3
Clover
2 apps.
0.89
(~1 in 7.85E+42
(~1 in 4
7 days
to ~1 in 4,300)
to ~1 in 2)
1 lb a.i./acre
~1 in 6.03E+17
Cotton
4 apps
5 days
1.29
(~1 in 7.85E+42
to ~1 in 4,300)
~1 in 1
2 lb a.i./acre
~1 in 6.03E+17
Kiwi
1 app
NA
1.21
(~1 in 7.85E+42
to ~1 in 4,300)
~1 in 1
0.25 lb a.i./acre
~1 in 6.03E+17
(~1 in 7.85E+42
to ~1 in 4,300)
~1 in 9.28E+10
Mango
5 apps.
21 days
0.17
(~1 in 8.3E+25
to ~1 in
2.83E+2)
10 lb a.i./A
~1 in 6.03E+17
Nursery
1 app
NA
6.03
(~1 in 7.85E+42
to ~1 in 4,300)
~1 in 1
0.5 lb a.i./acre
~1 in 6.03E+17
~1 in 148
Safflower, Sunflower
3 apps
0.52
(~1 in 7.85E+42
(~1 in 22,500
7 days
to ~1 in 4,300)
to ~1 in 6)
3 lb a.i./acre
~1 in 6.03E+17
Tree Fruit, Olives
1 app
NA
1.81
(~1 in 7.85E+42
to ~1 in 4,300)
~1 in 1
2 lb a.i./acre
~1 in 6.03E+17
Walnut
3 apps.
2 days
2.97
(~1 in 7.85E+42
to ~1 in 4,300)
~1 in 1
a Based on bobwhite quail acute dietary LC50 = 224 ppm
bThe effect probability was calculated using a probit dose-response slope of 8.7 (3.5 - 13.8) from the bobwhite quail
study.
cAcute RQs that exceed the acute endangered species LOC of 0.1 are in bold.
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Table 5.7. Summary of Direct Effect Chronic RQs for the Terrestrial-phase
CRLF for Spray Applications of Methidathion
Use
App. Rate (lb ai/A)
Number of Apps.
App. Interval (days)
Broadleaf Plants/
Small Insects
Chronic RQa
Artichoke
1 lb a.i./acre
8 apps
14 days
17.53b
Citrus
5 lb a.i./acre
2 apps
45 days
68.10
Clover
1 lb a.i./acre
2 apps.
7 days
19.97
Cotton
1 lb a.i./acre
4 apps
5 days
29.00
Kiwi
2 lb a.i./acre
1 app
NA
27.00
Nursery
10 lb a.i./A
1 app
NA
135.00
Mango
0.25 lb a.i./acre
5 apps.
21 days
3.79
Safflower, Sunflower
0.5 lb a.i./acre
3 apps
7 days
11.54
Tree Fruit, Olives
3 lb a.i./acre
1 app
NA
40.50
Walnut
2 lb a.i./acre
3 apps.
2 days
66.62
a Based on mallard duck chronic NOAEC =10 ppm
bChronic RQs that exceed the chronic LOC of 1.0 are in bold.
5.1.2.2 Indirect Effects to Terrestrial-Phase CRLF via Reduction in
Prey (terrestrial invertebrates, mammals, and frogs)
5.1.2.2.1 Terrestrial Invertebrates
In order to assess the risks of methidathion to terrestrial invertebrates, which are
considered prey of CRLF in terrestrial habitats, the honey bee is used as a surrogate for
terrestrial invertebrates. The toxicity value for terrestrial invertebrates is calculated by
multiplying the lowest available acute contact LD50 of 0.236 |ig/bee by 1 bee/0.128g,
which is based on the weight of an adult honey bee. EECs (ppm) calculated by T-REX
for small and large insects are divided by the calculated toxicity value for terrestrial
invertebrates, which is 1.84 ppm (Table 5.8). Based on acute LOC exceedances for all of
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the assessed uses, methidathion may affect the CRLF via reduction in terrestrial
invertebrate prey items.
Table 5.8 Summary of RQs For Indirect Effects to the Terrestrial-phase CRLF via
Direct Effects on Terrestrial Invertebrates as Dietary Food Items
Use
App. Rate (lb ai/A)
Number of Apps.
App. Interval (days)
Small
Insect
RQ
Large
Insect
RQ
Small Insect %
Expected Effect on
Prey Population at
RQb
Large Insect %
Expected Effect on
Prey Population at
RQb
Artichoke
1 lb a.i./acre
8 apps
14 days
95
10
100
100
Citrus
5 lb a.i./acre
2 apps
45 days
370
41
100
100
Clover
1 lb a.i./acre
2 apps.
7 days
109
12
100
100
Cotton
1 lb a.i./acre
4 apps
5 days
158
17
100
100
Kiwi
2 lb a.i./acre
1 app
NA
147
16
100
100
Mangos
0.25 lb a.i./acre
5 apps
21 days
21
2
100
100
Nursery
10 lb a.i./A
1 app
NA
734
82
100
100
Safflower,
Sunflower
0.5 lb a.i./acre
3 apps
7 days
63
7
100
100
Tree Fruit,
Olives
3 lb a.i./acre
1 app
NA
220
24
100
100
Walnut
2 lb a.i./acre
3 apps.
2 days
362
40
100
100
"Based on honey bee LD50 of 0.236 ug bee or 1.84 ppm
b The " o population was calculated using a probit dose-response slope of 9.06 from the honey bee study.
cAcute RQs that exceed the acute non-listed species LOC of 0.5 are in bold.
5.1.2.2.2 Mammals
Risks associated with ingestion of contaminated small mammals by large terrestrial-phase
CRLFs are derived for dietary-based and dose-based exposures modeled in T-REX for a
small mammal (15g) consuming short grass. Acute and chronic effects are estimated
using the most sensitive mammalian toxicity data. EECs are divided by the toxicity value
to estimate acute dose-based RQs as well as chronic dietary- and dose-based RQs. As
shown in Tables 5.9-5.10, the acute and chronic RQs exceed the LOC for all of the
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assessed methidathion uses. As a result, methidathion may affect the CRLF via reduction
in small mammal prey items.
Table 5.9. Summary of Acute RQs For Indirect Effects to the Terrestrial-phase
CRLF via Direct Effects on Small Mammals as Dietary Food Items
Use
App. Rate (lb ai/A)
Number of Apps.
App. Interval (days)
Acute Dose-Based
RQa
% Expected Effect on
Prey Population at RQb
Artichoke
1 lb a.i./acre
8 apps
14 days
11.27
100
Citrus
5 lb a.i./acre
2 apps
45 days
43.76
100
Clover
1 lb a.i./acre
2 apps.
7 days
12.84
100
Cotton
1 lb a.i./acre
4 apps
5 days
18.64
100
Kiwi
2 lb a.i./acre
1 app
NA
17.35
100
Mango
0.25 lb a.i./A
5 apps
21 days
2.44
96
Nursery
10 lb a.i./A
1 app
NA
86.76
100
Safflower, Sunflower
0.5 lb a.i./acre
3 apps
7 days
7.41
100
Tree Fruit, Olives
3 lb a.i./acre
1 app
NA
26.03
100
Walnut
2 lb a.i./acre
3 apps.
2 days
42.82
100
a Based on rat acute oral LD50 = 12 mg/kg-bw.
b The effect probability was calculated based on a default slope assumption of 4.5 with upper and lower 95% confidence intervals of 2
and 9 (Urban and Cook, 1986).
cAcute RQs that exceed the acute non-listed species LOC of 0.5 are in bold.
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Table 5.10. Summary of Chronic RQs For Indirect Effects to the Terrestrial-phase
CRLF via Direct Effects on Small Mammals as Dietary Food Items
Use
App. Rate (lb ai/A)
Number of Apps.
App. Interval (days)
Chronic Dose-Based
RQa
Chronic Dietary-Based
RQb
Artichoke
1 lb a.i./acre
8 apps
14 days
540.74
62.33
Citrus
5 lb a.i./acre
2 apps
45 days
2100.70
242.13
Clover
1 lb a.i./acre
2 apps.
7 days
616.11
71.01
Cotton
1 lb a.i./acre
4 apps
5 days
894.65
103.12
Kiwi
2 lb a.i./acre
1 app
NA
832.90
96.00
Mango
0.25 lb a.i./A
5 apps
21 days
117.00
13.49
Nursery
10 lb a.i./A
1 app
NA
4164.5
480.00
Safflower, Sunflower
0.5 lb a.i./acre
3 apps
7 days
355.92
41.02
Tree Fruit, Olives
3 lb a.i./acre
1 app
NA
1249.35
144.00
Walnut
2 lb a.i./acre
3 apps.
2 days
2055.21
236.88
a Based on rat NOAEL = 0.25 mg/kg-bw.
b Based on rat NOAEC = 5 mg/kg-diet.
c Chronic RQs that exceed the chronic LOC of 1 are in bold.
5.1.2.2.3 Frogs
An additional prey item of the adult terrestrial-phase CRLF is other species of frogs. In
order to assess risks to these organisms, dietary-based and dose-based exposures modeled
in T-REX for a small bird (20g) consuming small invertebrates are used. See Section
5.1.2.1 and associated tables (Tables 5.3 - 5.5) for results. Acute and chronic RQs
exceed the LOCs for all of the assessed methidathion uses. As a result, methidathion may
affect the CRLF via reduction in frogs as prey items.
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5.1.2.3 Indirect Effects to CRLF via Reduction in Terrestrial Plant
Community (Riparian and Upland Habitat)
Indirect effects to the CRLF via reduction in terrestrial plant community cannot be
quantitatively estimated because there are no vegetative vigor or seedling emergence
plant toxicity data available for methidathion. For a qualitative risk description, see
Section 5.2.3.2.
5.1.3 Primary Constituent Elements of Designated Critical Habitat
For methidathion use, the assessment endpoints for designated critical habitat PCEs
involve a reduction and/or modification of food sources necessary for normal growth and
viability of aquatic-phase CRLFs, and/or a reduction and/or modification of food sources
for terrestrial-phase juveniles and adults. Because these endpoints are also being
assessed relative to the potential for indirect effects to aquatic- and terrestrial-phase
CRLF, the effects determinations for indirect effects from the potential loss of food items
are used as the basis of the effects determination for potential modification to designated
critical habitat.
5.1.3.1 Aquatic-Phase (Aquatic Breeding Habitat and Aquatic Non-
Breeding Habitat)
Three of the four assessment endpoints for the aquatic-phase primary constituent
elements (PCEs) of designated critical habitat for the CRLF are related to potential
effects to aquatic and/or terrestrial plants:
• Alteration of channel/pond morphology or geometry and/or increase in sediment
deposition within the stream channel or pond: aquatic habitat (including riparian
vegetation) provides for shelter, foraging, predator avoidance, and aquatic
dispersal for juvenile and adult CRLFs.
• Alteration in water chemistry/quality including temperature, turbidity, and
oxygen content necessary for normal growth and viability of juvenile and adult
CRLFs and their food source.
• Reduction and/or modification of aquatic-based food sources for pre-metamorphs
(e.g., algae).
Risk estimations for potential effects to aquatic and/or terrestrial plants were not
conducted because no toxicity data are available for either aquatic or terrestrial plants.
Therefore, it cannot be estimated whether or not methidathion is likely to affect aquatic-
phase PCEs of designated habitat related to effects on aquatic and/or terrestrial plants.
Risks to aquatic and terrestrial plants will be discussed qualitatively in sections 5.2.2.1,
5.2.3.2 and 5.2.3.2.
The remaining aquatic-phase PCE is "alteration of other chemical characteristics
necessary for normal growth and viability of CRLFs and their food source." To assess
the impact of methidathion on this PCE, acute and chronic freshwater fish and
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invertebrate toxicity endpoints, as well endpoints for aquatic non-vascular plants, are
used as measures of effects. RQs for these endpoints were calculated in Sections 5.1.1.1
and 5.1.1.2. Based on exceedances of the listed and non-listed species LOCs following
both acute and chronic exposure for freshwater fish and invertebrates, methidathion is
likely to affect aquatic-phase PCEs of designated habitat related to effects of alteration of
other chemical characteristics necessary for normal growth and viability of CRLFs and
their food source. As stated previously, there are no aquatic plant toxicity data available
for methidathion. Therefore, effects to this aquatic-phase PCE based on effects to aquatic
non-vascular plants will be discussed qualitatively in Section 5.2.2.1.
5.1.3.2 Terrestrial-Phase (Upland Habitat and Dispersal Habitat)
Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical
habitat for the CRLF are related to potential effects to terrestrial plants:
• Elimination and/or disturbance of upland habitat; ability of habitat to support food
source of CRLFs: Upland areas within 200 ft of the edge of the riparian
vegetation or dripline surrounding aquatic and riparian habitat that are comprised
of grasslands, woodlands, and/or wetland/riparian plant species that provides the
CRLF shelter, forage, and predator avoidance
• Elimination and/or disturbance of dispersal habitat: Upland or riparian dispersal
habitat within designated units and between occupied locations within 0.7 mi of
each other that allow for movement between sites including both natural and
altered sites which do not contain barriers to dispersal
The risk estimation for terrestrial-phase PCEs of designated habitat related to potential
effects on terrestrial plants cannot be quantitatively addressed because there are no
vegetative vigor or seedling emergence plant toxicity data available for methidathion.
