Potential Risks of Disulfoton 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
June 20,2008
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Primary Authors:
Keara Moore, Brian Anderson
Secondary Review:
James Hetrick
Acting Branch Chief, Environmental Risk Assessment Branch 3:
Tom Bailey
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TABLE OF CONTENTS
1. Executive Summary 9
2. Problem Formulation 15
2.1 Purpose 16
2.2 Scope 18
2.3 Previous Assessments 19
2.4 Stressor Source and Distribution 20
2.4.1 Physical and Chemical Properties 20
2.4.2 Environmental Fate Properties 21
2.4.3 Environmental Transport Mechanisms 27
2.4.4 Mechanism of Action 28
2.4.5 Use Characterization 28
2.5 Assessed Species 32
2.5.1 Distribution 33
2.5.2 Reproduction 38
2.5.3 Diet38
2.5.4 Habitat 39
2.6 Designated Critical Habitat 40
2.7 Action Area 42
2.8 Assessment Endpoints and Measures of Ecological Effect 45
2.8.1. Assessment Endpoints for the CRLF 45
2.8.2 Assessment Endpoints for Designated Critical Habitat 47
2.9 Conceptual Model 49
2.9.1 Risk Hypotheses 49
2.9.2 Diagram 49
2.10 Analysis Plan 53
2.10.1 Measures to Evaluate the Risk Hypothesis and Conceptual Model 54
3. Exposure Assessment 58
3.1 Label Application Rates and Intervals 58
3.2 Aquatic Exposure Assessment 60
3.2.1 Modeling Approach 60
3.2.2 Model Inputs 60
3.2.3 Results 65
3.2.4 Existing Monitoring Data 66
3.3. Terrestrial Animal Exposure Assessment 68
3.4. Terrestrial Plant Exposure Assessment 71
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4. Effects Assessment 71
4.1 Toxicity of Disulfoton to Aquatic Organisms 73
4.1.1 Toxicity to Freshwater Fish 74
4.1.2 Toxicity to Freshwater Invertebrates 76
4.1.3 Toxicity to Aquatic Plants 78
4.2 Toxicity of Disulfoton to Terrestrial Organisms 78
4.2.1 Toxicity to Birds 80
4.2.2 Toxicity to Mammals 82
4.2.3 Toxicity to Terrestrial Invertebrates 83
4.2.4 Toxicity to Terrestrial Plants 83
4.3 Use of Probit Slope Response Relationship to Provide Information on the
Endangered Species Levels of Concern 84
4.4 Incident Database Review 84
5. Risk Characterization 86
5.1 Risk Estimation 86
5.1.1 Exposures in the Aquatic Habitat 86
5.1.2 Exposures in the Terrestrial Habitat 88
5.1.3 Primary Constituent Elements of Designated Critical Habitat 92
5.2 Risk Description 92
5.2.1 Direct Effects 95
5.2.2 Indirect Effects (via Reductions in Prey Base), Aquatic Phase CRLFs 99
5.2.3 Indirect Effects (via Habitat Effects) 102
5.2.4 Modification to Designated Critical Habitat 103
5.2.5 Distance From Treated Site Effects May Occur 103
6. Uncertainties 104
6.1 Exposure Assessment Uncertainties 106
6.1.1 Maximum Use Scenario 106
6.1.2 Aquatic Exposure Modeling of Disulfoton 107
6.1.3 Usage Uncertainties 109
6.1.4 Terrestrial Exposure Modeling of disulfoton 109
6.1.5 Spray Drift Modeling 110
6.2 Effects Assessment Uncertainties Ill
6.2.1 Age Class and Sensitivity of Effects Thresholds 111
6.2.2 Use of Surrogate Species Effects Data Ill
6.2.3 Sublethal Effects Ill
6.2.4 Location of Wildlife Species 112
7. Risk Conclusions 112
8. References 117
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List of Tables
Table 1.1a Effects Determination Summary for Direct and