The risk will be discussed qualitatively in Section 5.2.3.2.
The third terrestrial-phase PCE is "reduction and/or modification of food sources for
terrestrial phase juveniles and adults." To assess the impact of methidation on this PCE,
acute and chronic toxicity endpoints for birds, mammals, and terrestrial invertebrates are
used as measures of effects. RQs for these endpoints were calculated in Sections 5.1.2.1
and 5.1.2.2. Acute and chronic LOCs for listed and non-listed species were exceeded for
all uses for birds and mammals and acute LOCs for listed and non-listed terrestrial
invertebrates were also exceeded for all uses. Therefore, methidiathion is likely to affect
the third terrestrial-phase PCE.
The fourth terrestrial-phase PC is based on alteration of chemical characteristics
necessary for normal growth and viability of juvenile and adult CRLFs and their food
source. Direct acute and chronic RQs for terrestrial-phase CRLFs are presented in
Section 5.1.2.1. Acute and/or chronic LOCs for listed and non-listed species were
exceeded for freshwater fish and invertebrates, birds, mammals and terrestrial
invertebrates. No data are available for either aquatic or terrestrial plants. However,
based on the results with aquatic and terrestrial animals, methidathion is likely to affect
the fourth terrestrial - phase PCE.
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5.1.4 Spatial Extent of Potential Effects
Since this screening level risk assessment defines taxa that are predicted to be exposed
through runoff and drift to methidathion at concentrations above the Agency's Levels of
Concern (LOC), analysis of the potential spatial extent of effects requires expansion of
the area from the treated site to include all areas potentially impacted by this federal
action. Two methods are used to define these areas: (1) the spray drift assessment for
determining the extent of terrestrial and aquatic habitats potentially affected through drift
alone; and (2) the down stream dilution assessment for determining the extent of the
affected lotic aquatic habitats (flowing water). This analysis is described briefly below
and in more detail in Appendix D.
An LAA effects determination applies to those areas where it is expected that the
pesticide's use will directly or indirectly affect the CRLF or its designated critical habitat.
To determine this area, the footprint of methidathion's use pattern is identified, using
corresponding land cover data. The spatial extent of the effects determination also
includes areas beyond the initial area of concern that may be impacted by runoff and/or
spray drift. The identified direct and indirect effects and modification to critical habitat
are anticipated to occur only for those currently occupied core habitat areas, CNDDB
occurrence sections and designated critical habitat for the CRLF that overlap with the
initial area of concern plus greater than 1000 feet from its boundary. It is assumed that
non-flowing waterbodies (or potential CRLF habitat) are included within this area.
In addition to the spray drift buffer, the results of the downstream dilution extent analysis
result in a distance of 285 kilometers which represents the maximum continuous distance
of downstream dilution from the edge of the initial area of concern. If any of these
streams reaches flow into CRLF habitat, there is potential to affect either the CRLF or
modify its habitat. These lotic aquatic habitats within the CRLF core areas and critical
habitats potentially contain concentrations of methidathion sufficient to result in LAA
determination or modification of critical habitat.
The determination of the buffer distance and downstream dilution for spatial extent of the
effects determination is described below.
5.1.4.1 Spray Drift
In order to determine terrestrial and aquatic habitats of concern due to methidathion
exposures through spray drift, it is necessary to estimate the distance that spray
applications can drift from the treated area and still be present at concentrations that
exceed levels of concern. A quantitative analysis of spray drift distances was completed
using AgDrift (v. 2.01).
Spatial analysis of spray drift effects is limited to consideration of a single application
because, due to variable wind conditions, multiple applications are not likely to impact
the same location each time. Spray drift distances depend on both application rate and
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method, and so, in order to understand the range of possible impacts, modeling was done
for the uses with the highest single application rates for each method, which include tree
fruit and nuts treated by aerial spray, nursery stock treated by ground spray, and citrus
treated by spray that is presumed to be airblast. Use on mangos was modeled as well to
represent the lower boundary of potential effects.
Methidathion labels have specific application requirements in order to reduce potential
spray drift, including restrictions on wind speed, release height and droplet size. AgDrift
inputs are based on these requirements, as presented in Table 5.11. For inputs not
specified on the labels, default values are used. When Tier I models were used, it is
because no higher tier models are available.
Table 5.11. Input parameters for simulation of methidathion in spray drift using
AgE
rift (v. 2.01)
Parameter Description
Tree Fruit /
Nuts
Nursery Stock
Citrus
Mangos
Single Application Rate
3
lb a.i./A
10
lb a.i./A
5
lb a.i./A
0.5
lb a.i./A
Application method
Aerial
Ground
Airblast
Airblast
Droplet Size
Distribution (DSD)
Medium to
Coarse
Fine to
Medium1
NA
NA
Release height
NA
High Boom2
NA
NA
Orchard type
NA
NA
Dense Orchard
Dense Orchard
Labels require a medium to coarse DSD, but AgDrift does not include this as an option for Tier I Ground mode.
2 Labels require a release height for ground spray applications of less than 4 feet above the crop canopy. AgDrift does
not use specific release heights, so modeling for this use is based high boom spray, the option with the highest release
height, 4 ft (1.27 m).
The terrestrial analysis is based on the honey bee acute contact LD50 of 0.236 |ig/bee, or
1.84 ppm, which is the most sensitive terrestrial endpoint and is used as a surrogate for
terrestrial invertebrates. Based on this endpoint, the initial average deposition level that
would lead to exceedance of the terrestrial invertebrate LOC (0.05) is 0.00068 lb a.i./A,
where the deposition is calculated as the RQ divided by the LOC and multiplied by the
application rate. The most sensitive endpoint upon which the aquatic analysis is based is
the bluegill sunfish 96-hour acute LC50 of 2.2 |ig a.i./L. With this toxicity endpoint, the
endangered species LOC of 0.05 will be exceeded at any concentration greater than 0.11
|ig a.i./L.
Table 5.12 includes uses with the maximum single application rates for each application
method and presents a summary of the buffer distances at which spray drift deposition
from these uses drop below levels of concern (e.g., RQs will be below LOCs). These
distances represent the maximum extent where effects are possible using the most
sensitive data and the listed species LOC of 0.05 for either terrestrial invertebrates or
freshwater fish. For each application method, lower application rates will yield a smaller
buffer distance than reported here.
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Table 5.12 Summary of maximum predicted distances for potential spray drift
effects.
Application
Method
Application Rate
(lb aJ/A)
Uses Rep resented
Terrestrial
LD5o Distance (ft)
Aquatic
LCso Distance (ft)
Aerial
3
Almond &
Tree Fruit
> 1000
>1000
Ground
10
Nursery
> 1000
> 1000
Airblast
5
Citrus
> 1000
> 1000
Airblast
0.25
Mango
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23
The use of methidathion on almonds and tree fruit is predicted to have effects for
terrestrial invertebrates at distances greater than 1,000 ft (0.3 km), the limit of AgDrift
modeling. Given the uncertainty in AgDrift and the assumptions used in the modeling,
the potential for effects from drift cannot be quantified beyond 1000 ft and so this value
is used to represent the potential for effects to terrestrial invertebrates and to define the
spatial extent of the effects determination (i.e., this buffer distance is added to the initial
area of concern). Given the toxicity of methidathion and the calculated RQs, the extent
of spray drift effects may be at least an order of magnitude higher than this.
5.1.4.2 Downstream Dilution Analysis
The maximum downstream extent of methidathion exposure in streams and rivers where
the EEC can potentially be above levels that would exceed the most sensitive LOC may
be estimated by utilizing the greatest ratio of aquatic RQ to LOC. This is based on the
assumption that there is uniform runoff across the landscape in treated areas with streams
flowing through them (i.e. the initial area of concern), represented by the modeled EECs,
and that as those waters move downstream, the influx of non-impacted water will dilute
the concentrations of methidathion present.
Using an LC50 value of 2.2 ug/L for freshwater fish (the most sensitive species) and a
maximum peak EEC of 114.7 |ig/L for applications to nursery stock yields an RQ/LOC
ratio of 1042 (52.2/0.05). Using the downstream dilution approach (described in more
detail in Appendix D) yields a target percent crop area (PCA) of 27.8%. This value has
been input into the downstream dilution approach and results in a distance of 285
kilometers, which represents the maximum continuous distance of downstream dilution
from the edge of the initial area of concern. Similar to the spray drift buffer described
above, the LAA/NLAA determination is based on the area defined by the point where
concentrations exceed the LC50 value.
5.1.4.3 Overlap between CRLF habitat and Spatial Extent of
Potential Effects
An LAA effects determination is made for those areas where it is expected that the
pesticide's use will directly or indirectly affect the CRLF or its designated critical habitat
and the area overlaps with the core areas, critical habitat and available occurrence data
for CRLF.
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Figure 5.1 shows that there is some overlap between the initial area of concern mapped
according to methidathion's use pattern and the CRLF habitat, including currently
occupied core areas, CNDDB occurrence sections, and designated critical habitat. This
map does not include areas beyond the initial area of concern that may be impacted by
runoff and/or spray drift. It is expected that any additional areas of CRLF habitat that are
located >1000 ft (to account for offsite migration via spray drift) and 285 kilometers of
stream reach (to account for downstream dilution) outside the initial area of concern may
also be impacted and are part of the full spatial extent of the LAA/modification of critical
habitat effects determination.
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| Methidathion & CRLF overlap
CNDDB occurrence sections
Critical habitat
Core areas
County boundaries
Methidathion Use & CRLF Habitat Overlap
Kilometers
0 20 40 80 120 160
Compiled from California County boundaries (ESRI, 2002),
USD". Gap Analysis Program Orchard/Vineyard Landcover (GAP)
National Land Ccwer Database (NLCD) (MRLC, 2001)
Map created by US Environmental Protection Agency, Office
of Pesticides Programs, Environmental Fate and Effects Division.
Projection: Albers Equal Area Conic USGS, North American
Datum of 1933 (NAD 1983).
Figure 5.1. Overlap Map: CRLF Habitat and Methidathion Initial Area of Concern
5.2 Risk Description
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The risk description synthesizes an overall conclusion regarding the likelihood of adverse
impacts leading to an effects determination (i.e., "no effect," "may affect, but not likely
to adversely affect," or "likely to adversely affect") for the CRLF and its designated
critical habitat.
If the RQs presented in the Risk Estimation (Section 5.1) show no direct or indirect
effects for the CRLF, and no modification to PCEs of the CRLF's designated critical
habitat, a "no effect" determination is made, based on methidathion's use within the
action area. However, if direct or indirect effect LOCs are exceeded or effects may
modify the PCEs of the CRLF's critical habitat, the Agency concludes a preliminary
"may affect" determination for the FIFRA regulatory action regarding methidathion. A
summary of the results of the risk estimation results are provided in Table 5.13 for
direct and indirect effects to the CRLF and in Table 5.14 for the PCEs of designated
critical habitat for the CRLF.
These results represent risk from exposure to applied methidathion and do not include
the additional risk posed by any degradates of toxicological concern. Given that acute
and chronic risk quotients are exceeded for the parent compound alone, any contribution
in toxicity from any of the major degradates or from methidathion oxon or other OP
degradates would increase the risk estimates. Potential risk from methidathion oxon is
characterized in Section 5.2.1.3,
Table 5.13 Risk Estimation Summary for Methidathion - Direct and Indirect Effects to
CRLF
Assessment Endpoint
LOC
Exceedances
(Y/N)
Description of Results of Risk Estimation
Aquatic Phase
(eggs, larvae, tadpoles, juveniles, and adults)
Direct Effects
Survival, growth, and reproduction
of CRLF individuals via direct
effects on aquatic phases
Y
There are LOC exceedances for listed species
following both acute and chronic exposure using
freshwater fish as the surrogate for aquatic-phase
amphibians.
Indirect Effects
Survival, growth, and reproduction
of CRLF individuals via effects to
food supply (i.e., fish freshwater
invertebrates, non-vascular plants)
Y
The acute LOC for non-listed species (all uses) and
the chronic LOC (all uses except alfalfa, clover,
timothy and kiwi) are exceeded for freshwater fish.
There are LOC exceedances for non-listed species
following both acute and chronic exposure to
freshwater invertebrates. No aquatic plant data are
available. A qualitative discussion of risk will be
provided.
Indirect Effects
Survival, growth, and reproduction
of CRLF individuals via effects on
habitat, cover, and/or primary
productivity (i.e., aquatic plant
community)
Unknown
No aquatic plant data are available. A qualitative
discussion of risk to aquatic plants will be provided.
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Table 5.13 Risk Estimation Summary for Methidathion - Direct and Indirect Effects to
CRLF
Assessment Endpoint
LOC
Exceedances
(Y/N)
Description of Results of Risk Estimation
Indirect Effects
Survival, growth, and reproduction
of CRLF individuals via effects to
riparian vegetation, required to
maintain acceptable water quality
and habitat in ponds and streams
comprising the species' current
range.
Unknown
No terrestrial plant data are available. A qualitative
discussion of risk will be provided.
Terrestrial Phase
(Juveniles and adults)
Direct Effects
Survival, growth, and reproduction
of CRLF individuals via direct
effects on terrestrial phase adults and
juveniles
Y
There are LOC exceedances for listed species
following both acute and chronic exposure using birds
as the surrogate for terrestrial-phase amphibians.
Indirect Effects
Survival, growth, and reproduction
of CRLF individuals via effects on
prey (i.e., terrestrial invertebrates,
small terrestrial mammals and
terrestrial phase amphibians)
Y
There are LOC exceedances for non-listed species
following both acute and/or chronic exposure to
terrestrial invertebrates, mammals and birds (surrogate
to terrestrial-phase amphibians).
Indirect Effects
Survival, growth, and reproduction
of CRLF individuals via effects on
habitat (i.e., riparian vegetation)
Unknown
No terrestrial plant data are available. A qualitative
discussion of risk will be provided.
Table 5.14 Risk Estimation Summary for Methidath
for the CRL
ion - PCEs of Designated Critical Habitat
7
Assessment Endpoint
Habitat Modification
(Y/N)
Description of Results of Risk Estimation
Aquatic Phase PCEs
(Aquatic Breeding Habitat and Aquatic Non-Breeding Habitat)
Alteration of channel/pond morphology or
geometry and/or increase in sediment
deposition within the stream channel or
pond: aquatic habitat (including riparian
vegetation) provides for shelter, foraging,
predator avoidance, and aquatic dispersal
for juvenile and adult CRLFs.
Unknown
No aquatic and terrestrial plant data are available.
A qualitative discussion of risk will be provided.
Alteration in water chemistry/quality
including temperature, turbidity, and oxygen
content necessary for normal growth and
viability of juvenile and adult CRLFs and
their food source.
Unknown
No aquatic and terrestrial plant data are available.
A qualitative discussion of risk will be provided.
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Table 5.14 Risk Estimation Summary for Methidath
for the CRL
ion - PCEs of Designated Critical Habitat
7
Assessment Endpoint
Habitat Modification
(Y/N)
Description of Results of Risk Estimation
Alteration of other chemical characteristics
necessary for normal growth and viability of
CRLFs and their food source.
Y
LOC exceedances for both listed and non-listed
species following both acute and chronic exposure
for freshwater fish and invertebrates. No aquatic
plant toxicity data are available.
Reduction and/or modification of aquatic-
based food sources for pre-metamorphs
(e.g., algae)
Unknown
No aquatic plant data are available. A qualitative
discussion of risk will be provided.
Terrestrial Phase PCEs
(Upland Habitat and Dispersal Habitat)
Elimination and/or disturbance of upland
habitat; ability of habitat to support food
source of CRLFs: Upland areas within 200
ft of the edge of the riparian vegetation or
dripline surrounding aquatic and riparian
habitat that are comprised of grasslands,
woodlands, and/or wetland/riparian plant
species that provides the CRLF shelter,
forage, and predator avoidance
Unknown
No terrestrial plant data are available. A qualitative
discussion of risk will be provided.
Elimination and/or disturbance of dispersal
habitat: Upland or riparian dispersal habitat
within designated units and between
occupied locations within 0.7 mi of each
other that allow for movement between sites
including both natural and altered sites
which do not contain barriers to dispersal
Unknown
No terrestrial plant data are available. A qualitative
discussion of risk will be provided.
Reduction and/or modification of food
sources for terrestrial phase juveniles and
adults
Y
Acute and chronic LOCs for listed and non-listed
species were exceeded for all uses for birds and
mammals and acute LOCs for listed and non-listed
terrestrial invertebrates were also exceeded for all
uses.
Alteration of chemical characteristics
necessary for normal growth and viability of
juvenile and adult CRLFs and their food
source.
Y
Acute and/or chronic LOCs for listed and non-listed
species were exceeded for freshwater fish and
invertebrates, birds, mammals and terrestrial
invertebrates. No data are available for either
aquatic or terrestrial plants.
Following a "may affect" determination, additional information is considered to refine
the potential for exposure at the predicted levels based on the life history characteristics
{i.e., habitat range, feeding preferences, etc.) of the CRLF. Based on the best available
information, the Agency uses the refined evaluation to distinguish those actions that
"may affect, but are not likely to adversely affect" from those actions that are "likely to
adversely affect" the CRLF and its designated critical habitat.
The criteria used to make determinations that the effects of an action are "not likely to
adversely affect" the CRLF and its designated critical habitat include the following:
96
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• Significance of Effect: Insignificant effects are those that cannot be meaningfully
measured, detected, or evaluated in the context of a level of effect where "take"
occurs for even a single individual. "Take" in this context means to harass or
harm, defined as the following:
¦ Harm includes significant habitat modification or degradation that
results in death or injury to listed species by significantly impairing
behavioral patterns such as breeding, feeding, or sheltering.
¦ Harass is defined as actions that create the likelihood of injury to listed
species to such an extent as to significantly disrupt normal behavior
patterns which include, but are not limited to, breeding, feeding, or
sheltering.
• Likelihood of the Effect Occurring: Discountable effects are those that are
extremely unlikely to occur.
• Adverse Nature of Effect: Effects that are wholly beneficial without any adverse
effects are not considered adverse.
A description of the risk and effects determination for each of the established assessment
endpoints for the CRLF and its designated critical habitat is provided in Sections 5.2.1
through 5.2.3,
5.2.1 Direct Effects
5.2.1.1 Aquatic-Phase CRLF
The aquatic-phase considers life stages of the frog that are obligatory aquatic organisms,
including eggs and larvae. It also considers submerged terrestrial-phase juveniles and
adults, which spend a portion of their time in water bodies that may receive runoff and
spray drift containing methidathion.
As stated in the risk estimation section, acute RQs for the aquatic-phase CRLF exceed the
listed species LOC (0.05) for all of the assessed methidathion uses. The acute RQs range
from 2.1 for the alfalfa scenario to 52.5 for the nursery scenario. In addition, the lowest
acute RQ following a single application is estimated to be 0.20 for the lowest mango
scenario, which still exceeds the acute LOC for listed species.
Limited acute toxicity data are available to assess sensitivity across freshwater fish
species. The lowest and highest acute 96-hour LC50S values from the submitted studies
on bluegill sunfish, rainbow trout and goldfish are 2.2 (bluegill sunfish) and 14 |ig/L
(rainbow trout). Estimated acute RQs from these studies indicate that they would exceed
the listed species acute LOC for all of the assessed methidathion uses for any one of these
three surrogate freshwater fish species. It is noted that one study on the common eel
(Anguilla anguilla) was available in the open literature with a 96-hr acute LC50 that is
close to three orders of magnitude higher (1510 |ig/L) than the most sensitive submitted
freshwater fish species. If acute RQs were estimated based on that study, they would
range from 0.003 for the alfalfa scenario to 0.08 for the nursery scenario, the only
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scenario that exceeds the acute LOC for listed species. Therefore, there will be some fish
species that may not be at risk following acute exposure to some of the methidathion use
scenarios, and the relative sensitivity of aquatic amphibians to fish is unknown.
Based on a probit dose-response slope of 2.9 (1.9 - 4.0) from the bluegill sunfish acute
toxicity study with an acute LC50 of 2.2 |ig/L, the probability of an individual effect at the
LOC is estimated to be 1 in 12,400 and the probability of an individual effect at the RQ is
close to 1 in 1 for all scenarios. Sublethal effects were reported in the bluegill sunfish
acute toxicity study at concentration levels higher than the endpoints utilized in this risk
assessment (MRID 00011841). In this study, it was reported that test organisms in the
highest exposure group (1000 |ig/L) exhibited slight spastic motions and swam on their
sides.
As stated previously, the chronic NOAEC of 6.3 |ig/L for the fathead minnow is similar
to the acute 96-hour LC50 of several other test species, ranging from 2.2 to 14 |ig/L. A
chronic NOAEC for any of these other species would likely be lower than the available
fathead minnow NOAEC. Therefore, this assessment estimated a chronic NOAEC for
rainbow trout using an acute to chronic ratio (ACR) calculation from data from other
organophosphates. The most conservative acute to chronic ratio from the other
organophosphate data is 140. Using that ACR value with an acute LC50 value of 10 ppb
for rainbow trout, the chronic NOAEC used in this assessment is estimated to be 0.07
ppb. With this value, the chronic RQs exceed the LOC (1.0) for all of the assessed
methidathion uses (Table 5.2). If the chronic NOAEC of 6.3 ppb from the fathead
minnow study is used, then the chronic RQs would exceed the LOC (1.0) for all of the
assessed methidathion uses except for the alfalfa scenario (RQ = 0.40: alfalfa, clover and
timothy); the grape scenario (RQ = 0.51: kiwi) and the tree fruit scenario (RQ = 0.79:
apple, apricot, cherry, nectarine, peach, pear, plum and prune). Therefore, even with the
endpoint from the chronic study in fathead minnows, the chronic LOC is still exceeded
with many methidathion uses.
Sublethal effects in the chronic study include effects on total length and wet weight at the
highest measured treatment level (12 |ig ai/L) relative to the pooled control.
Surface water monitoring data support the conclusion of risk determined based on
calculated RQs. Monitoring in the Sacramento Valley found 46 detections out of 472
samples with a maximum of 15.1 ug/L and an average across the detections of 1.2 ug/L.
In the San Joaquin Valley, the maximum of 30 detections out of 738 samples was 2.4
ug/L and the average was 0.3 ug/L. Atmospheric monitoring detected methidathion
concentrations in fog of 15.5 ug/L. Compared to 96-hr acute LC50 values of 2.2 ug/L to
14 ug/L, these data demonstrate the potential for aquatic exposures that could lead to
direct effects, particularly because monitoring is generally not expected to capture peak
values.
In 1976, approximately 3,000 fish of unknown species in a nearby creek were killed
following an aerial application of methidathion to an agricultural area in Sacramento
County, California. The incident was attributed to accidental misuse of methidathion.
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Samples of the water collected from the field drain six days after the material was applied
to the field showed methidathion concentrations of 60 ppb. The certainty index for this
incident is highly probable.
In summary, methidathion is very highly toxic to freshwater fish, the surrogate for the
aquatic-phase CRLF. The acute LOC (listed species) and the chronic LOC are exceeded
for all of the methidathion uses. There is one study in the open literature that indicates
that there will be some fish species that may not be at risk following acute exposure from
some of the methidathion use scenarios. This is an uncertainty. The probability of an
individual effect on an acute basis approaches 100% at the RQ levels. Incident data
indicate that freshwater fish are vulnerable, particularly if there are misuses. Spatial
analyses indicate that a significant overlap will exist between the use sites for
methidathion and the CRLF habitat, particularly when the spraydrift and downstream
dilution buffers are applied to the "footprint" of the methidathion use area. Therefore,
based on the weight-of-evidence, there is a potential for direct impact to the aquatic-
phase CRLF based on the endpoints generated from the freshwater fish data and the
effects determination is Likely to adversely affect (LAA).
As stated previously, the only detected degradate of toxicological concern is the minor
degradate methidathion oxon, which may be more toxic than the parent. However, no
data are available on the environmental fate, toxicity, and occurrence of methidathion
oxon in the environment and so it is not considered directly in calculated RQs. Given
that acute and chronic risk quotients are exceeded for the parent compound alone, any
contribution in toxicity from any of the major degradates or from methidathion oxon or
other OP degradates would increase the risk estimates. Further characterization of risk
from methidathion oxon is provided in Section 5.2.1.3,
5.2.1.2 Terrestrial-Phase CRLF
As stated in the risk estimation section, acute dose-based and acute dietary-based RQs for
the terrestrial-phase CRLF exceed the listed species LOC (0.1) for all of the assessed
methidathion uses. The acute RQs range from 0.17 for mangos on a dietary basis to 442.0
for nursery uses on a dose basis. Limited acute toxicity data are available to assess
sensitivity across avian species on a dose-basis. The acute oral LD50's range from 6.7
mg/kg for mallard ducks (Anasplatyrhynchos) (MRID 000159201) to 225 mg/kg for
Chukar (Alectoris chukar) (MRID 00060823). Other species fall within that range. Even
with the highest LD50 for the Chukar, the dose-based acute RQ for avian species is still
over 150 times higher than the acute LOC of 0.1 for listed terrestrial animals with the
nursery uses. With the lowest application rate of 0.25 lb a.i./A for mangos and the
highest acute LD50 value for the Chukar, the acute dose-based RQ for small birds eating
small insects (0.29) is still greater than the acute LOC of 0.1 for endangered birds. On a
dietary basis, there are only two studies with the technical material, one with bobwhite
quail and one with mallard ducks. The acute dietary LC50s are 224 and 543 ppm for
bobwhite quail and mallard ducks, respectively. Again, with the lowest application rate
of 0.25 lb a.i./A for mangos and using the bobwhite quail LC50 value of 224 ppm, the
acute RQ on a dietary basis for small birds eating small insects is 0.17, which exceeds the
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LOC for listed species. Using the mallard duck LC50 value of 543 ppm, the acute RQ on
a dietary basis for small birds eating small insects is 0.07, which is just under the LOC
for endangered species. However, with the next highest application rate of 0.5 lbs a.i./A
for safflower, even when applied only once per year, the RQ using the highest LC50 of
543 ppm for mallard ducks, the RQ would be 0.12, which is greater than the acute LOC
for endangered birds. Therefore, even within the range of dose- and dietary-based acute
toxicity values for avian species, there is potential acute risk to endangered birds for all
uses with the possible exception of mangos with some of the less sensitive birds.
Chronic RQs also exceed the chronic LOC for all of the assessed methidathion uses
(Table 5.7). The chronic endpoint used in this assessment (10 ppm) is based on a study
with mallard ducks (MRID 44381602). In bobwhite quail, no effects were observed in
two studies at the highest concentrations tested (30 and 35 ppm; MRIDs 44381601 and
44381602, respectively). Using the highest bobwhite quail NOAEC value of 35 ppm, the
chronic RQ with the lowest application rate (mangos) is 1.08, which is still greater than
the chronic LOC of 1. Therefore, within the available range of dietary-based chronic
toxicity values for avian species (two), there is potential risk following chronic exposure
for all of the assessed methidathion uses.
In an effort to refine the acute dose-based risk estimates, the T-REX model was modified
to account for the lower metabolic rate and lower caloric requirement of amphibians
(compared to birds). Acute dose-based RQs were recalculated using the T-HERPS
(Version 1.0) model for small (1 g), medium (37 g), and large (238 g) frogs (Table 5.15,
Appendix M). Using this refinement, the acute dose-based RQs still exceed the LOC
(0.1) for nearly all of the modeled scenarios for methidathion. The T-HERPS model can
only be used to refine dose-based risk estimates at this time; thus, further refinement of
the acute or chronic dietary-based RQs is not possible.
Table 5.15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients
Size
Class
(grams)
Adjusted
LD50
EECs and RQs
Broadleaf
Plants/
Small
Insects
Fruits/Pods/
Seeds/
Large Insects
Small
Herbivore
Mammals
Small
Insectivore
Mammal
Small
Amphibians
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
ARTICHOKE
1.4
6.70
6.81
1.02
0.76
0.11
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
6.69
1.00
0.74
0.11
194.24
28.99
12.14
1.81
0.23
0.03
238
6.70
4.39
0.65
0.49
0.07
30.20
4.51
1.89
0.28
0.15
0.02
CITRUS
1.4
6.70
26.46
3.95
2.94
0.44
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
26.00
3.88
2.89
0.43
754.62
112.63
47.16
7.04
0.90
0.13
238
6.70
17.04
2.54
1.89
0.28
117.31
17.51
7.33
1.09
0.59
0.09
CLOVER
1.4
6.70
7.76
1.16
0.86
0.13
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
7.63
1.14
0.85
0.13
221.32
33.03
13.83
2.06
0.26
0.04
100
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Table 5.15. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients
Size
Class
(grams)
Adjusted
LD50
EECs and RQs
Broadleaf
Plants/
Small
Insects
Fruits/Pods/
Seeds/
Large Insects
Small
Herbivore
Mammals
Small
Insectivore
Mammal
Small
Amphibians
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
238
6.70
5.00
0.75
0.56
0.08
34.41
5.14
2.15
0.32
0.17
0.03
COTTON
1.4
6.70
11.27
1.68
1.25
0.19
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
11.07
1.65
1.23
0.18
321.38
47.97
20.09
3.00
0.38
0.06
238
6.70
7.26
1.08
0.81
0.12
49.96
7.46
3.12
0.47
0.25
0.04
KIWI
1.4
6.70
10.49
1.57
1.17
0.17
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
10.31
1.54
1.15
0.17
299.20
44.66
18.70
2.79
0.36
0.05
238
6.70
6.76
1.01
0.75
0.11
46.51
6.94
2.91
0.43
0.23
0.04
MANGO
1.4
6.70
1.47
0.22
0.16
0.02
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
1.45
0.22
0.16
0.02
42.03
6.27
2.63
0.39
0.05
0.01
238
6.70
0.95
0.14
0.11
0.02
6.53
0.98
0.41
0.06
0.03
0.00
NURSERY
1.4
6.70
2.62
0.39
0.29
0.04
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
2.58
0.38
0.29
0.04
74.80
11.16
4.67
0.70
0.09
0.01
238
6.70
1.69
0.25
0.19
0.03
11.63
1.74
0.73
0.11
0.06
0.01
SAFFLOWER, SUNFLOWER
1.4
6.70
4.48
0.67
0.50
0.07
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
4.41
0.66
0.49
0.07
127.85
19.08
7.99
1.19
0.15
0.02
238
6.70
2.89
0.43
0.32
0.05
19.88
2.97
1.24
0.19
0.10
0.01
TREE FRUIT, OLIVES
1.4
6.70
15.73
2.35
1.75
0.26
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
15.46
2.31
1.72
0.26
448.79
66.98
28.05
4.19
0.54
0.08
238
6.70
10.14
1.51
1.13
0.17
69.77
10.41
4.36
0.65
0.35
0.05
WALNUT
1.4
6.70
25.88
3.86
2.88
0.43
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
25.44
3.80
2.83
0.42
738.28
110.19
46.14
6.89
0.88
0.13
238
6.70
16.67
2.49
1.85
0.28
114.77
17.13
7.17
1.07
0.58
0.09
aAcute RQs that exceed the acute endangered species LOC of 0.1 are in bold.
On a dose basis, based on a default probit dose-response slope of 4.5 (2 - 9) from the
mallard duck acute oral toxicity study with an acute LD50 of 6.7 mg/kg bw, the
probability of an individual effect at the RQ is approximately 1 in 1 for all scenarios. On
a dietary basis, the probit dose-response slope was 8.7 (3.5-13.8) from the bobwhite quail
dietary study with an LC50 of 224 ppm. The probability of an individual effect at the RQ
ranged from 1 in 1 for 5 use scenarios to 1 in 9.28E+10 for use on mangos (see Tables
5.5 and 5.6).
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Sublethal effects were reported in the mallard acute oral toxicity study (MRID
00159201). Toxic symptoms included depression, reduced reaction to external stimuli,
wing droop, convulsions, and salivation. In ducks, pheasants and/or partridges (MRID
00060823), toxic symptoms included goose-stepping, ataxia, dyspnea, lacrimation,
salivation, ataxia, wing-spread seizures, and terminal opisthotonos.
Symptoms appeared as soon as 20 minutes after dosage; recovery of
survivors took 1-2 days. The NOAELs and LOAELs for these sublethal effects were not
reported.
There are no other acute or chronic toxicity data with birds that have lower endpoints
than the studies used in this assessment.
Four incidents were reported in which red-tailed hawks were found either dead or injured
after potential exposure to methidathion. One incident was classified as possible, two as
probable and the fourth as highly probable. The incidents were all reported in 1994 in
several counties in California (Colusa, Stanislaus and Merced). In one case, blood
plasma cholinesterase and acetlycholinesterase levels were within the normal range;
however, the results of the footwash showed detections of methidathion at 2.7 ppb (other
organophosphates, including chlorpyrifos and diazinon were also detected in the
footwash sample at respective concentrations of 0.2 ppb and 4.1 ppb). In a second case,
the hawk recovered. Plasma cholinesterase and acetylcholinesterase levels were found
significantly below the normal range. Footwash analysis results show that methidathion
was detected at a concentration of 2.7 ppb. Again, chlorpyrifos and diazinon were also
detected in the footwash sample at respective concentrations of 0.7 ppb and 0.4 ppb. In
the third case where the hawk died, the plasma cholinesterase level was low and the brain
cholinesterase level was severely depressed. Methidathion was detected at a
concentration of 0.7 ppm in the footskin. Chloropyrifos and diazinon were detected in
both the feathers and footskin at concentrations ranging from 0.02 to 0.08 ppb. It is
likely that all three organophosphates contributed to the death of this bird.
In summary, the acute dose-based, acute dietary-based and/or chronic dietary-based RQs
for the terrestrial-phase CRLF, using avian data as a surrogate, exceed the acute listed
species LOC (0.1) and/or the chronic non-listed species LOC (1) for all of the assessed
methidathion uses. In addition, the probability of an individual effect on an acute basis is
high at the RQ levels. Incident data indicate that birds are vulnerable. Spatial analyses
indicate that a significant overlap will exist between the use sites for methidathion and
the CRLF habitat, particularly when the spraydrift buffers are applied to the "footprint"
of the methidathion use area. Therefore, based on the weight-of-evidence, there is a
potential for direct impact to the terrestrial-phase CRLF using the endpoints generated
from the avian data and the effects determination is LAA.
As stated previously, the only detected degradate of toxicological concern is the minor
degradate methidathion oxon, which may be more toxic than the parent. However, no
data are available on the environmental fate, toxicity, and occurrence of methidathion
oxon in the environment and so it is not considered directly in calculated RQs. Given
that acute and chronic risk quotients are exceeded for the parent compound alone, any
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contribution in toxicity from any of the major degradates or from methidathion oxon or
other OP degradates would increase the risk estimates. Further characterization of risk
from methidathion oxon is provided in Section 5.2.1.3,
5.2.1.3 Direct Effects from Methidathion Oxon to Aquatic-Phase and
Terrestrial-Phase CRLF
As described in Section 2.1.4, methidathion oxon formation in soil was demonstrated in
soil photolysis, aerobic soil metabolism, and terrestrial field dissipation studies, and
formation on foliage was demonstrated in dislodgeable residue studies. Long range
transport has also been shown to occur, with methidathion oxon detected in air and on
pine needles at more than 20 km from the nearest use site. Although formed at less than
10%, methidation oxon is a degradate of concern based on comparison with other OP
pesticides (e.g., diazinon, chlorpyrifos), for which oxon degradates have been shown in
many cases to be of equal or greater toxicity than the parent. This analysis does not
include consideration of other oxon degradates which, based on the degradation pathway
for methidathion, may have been formed but were not detected in fate studies due to the
lack of appropriate labeling.
No environmental fate or ecological effects data are available for methidathion oxon.
Therefore, it is difficult to quantify the increased risk which may result from potential
exposure to this degradate; however, based on conservative assumptions, some broad
conclusions can be reached. In laboratory studies, transformation to the oxon occurred at
maximum amounts under soil photolysis conditions, where 5% of the applied parent was
present as methidathion oxon at study termination. For quantitative modeling purposes,
then, the assumption can be made that methidathion oxon is initially present on soil and
on foliage at 5% of the initially applied parent compound.
For both aquatic modeling using PRZM/EXAMS and terrestrial modeling using T-REX,
EECs are linear with the application rate. Therefore, based on an application rate of 5%
of the applied methidathion for the methidathion oxon, and assuming that its fate
properties are similar to the parent, aquatic and terrestrial EECs for the oxon alone would
be approximately 5% of those estimated for the parent7. If the oxon were of equal
toxicity to the parent, then the acute aquatic and terrestrial RQs for the oxon alone would
be 5% of those calculated for the parent. For acute aquatic risk, the acute RQs would
range from 0.01 for the use on mangos to 1.12 for almonds and 2.62 for the nursery use,
as shown in Table 5.16. For terrestrial risk, acute dose-based RQs would range from
0.62 for mangos to 11.1 for citrus and 22.1 for the nursery use, as shown in Table 5.17.
Therefore, at equal toxicity to the parent, risk from the oxon degradate alone would
exceed the endangered species LOCs for acute risk to both the aquatic phase CRLF (LOC
= 0.05) and the terrestrial phase CRLF (LOC = 0.1) for all except the lowest application
rates.
7 Modeled aquatic EECs for the oxon would not be exactly 5% because spray drift would not be a factor
and so model inputs would change to 100% application efficiency and 0% drift, rather than 95% and 2%,
respectively. This, along with a correction for molecular weight, would be likely to change the percentage
slightly but not enough to affect conclusions.
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However, oxons can be considerably more toxic than the parent and risk from
methidathion oxon alone could be even greater than that from the parent. In the
cumulative assessment for OP insecticides8, if there were no toxicity data available for a
given OP-oxon, high-end adjustment factors of 10X to 100X were applied to account for
the presumed increased toxicity of the oxon relative to the parent chemical. To explore
the potential risk if methidathion oxon were more toxic than the parent, RQs based on
these factors are reported as well in Tables 5.16 and 5.17, for direct effects to the aquatic
and terrestrial phase CRLF, respectively.
Table 5.16 Summary of Direct Effect Acute RQs for the Aquatic-phase CRLF,
based on exposure to methidathion oxon degradate
Use
Parent Onlya
Oxon Only (no parent) b
Equal Toxicity
10X Toxicity
100X Toxicity
Mango
0.20
0.01
0.1
1.0
Alfalfa
2.1
0.10
1.0
10
Almond
22.4
1.12
11.2
112
Nursery
52.5
2.62
26.2
262
a Previously calculated in Table 5.1 using bluegill sunfish (Lepomis macrochirus) acute 96-hour LC50 = 2.2 (ig/L
b Assumes EECs to be 5% of those for the parent, based on the maximum percent formation observed in laboratory
studies.
Table 5.17 Summary of Di
Terrestrial-phase CRLF, baset
rect Effect Acute Dose-based RQs for the
on exposure to methidathion oxon degradate
Use
Parent Onlya
Oxon Only (no parent) b
Equal Toxicity
10X Toxicity
100X Toxicity
Mango
12.4
0.62
6.2
60
Clover
65.4
3.3
33
330
Citrus
222.9
11.1
111
1110
Nursery
442.0
22.1
221
2210
a Previously calculated in Table 5.5 based on mallard duck acute oral LD50 = 6.7 mg/kg.
b Assumes EECs to be 5% of those for the parent, based on the maximum percent formation observed in laboratory
studies.
There is a great deal of uncertainty in these estimates because of the assumptions on
which they are based, including those for formation (e.g., fate properties) and toxicity of
the methidathion oxon. For the aquatic-phase CRLF, exposure estimates are based on the
assumptions that all of the applied methidathion is available on the soil surface for
photolysis and that all of the transformation product is available for runoff at one time.
For the terrestrial-phase CRLF, exposure estimates assume that laboratory soil photolysis
rates are also reflective of the photolysis rates that may occur on foliage. Additionally,
the fate properties of methidathion oxon are unknown, and assuming the same fate
behavior of the parent may lead to overestimation because OP oxons tend to be more
g
More detail on the OP cumulative assessment and the characterization of additional risk due to oxon
occurrence may be found at http://www.epa.gov/pesticides/cmnulative/2006-op/op era appendices part2.pdf
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transient than the parent. Methidathion oxon has not been detected in surface water
monitoring done in two studies, even in samples in which the parent methidathion was
detected. Despite the uncertainty, the potential for acute direct effects to aquatic-phase
and terrestrial-phase CRLF from exposure to methidathion oxon alone cannot be
precluded, even if the toxicity is no greater than that of the parent. Exposure from the
parent methidathion already leads to conclusions of risk; thus, consideration of the oxon
degradate as well increases the likelihood of direct effects.
5.2.2 Indirect Effects (via Reductions in Prey Base)
5.2.2.1 Algae (non-vascular plants)
As discussed in Section 2.5.3, the diet of CRLF tadpoles is composed primarily of
unicellular aquatic plants (i.e., algae and diatoms) and detritus. Due to a lack of aquatic
plant toxicity data indirect effects to the CRLF via direct toxicity to aquatic plants cannot
be quantitatively estimated for methidathion. There are no incident data for aquatic
plants and no relevant data in the open literature. An examination of the completed
CRLF assessments for nine other organophosphates indicates that nearly all of the effects
determinations for aquatic non-vascular plants were either "no effect" or "not likely to
adversely affect". In addition, the mode of action for methidathion as an
organophosphate insecticide is by disrupting nervous system function of exposed animals
via acetylcholinesterase inhibition. Unless there is a separate herbicidal mode of action
as with bensulide, a pre-emergent organophosphate herbicide which inhibits cell division
in root tips and inhibits seedling growth by conjugation of acetyl co-enzyme A,
this mode of action on animals is not expected to affect plants. Methidathion also has a
history of being applied to a myriad of agricultural crops (as per the label), with no
known incidents of adverse phytotoxic effects to aquatic plants. Therefore, based on the
weight-of-evidence, the potential indirect impact to the CRLF is expected to be minimal.
The effects determination is may affect, not likely to adversely affect (NLAA).
5.2.2.2 Aquatic Invertebrates
The potential for methidathion to elicit indirect effects to the CRLF via effects on
freshwater invertebrate food items is dependent on several factors including: (1) the
potential magnitude of effect on freshwater invertebrate individuals and populations; and
(2) the number of prey species potentially affected relative to the expected number of
species needed to maintain the dietary needs of the CRLF. Together, these data provide a
basis to evaluate whether the number of individuals within a prey species is likely to be
reduced such that it may indirectly affect the CRLF.
The acute RQs for aquatic invertebrates range from 1.5 to 38.5, thus exceeding the acute
risk LOC for non-listed species for all of the assessed uses (LOC = 0.5; see Table 5.3).
Chronic RQs for freshwater invertebrates range from 8.9 tol45.0, thus exceeding the
LOC (1.0) for all of the assessed methidathion uses (see Table 5.4). Only one other
acute toxicity study with the same species {daphnia magna, MRID 00011350) provides
any other data relevant to this assessment, either from submitted studies or from the open
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literature. With the endpoint from that study, the lowest acute RQ would be 1.0, which
still exceeds the acute LOC for non-listed species. At the lowest application rate with the
default slope of 4.5 and the lowest RQ of 1.5, the probability of an individual effect is
approximately 1 in 1.3 and the percentage effect to the aquatic invertebrate prey base is
79% (e.g., the percentage of the aquatic invertebrate population that is expected to be
affected following exposure to methidathion). Based on the weight-of-evidence, there is
a potential indirect impact to the CRLF via effects on freshwater invertebrate food items.
The effects determination is LAA.
5.2.2.3 Fish and Aquatic-phase Frogs
Methidathion is very highly toxic to freshwater fish, the surrogate for the aquatic-phase
CRLF. The acute LOC for non-listed species (all uses) and the chronic LOC (all uses)
are exceeded; however, one study in the open literature indicates that there will be some
fish species that may not be at risk following acute exposure from some of the
methidathion use scenarios. At the lowest application rate, the probability of an
individual effect on an acute basis at the RQ is approximately 1 in 1 and the percentage
effect to the fish and aquatic-phase frog prey base is 93.4%. In addition, the incident data
indicate that freshwater fish are vulnerable. Therefore, based on the weight-of-evidence,
there is a potential indirect impact to the aquatic-phase CRLF based on the endpoints
generated from the freshwater fish data. The effects determination is LAA.
5.2.2.4 Terrestrial Invertebrates
When the terrestrial-phase CRLF reaches juvenile and adult stages, its diet is mainly
composed of terrestrial invertebrates. The RQs for both small and large insects exceed
the acute LOC for non-listed species for all methidathion uses, even with only a single
use. The acute RQs for small insects range from 21 (mangos) to 734 (nursery) and the
acute RQs for large insects range from 2.3 (mangos) to 82 (nursery). Following a single
use, the lowest RQ is 2.0 for large insects (mangos). There are insufficient studies to
conduct a sensitivity analysis; however, the highest terrestrial invertebrate endpoint is
from the open literature with an acute toxicity value of 0.416 |ig/bee (ECOTOX reference
70351). The lowest RQ, using the endpoint from this study is 1.1 (mangos), which still
exceeds the acute list species LOC of 0.05. The probability of an individual effect at the
LOC of 0.5 is 1 in 4.41E+31 and the probability of an individual effect at the RQ for all
uses is 1 in 1. In addition, the percentage effect to the terrestrial invertebrate prey base is
99.9 - 100%.
Two incidents were reported following application of methidathion in California. The
first involved spraying of plum trees near some pollination hives. Dead bees (number not
specified) were found along with residues of methidathion and diazinon. This incident
was rated as possible. The second incident reported that an unknown number of bees
were killed in an unknown county. The incident was attributed to accidental misuse. No
residue analysis was completed; however, the certainty index for this incident is probable.
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Based on the weight-of-evidence, there is a potential indirect impact to the CRLF based
on this endpoint. The effects determination is LAA.
5.2.2.5 Mammals
Life history data for terrestrial-phase CRLFs indicate that large adult frogs consume
terrestrial vertebrates, including mice. The acute and chronic RQs exceed the listed
species acute LOC and the chronic LOC for all uses. The acute RQs range from 2.4
(mangos) to 86.8 (nursery). The chronic dietary RQs range from 117 (mangos) to 4164.5
(nursery, dose-based) and from 13.5 (mangos) to 480.0 (nursery, dietary-based). There is
one study from the open literature conducted with the mouse (mus musculus) that appears
to have a lower endpoint (E85173); however, the study is not available for review at this
time. The acute LD50 for this study is 10.5 mg a.i./kg. Using the endpoint from this
study and assuming that a mouse weighs 20 g, the lowest RQ (mangos) would be 5.70,
which exceeds the non-listed species LOC by 10 times. No other relevant studies are
available in the open literature. At the lowest application rate (0.25 lb a.i./A for mangos),
with the default slope of 4.5 and the lowest RQ of 2.4, the probability of an individual
effect is approximately 1 in 1 and the percentage effect to the mammalian prey base is
95.6%. Based on the weight-of-evidence, the labeled uses for methidathion may
indirectly impact the CRLF through effects to the mammalian prey base. The effects
determination is LAA.
5.2.2.6 Terrestrial-phase Amphibians
Terrestrial-phase adult CRLFs also consume frogs. RQ values representing direct
exposures of methidathion to terrestrial-phase CRLFs are used to represent exposures of
methidathion to frogs in terrestrial habitats. With the exception of mangos on an acute
dietary basis, the acute dose-based, acute dietary-based and/or chronic dietary-based RQs
for terrestrial-phase amphibians, using avian data as a surrogate, exceed the acute non-
listed species LOC and the chronic LOC for all of the assessed methidathion uses.
Refining the RQ values with T-HERPS, the acute dose-based RQs still exceed the non-
listed LOC (0.5) for at least one food category for all of the modeled scenarios for
methidathion.
Table 5.18. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients
Size
Class
(grams)
Adjusted
LD50
EECs and RQs
Broadleaf
Plants/
Small
Insects
Fruits/Pods/
Seeds/
Large Insects
Small
Herbivore
Mammals
Small
Insectivore
Mammal
Small
Amphibians
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
ARTICHOKE
1.4
6.70
6.81
1.02
0.76
0.11
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
6.69
1.00
0.74
0.11
194.24
28.99
12.14
1.81
0.23
0.03
238
6.70
4.39
0.65
0.49
0.07
30.20
4.51
1.89
0.28
0.15
0.02
CITRUS
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Table 5.18. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk Quotients
Size
Class
(grams)
Adjusted
LD50
EECs and RQs
Broadleaf
Plants/
Small
Insects
Fruits/Pods/
Seeds/
Large Insects
Small
Herbivore
Mammals
Small
Insectivore
Mammal
Small
Amphibians
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
EEC
RQ1
1.4
6.70
26.46
3.95
2.94
0.44
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
26.00
3.88
2.89
0.43
754.62
112.63
47.16
7.04
0.90
0.13
238
6.70
17.04
2.54
1.89
0.28
117.31
17.51
7.33
1.09
0.59
0.09
CLOVER
1.4
6.70
7.76
1.16
0.86
0.13
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
7.63
1.14
0.85
0.13
221.32
33.03
13.83
2.06
0.26
0.04
238
6.70
5.00
0.75
0.56
0.08
34.41
5.14
2.15
0.32
0.17
0.03
COTTON
1.4
6.70
11.27
1.68
1.25
0.19
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
11.07
1.65
1.23
0.18
321.38
47.97
20.09
3.00
0.38
0.06
238
6.70
7.26
1.08
0.81
0.12
49.96
7.46
3.12
0.47
0.25
0.04
KIWI
1.4
6.70
10.49
1.57
1.17
0.17
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
10.31
1.54
1.15
0.17
299.20
44.66
18.70
2.79
0.36
0.05
238
6.70
6.76
1.01
0.75
0.11
46.51
6.94
2.91
0.43
0.23
0.04
MANGO
1.4
6.70
1.47
0.22
0.16
0.02
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
1.45
0.22
0.16
0.02
42.03
6.27
2.63
0.39
0.05
0.01
238
6.70
0.95
0.14
0.11
0.02
6.53
0.98
0.41
0.06
0.03
0.00
NURSERY
1.4
6.70
2.62
0.39
0.29
0.04
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
2.58
0.38
0.29
0.04
74.80
11.16
4.67
0.70
0.09
0.01
238
6.70
1.69
0.25
0.19
0.03
11.63
1.74
0.73
0.11
0.06
0.01
SAFFLOWER, SUNFLOWER
1.4
6.70
4.48
0.67
0.50
0.07
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
4.41
0.66
0.49
0.07
127.85
19.08
7.99
1.19
0.15
0.02
238
6.70
2.89
0.43
0.32
0.05
19.88
2.97
1.24
0.19
0.10
0.01
TREE FRUIT, OLIVES
1.4
6.70
15.73
2.35
1.75
0.26
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
15.46
2.31
1.72
0.26
448.79
66.98
28.05
4.19
0.54
0.08
238
6.70
10.14
1.51
1.13
0.17
69.77
10.41
4.36
0.65
0.35
0.05
WALNUT
1.4
6.70
25.88
3.86
2.88
0.43
N/A
N/A
N/A
N/A
N/A
N/A
37
6.70
25.44
3.80
2.83
0.42
738.28
110.19
46.14
6.89
0.88
0.13
238
6.70
16.67
2.49
1.85
0.28
114.77
17.13
7.17
1.07
0.58
0.09
aAcute RQs that exceed the acute non-listed species LOC of 0.5 are in bold.
In addition, on an acute dose-basis, the probability of an individual effect is 1 in 1 at the
lowest RQ level and the percentage effect to the avian/terrestrial-phase amphibian prey
base is 100%. Incident data indicate that birds are vulnerable. Therefore, based on the
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weight-of-evidence, there is a potential indirect impact to the terrestrial-phase CRLF
based on these endpoints. The effects determination is LAA.
5.2.3 Indirect Effects (via Habitat Effects)
5.2.3.1 Aquatic Plants (Vascular and Non-vascular)
Aquatic plants serve several important functions in aquatic ecosystems. Non-vascular
aquatic plants are primary producers and provide the autochthonous energy base for
aquatic ecosystems. Vascular plants provide structure as attachment sites and refugia for
many aquatic invertebrates, fish, and juvenile organisms, such as fish and frogs. In
addition, vascular plants also provide primary productivity and oxygen to the aquatic
ecosystem. Rooted plants help reduce sediment loading and provide stability to
nearshore areas and lower streambanks. In addition, vascular aquatic plants are important
as attachment sites for egg masses of CRLFs.
Potential indirect effects to the CRLF based on impacts to habitat and/or primary
production would normally be assessed using RQs from freshwater aquatic vascular and
non-vascular plant data; however, there are no aquatic plant data for methidathion and
quantitative risk estimations cannot be conducted. There are no incident data for aquatic
plants and no relevant data in the open literature. An examination of the completed
CRLF assessments for nine other organophosphates indicates that nearly all of the effects
determinations for aquatic vascular and non-vascular plants were either no effect or not
likely to adversely affect. Not all of the other CRLF organophosphate chemicals have
aquatic vascular plant data. In those cases, determinations were based on the non-
vascular plant data and any available terrestrial plant data. As stated previously, the
mechanism of action for methidathion as an organophosphate insecticide is by disrupting
nervous system function of exposed animals via acetylcholinesterase inhibition, which is
not a mode of action that is expected to affect plants. Methidathion also has a history of
being applied to a myriad of agricultural crops (as per the label), with no known incidents
of adverse phytotoxic effects to aquatic plants. Therefore, based on the weight-of-
evidence, the potential indirect impact to the CRLF is minimal. The effects
determination is may affect, NLAA.
5.2.3.2 Terrestrial Plants
Terrestrial plants serve several important habitat-related functions for the CRLF. In
addition to providing habitat and cover for invertebrate and vertebrate prey items of the
CRLF, terrestrial vegetation also provides shelter for the CRLF and cover from predators
while foraging. Terrestrial plants also provide energy to the terrestrial ecosystem through
primary production. Upland vegetation including grassland and woodlands provides
cover during dispersal. Riparian vegetation helps to maintain the integrity of aquatic
systems by providing bank and thermal stability, serving as a buffer to filter out sediment,
nutrients, and contaminants before they reach the watershed, and serving as an energy
source.
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Again, as with aquatic plants, there are no registrant-submitted terrestrial plant toxicity
data for methidathion for assessment of the potential for indirect effects to the aquatic-
and terrestrial-phase CRLF via effects to riparian vegetation or effects to the primary
constituent elements (PCEs) relevant to the aquatic- and terrestrial-phase CRLF.
However, there is limited evidence in the open literature that methidathion has the
potential to elicit phototoxic effects (Ecotox ref. 64451). This paper reported that
methidathion significantly affected corn photosynthesis when applied at 0.5 lbs a.i./A, a
rate that is lower than many of the currently registered rates. In addition, potential
phytotoxic effects have been highlighted on methidathion labels (i.e., spotting, reddening,
or chlorosis of the leaves) in certain sorghum varieties. An examination of the completed
CRLF assessments for nine other organophosphates indicates that the majority of the
effects determinations for terrestrial plants were either "no effect" or "not likely to
adversely affect". Two were determined to be "LAA"; however, one had herbicidal
activity from a known mechanism and the other used surrogate data from another
pesticide. For some of these organophosphates, as with the study mentioned above, there
is the potential for some damage to plants. Nevertheless, the conclusions in those cases
were generally that while effects to terrestrial plants may affect the CRLF via habitat
modification, they are not likely to adversely affect the CRLF based on the type and
extent of damage as observed. The mode of action for methidathion as an
organophosphate insecticide is not one known to affect plants. Therefore, based on the
weight-of-evidence, methidathion is not likely to impact plants to an extent that is
expected to adversely affect the CRLF at the labeled application rates. Although some
phytotoxicity could occur, these effects are considered to be insignificant in the context
of a "take". The effects determination is may affect, NLAA.
5.2.4 Modification to Designated Critical Habitat
The risk conclusions for the designated critical habitat are based on conclusions described
for indirect effects previously described. Potential habitat modification is described
below.
5.2.4.1 Aquatic-Phase PCEs
Three of the four assessment endpoints for the aquatic-phase primary constituent
elements (PCEs) of designated critical habitat for the CRLF are related to potential
effects to aquatic and/or terrestrial plants:
• Alteration of channel/pond morphology or geometry and/or increase in sediment
deposition within the stream channel or pond: aquatic habitat (including riparian
vegetation) provides for shelter, foraging, predator avoidance, and aquatic
dispersal for juvenile and adult CRLFs.
• Alteration in water chemistry/quality including temperature, turbidity, and
oxygen content necessary for normal growth and viability of juvenile and adult
CRLFs and their food source.
• Reduction and/or modification of aquatic-based food sources for pre-metamorphs
(e.g., algae).
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Conclusions for potential indirect effects to the CRLF via direct effects to aquatic and
terrestrial plants are used to determine whether modification to critical habitat may occur.
The potential for habitat medication via impacts to aquatic plants (Sections 5.2.2.1 and
5.2.3.1) and terrestrial plants (5.2.3.2) is not considered to be significant. No habitat
modification is expected.
The remaining aquatic-phase PCE is "alteration of other chemical characteristics
necessary for normal growth and viability of CRLFs and their food source." Other than
impacts to algae as food items for tadpoles (discussed above), this PCE is assessed by
considering direct and indirect effects to the aquatic-phase CRLF via acute and chronic
freshwater fish and invertebrate toxicity endpoints as measures of effects. Based on the
potential direct impact to the aquatic-phase CRLF (Section 5.2.1.1) and impacts to
freshwater invertebrates and fish as food items (Sections 5.2.2.2 and 5.2.2.3), there is a
potential for habitat modification.
5.2.4.2 Terrestrial-Phase PCEs
Two of the four assessment endpoints for the terrestrial-phase PCEs of designated critical
habitat for the CRLF are related to potential effects to terrestrial plants:
• Elimination and/or disturbance of upland habitat; ability of habitat to support food
source of CRLFs: Upland areas within 200 ft of the edge of the riparian
vegetation or drip line surrounding aquatic and riparian habitat that are comprised
of grasslands, woodlands, and/or wetland/riparian plant species that provides the
CRLF shelter, forage, and predator avoidance.
• Elimination and/or disturbance of dispersal habitat: Upland or riparian dispersal
habitat within designated units and between occupied locations within 0.7 mi of
each other that allow for movement between sites including both natural and
altered sites which do not contain barriers to dispersal.
No habitat modification is expected through impacts to terrestrial plants (5.2.3.2).
The third terrestrial-phase PCE is "reduction and/or modification of food sources for
terrestrial phase juveniles and adults." To assess the impact of methidathion on this PCE,
acute and chronic toxicity endpoints for terrestrial invertebrates, mammals, and
terrestrial-phase frogs are used as measures of effects. There is a potential for habitat
modification based on potential reductions in prey base (Section 5.2.2.4 for terrestrial
invertebrates, Section 5.2.2.5 for mammals, and 5.2.2.6 for frogs).
The fourth terrestrial-phase PCE is based on alteration of chemical characteristics
necessary for normal growth and viability of juvenile and adult CRLFs and their food
source. There is a potential for habitat modification based on potential direct (Section
5.2.1.2) and indirect effects (Sections 5.2.2.4, 5.2.2.5, and 5.2.2.6) to terrestrial-phase
CRLFs.
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6.
Uncertainties
6.1 Exposure Assessment Uncertainties
6.1.1 Maximum Use Scenario
The screening-level risk assessment focuses on characterizing potential ecological risks
resulting from a maximum use scenario, which is determined from labeled statements of
maximum application rate and number of applications with the shortest time interval
between applications. The frequency at which actual uses approach this maximum use
scenario may be dependant on pest resistance, timing of applications, cultural practices,
and market forces. In particular, the application rate assumed for use on nursery stock is
conservative. The label does not provide explicit application rates for this use and so it is
assessed based on an assumption of application at the general maximum application rate
required by the labels.
6.1.2 Aquatic Exposure Modeling of Methidathion
The standard ecological water body scenario (EXAMS pond) used to calculate potential
aquatic exposure to pesticides is intended to represent conservative estimates, and to
avoid underestimations of the actual exposure. The standard scenario consists of
application to a 10-hectare field bordering a 1-hectare, 2-meter deep (20,000 m3) pond
with no outlet. Exposure estimates generated using the EXAMS pond are intended to
represent a wide variety of vulnerable water bodies that occur at the top of watersheds
including prairie pot holes, playa lakes, wetlands, vernal pools, man-made and natural
ponds, and intermittent and lower order streams. As a group, there are factors that make
these water bodies more or less vulnerable than the EXAMS pond. Static water bodies
that have larger ratios of pesticide-treated drainage area to water body volume would be
expected to have higher peak EECs than the EXAMS pond. These water bodies will be
either smaller in size or have larger drainage areas. Smaller water bodies have limited
storage capacity and thus may overflow and carry pesticide in the discharge, whereas the
EXAMS pond has no discharge. As watershed size increases beyond 10-hectares, it
becomes increasingly unlikely that the entire watershed is planted with a single crop that
is all treated simultaneously with the pesticide. Headwater streams can also have peak
concentrations higher than the EXAMS pond, but they likely persist for only short
periods of time and are then carried and dissipated downstream.
The Agency acknowledges that there are some unique aquatic habitats that are not
accurately captured by this modeling scenario and modeling results may, therefore,
under- or over-estimate exposure, depending on a number of variables. For example,
aquatic-phase CRLFs may inhabit water bodies of different size and depth and/or are
located adjacent to larger or smaller drainage areas than the EXAMS pond. The Agency
does not currently have sufficient information regarding the hydrology of these aquatic
habitats to develop a specific alternate scenario for the CRLF. CRLFs prefer habitat with
perennial (present year-round) or near-perennial water and do not frequently inhabit
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vernal (temporary) pools because conditions in these habitats are generally not suitable
(Hayes and Jennings 1988). Therefore, the EXAMS pond is assumed to be representative
of exposure to aquatic-phase CRLFs. In addition, the Services agree that the existing
EXAMS pond represents the best currently available approach for estimating aquatic
exposure to pesticides (USFWS/NMFS 2004).
In general, the linked PRZM/EXAMS model produces estimated aquatic concentrations
that are expected to be exceeded once within a ten-year period. The Pesticide Root Zone
Model is a process or "simulation" model that calculates what happens to a pesticide in
an agricultural field on a day-to-day basis. It considers factors such as rainfall and plant
transpiration of water, as well as how and when the pesticide is applied. It has two major
components: hydrology and chemical transport. Water movement is simulated by the use
of generalized soil parameters, including field capacity, wilting point, and saturation
water content. The chemical transport component can simulate pesticide application on
the soil or on the plant foliage. Dissolved, adsorbed, and vapor-phase concentrations in
the soil are estimated by simultaneously considering the processes of pesticide uptake by
plants, surface runoff, erosion, decay, volatilization, foliar wash-off, advection,
dispersion, and retardation.
Uncertainties associated with each of these individual components add to the overall
uncertainty of the modeled concentrations. Additionally, model inputs from the
environmental fate degradation studies are chosen to represent the upper confidence
bound on the mean values that are not expected to be exceeded in the environment
approximately 90 percent of the time. Mobility input values are chosen to be
representative of conditions in the environment. The natural variation in soils adds to the
uncertainty of modeled values. Factors such as application date, crop emergence date,
and canopy cover can also affect estimated concentrations, adding to the uncertainty of
modeled values. Factors within the ambient environment such as soil temperatures,
sunlight intensity, antecedent soil moisture, and surface water temperatures can cause
actual aquatic concentrations to differ for the modeled values.
Unlike spray drift, tools are currently not available to evaluate the effectiveness of buffers
on runoff and loadings. The effect of buffers on runoff is highly dependent on the
condition of the vegetative strip. For example, a well-established, healthy vegetative
setback can be a very effective means of reducing runoff and erosion from agricultural
fields. Alternatively, a setback of poor vegetative quality or a setback that is channelized
can be ineffective at reducing loadings. Until such time as a quantitative method to
estimate the effect of vegetative setbacks on various conditions on pesticide loadings
becomes available, the aquatic exposure predictions are likely to overestimate exposure
where healthy vegetative setbacks exist and underestimate exposure where poorly
developed, channelized, or bare setbacks exist.
In order to account for uncertainties associated with modeling, available monitoring data
were compared to PRZM/EXAMS estimates of peak EECs. The surface water dataset for
methidathion has several thousand samples collected from 135 sites primarily in
agricultural areas, and includes several monitoring studies targeted to water bodies
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receiving runoff from high OP use areas and with a high frequency of sampling, factors
which reduce the uncertainty inherent in monitoring data. This monitoring detected
methidathion in surface waters at concentrations of up to 15.1 ug/L. Peak modeling
EECs range from 7.3 ug/L for alfalfa to 52.4 ug/L for the almond use. (The maximum
modeled EEC is 114.7 ug/L for the use on nursery stock, but this result is based on
conservative assumptions about application rates.) The highest monitoring detection of
15.1 ug/L is within this range, below the peak EECs for some uses but higher than EECs
for others. Although the use in the location with the highest detection is unknown, the
majority of the sampling was conducted in areas with high acreage of fruit and nut
orchards, which had modeled EECs of 17.8 ug/L for fruit trees, 20.0 ug/L for olives, and
52.4 ug/L, suggesting that the detection is not in excess of relevant modeled EECs.
Additionally, the next highest monitored detection from this relatively large dataset is 2.4
ug/L, suggesting that the modeled EECs are reasonably conservative representations of 1-
in-10 year concentrations.
6.1.3 Potential Groundwater Contributions to Surface Water
Chemical Concentrations
Although the potential impact of discharging ground water on CRLF populations is not
explicitly delineated, it should be noted that ground water could provide a source of
pesticide to surface water bodies - especially low-order streams, headwaters, and ground
water-fed pools. Terrestrial field dissipation studies did not find leaching of
methidathion below 12 inches, but given the fact that methidathion is soluble and
classified as moderatley mobile, and that its abiotic and biotic degradation in anaerobic
aquatic environments may be slow, the possibility of methidathion reaching ground water
cannot be precluded. Much of available ground water will eventually be discharged to
the surface - often supporting stream flow in the absence of rainfall. Continuously
flowing low-order streams in particular are sustained by ground water discharge, which
can constitute 100% of stream flow during baseflow (no runoff) conditions. Thus, it is
important to keep in mind that pesticides in groundwater may have a major (detrimental)
impact on surface water quality, and on CRLF habitats.
Concentrations in a receiving water body resulting from groundwater discharge cannot be
explicitly quantified, but it can be assumed that significant attenuation and retardation of
the chemical will have occurred prior to discharge. Nevertheless, groundwater could still
be a significant consistent source of chronic background concentrations in surface water,
and may also add to surface runoff during storm events (as a result of enhanced
groundwater discharge typically characterized by the 'tailing limb' of a storm
hydrograph). In this assessment, long-term chronic concentrations derived from the
PRZM-EXAMS model are assumed to reflect background concentrations that might be
found in discharged groundwater/stream baseflow, but this factor remains an uncertainty.
6.1.4 Action Area Uncertainties
An example of an important simplifying assumption that may require future refinement is
the assumption of uniform runoff characteristics throughout a landscape. It is well
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documented that runoff characteristics are highly non-uniform and anisotropic, and
become increasingly so as the area under consideration becomes larger. The assumption
made for estimating the aquatic action area (based on predicted in-stream dilution) was
that the entire landscape exhibited runoff properties identical to those commonly found in
agricultural lands in this region. However, considering the vastly different runoff
characteristics of: a) undeveloped (especially forested) areas, which exhibit the least
amount of surface runoff but the greatest amount of groundwater recharge; b)
suburban/residential areas, which are dominated by the relationship between
impermeable surfaces (roads, lots) and grassed/other areas (lawns) plus local drainage
management; c) urban areas, that are dominated by managed storm drainage and
impermeable surfaces; and d) agricultural areas dominated by Hortonian and focused
runoff (especially with row crops), a refined assessment should incorporate these
differences for modeled stream flow generation. As the zone around the immediate
(application) target area expands, there will be greater variability in the landscape; in the
context of a risk assessment, the runoff potential that is assumed for the expanding area
will be a crucial variable (since dilution at the outflow point is determined by the size of
the expanding area). Thus, it important to know at least some approximate estimate of
types of land use within that region. Runoff from forested areas ranges from 45 -
2,700% less than from agricultural areas; in most studies, runoff was 2.5 to 7 times higher
in agricultural areas (e.g., Okisaka et al., 1997; Karvonen et al., 1999; McDonald et al.,
2002; Phuong and van Dam 2002). Differences in runoff potential between
urban/sub urban areas and agricultural areas are generally less than between agricultural
and forested areas. In terms of likely runoff potential (other variables - such as
topography and rainfall - being equal), the relationship is generally as follows (going
from lowest to highest runoff potential):
Three-tiered forest < agroforestry < suburban < row-crop agriculture < urban.
There are, however, other uncertainties that should serve to counteract the effects of the
aforementioned issue. For example, the dilution model considers that 100% of the
agricultural area has the chemical applied, which is almost certainly a gross over-
estimation. Thus, there will be assumed chemical contributions from agricultural areas
that will actually be contributing only runoff water (dilutant); so some contributions to
total contaminant load will really serve to lessen rather than increase aquatic
concentrations. In light of these (and other) confounding factors, Agency believes that
this model gives us the best available estimates under current circumstances.
6.1.5 Usage Uncertainties
County-level usage data were obtained from California's Department of Pesticide
Regulation Pesticide Use Reporting (CDPR PUR) database. Eight years of data (1999 -
2006) were included in this analysis. CDPR PUR documentation indicates that errors in
the data may include the following: a misplaced decimal; incorrect measures, area
treated, or units; and reports of diluted pesticide concentrations. In addition, it is possible
that the data may contain reports for pesticide uses that have been cancelled. The CPDR
PUR data does not include home owner applied pesticides; therefore, residential uses are
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not likely to be reported. As with all pesticide usage data, there may be instances of
misuse and misreporting. The Agency made use of the most current, verifiable
information; in cases where there were discrepancies, the most conservative information
was used.
6.1.6 Terrestrial Exposure Modeling of Methidathion
The Agency relies on the work of Fletcher et al. (1994) for setting the assumed pesticide
residues in wildlife dietary items. These residue assumptions are believed to reflect a
realistic upper-bound residue estimate, although the degree to which this assumption
reflects a specific percentile estimate is difficult to quantify. It is important to note that
the field measurement efforts used to develop the Fletcher estimates of exposure involve
highly varied sampling techniques. It is entirely possible that much of these data reflect
residues averaged over entire above ground plants in the case of grass and forage
sampling.
It was assumed that ingestion of food items in the field occurs at rates commensurate
with those in the laboratory. Although the screening assessment process adjusts dry-
weight estimates of food intake to reflect the increased mass in fresh-weight wildlife food
intake estimates, it does not allow for gross energy differences. Direct comparison of a
laboratory dietary concentration- based effects threshold to a fresh-weight pesticide
residue estimate would result in an underestimation of field exposure by food
consumption by a factor of 1.25 - 2.5 for most food items.
Differences in assimilative efficiency between laboratory and wild diets suggest that
current screening assessment methods do not account for a potentially important aspect of
food requirements. Depending upon species and dietary matrix, bird assimilation of wild
diet energy ranges from 23 - 80%, and mammal's assimilation ranges from 41 - 85%
(U.S. Environmental Protection Agency, 1993). If it is assumed that laboratory chow is
formulated to maximize assimilative efficiency (e.g., a value of 85%), a potential for
underestimation of exposure may exist by assuming that consumption of food in the wild
is comparable with consumption during laboratory testing. In the screening process,
exposure may be underestimated because metabolic rates are not related to food
consumption.
For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal
was assumed to occupy either the treated field or adjacent areas receiving a treatment rate
on the field. Actual habitat requirements of any particular terrestrial species were not
considered, and it was assumed that species occupy, exclusively and permanently, the
modeled treatment area. Spray drift model predictions suggest that this assumption leads
to an overestimation of exposure to species that do not occupy the treated field
exclusively and permanently.
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6.1.7 Spray Drift Modeling
Although there may be multiple methidathion applications at a single site, it is unlikely
that the same organism would be exposed to the maximum amount of spray drift from
every application made. In order for an organism to receive the maximum concentration
of methidathion from multiple applications, each application of methidathion would have
to occur under identical atmospheric conditions (e.g., same wind speed and - for plants -
same wind direction) and (if it is an animal) the animal being exposed would have to be
present directly downwind at the same distance after each application. Although there
may be sites where the dominant wind direction is fairly consistent (at least during the
relatively quiescent conditions that are most favorable for aerial spray applications), it is
nevertheless highly unlikely that plants in any specific area would receive the maximum
amount of spray drift repeatedly. It appears that in most areas (based upon available
meteorological data) wind direction is temporally very changeable, even within the same
day. Additionally, other factors, including variations in topography, cover, and
meteorological conditions over the transport distance are not accounted for by the
AgDRIFT/AGDISP model (i.e., it models spray drift from aerial and ground applications
in a flat area with little to no ground cover and a steady, constant wind speed and
direction). Therefore, in most cases, the drift estimates from AgDRIFT/AGDISP may
overestimate exposure even from single applications, especially as the distance increases
from the site of application, since the model does not account for potential obstructions
(e.g., large hills, berms, buildings, trees, etc.). Furthermore, conservative assumptions
are often made regarding the application method (e.g., aerial), release heights and wind
speeds. Alterations in any of these inputs would change the area of potential effect.
6.2 Effects Assessment Uncertainties
6.2.1 Age Class and Sensitivity of Effects Thresholds
It is generally recognized that test organism age may have a significant impact on the
observed sensitivity to a toxicant. The acute toxicity data for fish are collected on
juvenile fish between 0.1 and 5 grams. Aquatic invertebrate acute testing is performed on
recommended immature age classes (e.g., first instar for daphnids, second instar for
amphipods, stoneflies, mayflies, and third instar for midges).
Testing of juveniles may overestimate toxicity at older age classes for pesticide active
ingredients that act directly without metabolic transformation because younger age
classes may not have the enzymatic systems associated with detoxifying xenobiotics. In
so far as the available toxicity data may provide ranges of sensitivity information with
respect to age class, this assessment uses the most sensitive life-stage information as
measures of effect for surrogate aquatic animals, and is therefore, considered as
protective of the CRLF.
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6.2.2 Use of Surrogate Species Effects Data
Guideline toxicity tests and open literature data on methidathion are not available for
frogs or any other aquatic-phase amphibian; therefore, freshwater fish are used as
surrogate species for aquatic-phase amphibians. Efforts are made to select the organisms
most likely to be affected by the type of compound and usage pattern; however, there is
an inherent uncertainty in extrapolating across phyla. In addition, the Agency's LOCs
are intentionally set very low, and conservative estimates are made in the screening level
risk assessment to account for these uncertainties.
6.2.3 Sublethal Effects
When assessing acute risk, the screening risk assessment relies on the acute mortality
endpoint as well as a suite of sublethal responses to the pesticide, as determined by the
testing of species response to chronic exposure conditions and subsequent chronic risk
assessment. Consideration of additional sublethal data in the effects determination is
exercised on a case-by-case basis and only after careful consideration of the nature of the
sublethal effect measured and the extent and quality of available data to support
establishing a plausible relationship between the measure of effect (sublethal endpoint)
and the assessment endpoints. However, the full suite of sublethal effects from valid
open literature studies is considered for the purposes of defining the action area.
When assessing acute risk, the screening risk assessment relies on the acute mortality
endpoint as well as a suite of sublethal responses to the pesticide, as determined by the
testing of species response to chronic exposure conditions and subsequent chronic risk
assessment. Consideration of additional sublethal data in the effects determination is
exercised on a case-by-case basis and only after careful consideration of the nature of the
sublethal effect measured and the extent and quality of available data to support
establishing a plausible relationship between the measure of effect (sublethal endpoint)
and the assessment endpoints. However, the full suite of sublethal effects from valid
open literature studies is considered for the purposes of defining the action area.
In fish, slight spastic motions and swimming on their sides were observed following
acute exposure at 1000 |ig/L (MRID 00011841). Following chronic exposure, total length
and wet weight are affected (MRID 00015730 or 45822701) with a NOAEC/LOAEC of
6.3/12 |ig ai/L respectively. No sublethal effects were reported in the freshwater
invertebrate studies.
In birds, depression, reduced reaction to external stimuli, wing droop, convulsions, and
salivation were observed following acute exposure via gavage. In subacute dietary
studies, toxic symptoms including depression, reduced reaction to external stimuli, wing
droop, loss of coordination, lower limb weakness, ruffled appearance, prostrate posture,
and loss of righting reflex were observed at 178 ppm a.i. and above. Reductions on body
weight gain and food consumption were reduced at levels above 316 ppm a.i.. Following
chronic exposure (MRID 44381602), none of the ducks showed symptoms of toxicity or
behavioral abnormalities during the experiment. An increase in the number of eggs
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cracked and a decrease in the number of eggs not cracked/eggs laid were observed at 10
ppm. Brain cholinesterase was inconsistently affected at 30 ppm.
The study used to define the action area examined the effects of subchronic methidathion
administration on vascular wall damage in rats (ECOTOX ref. 80451). At the lowest dose
tested (5 mg/kg/day), the levels of malondialdehyde (MDA), a biomarker for oxidative
stress was significantly higher and cholinesterase activity was significantly lower. There
were irregular, prominent breaks and fragmentation of the elastic fibers in the aortic wall.
In a 2-generation reproduction study in rats (MRID 40079812, -13), the parental systemic
NOAEC was 5 ppm and the LOAEC was 25 ppm, based on tremors and decreased food
consumption during lactation, and decreased ovarian weight. In addition, there was also a
slight decrease in body weight early in the F1 growth phase at 50 ppm. The reproductive
NOAEC was 5 ppm and the LOAEC was 25 ppm based on a decreased mating index and
a generalized indication of pup unthriftyness while nursing. In addition, there was an
increase in stillbirths and decreased pup survival at birth and during lactation at the 50
ppm treatment level.
To the extent to which sublethal effects are not considered in this assessment, the
potential direct and indirect effects of methidathion on CRLF may be underestimated.
6.2.4 Location of Wildlife Species
For the terrestrial exposure analysis of this risk assessment, a generic bird or mammal
was assumed to occupy either the treated field or adjacent areas receiving a treatment rate
on the field. Actual habitat requirements of any particular terrestrial species were not
considered, and it was assumed that species occupy, exclusively and permanently, the
modeled treatment area. Spray drift model predictions suggest that this assumption leads
to an overestimation of exposure to species that do not occupy the treated field
exclusively and permanently.
7. Risk Conclusions
In fulfilling its obligations under Section 7(a)(2) of the Endangered Species Act, the
information presented in this endangered species risk assessment represents the best data
currently available to assess the potential risks of methidathion to the CRLF and its
designated critical habitat.
Based on the best available information, the Agency makes a Likely to Adversely Affect
determination for the CRLF from the use of methidathion. Additionally, the Agency has
determined that there is the potential for modification of CRLF designated critical habitat
from the use of the chemical.
There is potential for direct risk to the aquatic- and terrestrial-phase CRLF following
either acute or chronic exposure to methidathion with its current use patterns in
California. The acute listed species LOC for freshwater fish (surrogate to the aquatic-
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phase CRLF) is exceeded for all uses; however, there are limited data which indicate that
there will be some fish species that may not be at risk following acute exposure from
some of the methidathion use scenarios. The chronic LOC for freshwater fish is also
exceeded for all uses. For the terrestrial-phase CRLF, using avian data as a surrogate,
with the exception of mangos on a dietary basis for non-listed species, the acute dose-
based, acute dietary-based and chronic dietary-based RQs for the terrestrial-phase CRLF,
exceed both the acute listed and non-listed species LOC and chronic LOC for all of the
assessed uses.
There is a potential for indirect effects to both the aquatic- and terrestrial-phase CRLF.
For aquatic invertebrates (food source), the acute and chronic RQs exceed the acute non-
listed LOC for aquatic animals and chronic LOC for aquatic animals respectively, for all
assessed uses. For fish and aquatic-phase amphibians (food source), the acute non-listed
species LOC is exceeded for all uses. The chronic LOC is also exceeded for all uses. No
studies are available for aquatic vascular and non-vascular plants (food source and
habitat); however, the weight of the evidence indicates an effect determination of may
affect, not likely to adversely affect (NLAA). For terrestrial invertebrates (food source),
the RQs for both small and large insects exceed acute LOC for non-listed species for all
methidathion uses, even with only a single use. For terrestrial-phase amphibians (food
source), see the direct effects paragraph. For mammals (food source), the acute and
chronic RQs exceed the non-listed species acute LOC and the chronic LOC, respectively
for all uses. No studies are available for terrestrial plants (habitat). There is some
evidence for potential for terrestrial plant damage; however, the weight of the evidence
indicates that the effect to terrestrial plants is may affect, not likely to adversely affect
(NLAA). Habitat modification is expected for both the aquatic- and terrestrial-phase
CRLF based on indirect effects from the reduction in prey base. A significant overlap is
expected to exist between the use sites and CRLF habitat, particularly when the spraydrift
and downstream dilution buffers are added to the methidathion use area. Spraydrift
distances range from 23 to greater than 1000 feet and the downstream dilution buffer is
285 km. Figure 5.1 references the overlap of frog habitat with NLCD land cover data for
use patterns that result in LAA determinations. Given the LAA determination for the
CRLF and potential modification of designated critical habitat, a description of the
baseline status and cumulative effects for the CRLF is provided in Attachment 2.
A summary of the risk conclusions and effects determinations for the CRLF and its
critical habitat, given the uncertainties discussed in Section 6, is presented in Tables 7.1
and 7.2.
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Table 7.1 Effects Determination Summary for Methidathion Use and the CRLF
Assessment
Endpoint
Effects
Determination 1
Basis for Determination
Survival, growth,
and/or reproduction
of CRLF
individuals
LAA1
Potential for Direct Effects
Aquatic-phase (Eggs, Larvae, and Adults) :
Highly toxic to freshwater fish. Acute listed species LOC exceeded for all uses
for three tested surrogate species. Chronic effects based on survival and growth.
Chronic LOC exceeded for all uses. Probability of an individual effect on an
acute basis is high, both at the acute LOC and at the RQ levels. Incident data
also indicate freshwater fish vulnerability, and monitoring has detected surface
water concentrations above the acute endpoint. Significant overlap expected to
exist between use sites and CRLF habitat, particularly when the spraydrift and
downstream dilution buffers are added to the methidathion use area. Spraydrift
distances range from 23 to over 1000 feet and the downstream dilution buffer is
285 km.
Terrestrial-phase (Juveniles and Adults) :
Moderately toxic to very highly toxic to avian species. Acute dose-based, acute
dietary-based and chronic dietary-based RQs for the terrestrial-phase CRLF,
using avian data as a surrogate, exceed acute listed species LOC and chronic
LOC for all of the assessed uses. Chronic effects based on reduction in number
of normal hatchlings/live 3-week embryos. Limited data indicate that on a dose
basis, the acute LOC is exceeded for all uses, even with the least sensitive
species at the lowest application rate (mangos). On a dietary basis, the same is
true for all uses except mangos. Probability of an individual effect on an acute
basis is high at the RQ levels. Incident data indicate that birds are vulnerable.
Significant overlap exists between use sites and CRLF habitat, particularly when
the spraydrift buffers are applied to the methidathion use area. Spraydrift
distances range from 194 to over 1000 feet.
Potential for Indirect Effects
Aquatic prey items, aquatic habitat, cover and/or primary productivity
Very highly toxic to freshwater invertebrates. Acute and chronic RQs exceed
acute LOC for non-listed species and chronic LOC, respectively for all assessed
uses. The percentage effect to the aquatic invertebrate prey base for all uses is
very high. For fish (aquatic-phase amphibians), acute non-listed species LOC
exceeded for all uses. Chronic LOC exceeded for all uses. Percentage effect to
the freshwater fish/aquatic-phase amphibian prey base is very high. No studies
are available for aquatic vascular and non-vascular plants.
Terrestrial prey items, riparian habitat
RQs for both small and large insects exceed acute LOC for non-listed species for
all methidathion uses, even with only a single use. Insufficient studies to
conduct a sensitivity analysis; however, lowest RQ, using highest terrestrial
invertebrate endpoint still exceeds the acute list species LOC. Percentage effect
to the terrestrial invertebrate prey base for all uses is very high. For terrestrial-
phase amphibians using avian data as a surrogate, with the exception of mangos
on an acute dietary basis, the acute dose-based, acute dietary-based and chronic
dietary-based RQs exceed the acute non-listed species LOC and the chronic LOC
for all of the assessed methidathion uses, respectively. Percentage effect to the
avian/terrestrial-phase amphibian prey base is very high. For mammals, the
acute and chronic RQs exceed the non-listed species acute LOC and the chronic
LOC, respectively for all uses. At the lowest RQ, the percentage effect to the
mammalian prey base is very high. No studies are available for terrestrial plants.
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1 No effect (NE); May affect, but not likely to adversely affect (NLAA); May affect, likely to adversely
affect (LAA)
Table 7.2 Effects Determination Summary for Methidathion Use and CRLF Critical Habitat
Impact Analysis
Assessment
Endpoint
Effects
Determination 1
Basis for Determination
Modification of
aquatic-phase PCE
Habitat
Modification1
No studies available for aquatic vascular and non-vascular plants and
terrestrial plants.
For the aquatic-phase CRLF, acute non-listed species LOC for freshwater
fish (aquatic-phase amphibians) exceeded for all uses. Chronic LOC for
freshwater fish exceeded for all uses. Probability of an individual effect and
percentage effect to the freshwater fisli/aquatic-phase amphibian prey base
on an acute basis at the RQ are high. Incident data support freshwater fish
vulnerability. For freshwater invertebrates, acute and chronic RQs exceed
acute LOC (non-listed species) and chronic LOC, respectively for all
assessed uses. Again the percentage effect to the aquatic invertebrate prey
base is very high. Significant overlap expected between use sites and CRLF
habitat, particularly when the spraydrift and downstream dilution buffers are
added to the use area. Spraydrift distances range from 23 (mangos) to over
1000 feet and the downstream dilution buffer is 285 km.
Modification of
terrestrial-phase
PCE
No studies are available for terrestrial plants.
For the terrestrial-phase CRLF, using avian data as a surrogate, with the
exception of mangos on a dietary basis for non-listed species, the acute dose-
based, acute dietary-based and chronic dietary-based RQs for the terrestrial-
phase CRLF, exceed both the acute listed and non-listed species LOC and
chronic LOC for all of the assessed uses. Probability of an individual effect
on an acute basis at the lowest RQ level and percentage effect to the
avian/terrestrial-phase amphibian prey base are very high. Incident data
indicate that birds are vulnerable. RQs for both small and large insects
exceed acute LOC for non-listed species for all methidathion uses, even with
only a single use. Percentage effect to terrestrial invertebrate prey base for
all uses are very high. For mammals, the acute and chronic RQs exceed the
non-listed species acute LOC and the chronic LOC, respectively for all uses.
The percentage effect to the mammalian prey base is very high for all uses.
Significant overlap exists between use sites and CRLF habitat, particularly
when the spraydrift buffers are applied to the methidathion use area.
Spraydrift distances range from 194 (mangos) to over 1000 feet.
1 Habitat Modification or No effect (NE)
Based on the conclusions of this assessment, a formal consultation with the U. S. Fish
and Wildlife Service under Section 7 of the Endangered Species Act should be initiated
to seek concurrence with the LAA determinations and to determine whether there are
reasonable and prudent alternatives and/or measures to reduce and/or eliminate potential
incidental take.
When evaluating the significance of this risk assessment's direct/indirect and adverse
habitat modification effects determinations, it is important to note that pesticide
exposures and predicted risks to the species and its resources (i.e., food and habitat) are
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not expected to be uniform across the action area. In fact, given the assumptions of drift
and downstream transport (i.e., attenuation with distance), pesticide exposure and
associated risks to the species and its resources are expected to decrease with increasing
distance away from the treated field or site of application. Evaluation of the implication
of this non-uniform distribution of risk to the species would require information and
assessment techniques that are not currently available. Examples of such information and
methodology required for this type of analysis would include the following:
• Enhanced information on the density and distribution of CRLF life stages
within specific recovery units and/or designated critical habitat within the
action area. This information would allow for quantitative extrapolation
of the present risk assessment's predictions of individual effects to the
proportion of the population extant within geographical areas where those
effects are predicted. Furthermore, such population information would
allow for a more comprehensive evaluation of the significance of potential
resource impairment to individuals of the species.
• Quantitative information on prey base requirements for individual aquatic-
and terrestrial-phase frogs. While existing information provides a
preliminary picture of the types of food sources utilized by the frog, it
does not establish minimal requirements to sustain healthy individuals at
varying life stages. Such information could be used to establish
biologically relevant thresholds of effects on the prey base, and ultimately
establish geographical limits to those effects. This information could be
used together with the density data discussed above to characterize the
likelihood of adverse effects to individuals.
• Information on population responses of prey base organisms to the
pesticide. Currently, methodologies are limited to predicting exposures
and likely levels of direct mortality, growth or reproductive impairment
immediately following exposure to the pesticide. The degree to which
repeated exposure events and the inherent demographic characteristics of
the prey population play into the extent to which prey resources may
recover is not predictable. An enhanced understanding of long-term prey
responses to pesticide exposure would allow for a more refined
determination of the magnitude and duration of resource impairment, and
together with the information described above, a more complete prediction
of effects to individual frogs and potential modification to critical habitat.
8. References
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R.W. McDiarmid and R. Altig (Eds.), Tadpoles: The Biology of Anuran
Larvae.University of Chicago Press, Chicago, pp. 24-51.
Alvarez, J. 2000. Letter to the U.S. Fish and Wildlife Service providing comments on
the Draft California Red-legged Frog Recovery Plan.
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Aston, L.S. and J. N. Seiber. 1997. Fate of summertime airborne organophosphate
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Burns, L.A. 1997. Exposure Analysis Modeling System (EXAMSII) Users Guide for
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Fletcher, J.S., J.E. Nellessen, and T.G. Pfleeger. 1994. Literature review and evaluation
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(9): 1383-1391.
Ganapathy, C., C. Nordmark, K. Bennet, and A. Bradley. 1997. Temporal distribution
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http://www.cdpr.ca.gov/docs/sw/swemreps.htm .
Hayes, M.P. and M.R. Jennings. 1988. Habitat correlates of distribution of the
California red-legged frog (Rana aurora draytonii) and the foothill yellow-legged
frog (Rana boylii): Implications for management. Pp. 144-158. In Proceedings
of the symposium on the management of amphibians, reptiles, and small
mammals in North America. R. Sarzo, K.E. Severson, and D.R. Patton (technical
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Hayes, M.P. and M.M. Miyamoto. 1984. Biochemical, behavioral and body size
differences between Rana aurora aurora and R. a. draytonii. Copeia 1984(4):
1018-22.
Hayes and Tennant. 1985. Diet and feeding behavior of the California red-legged frog.
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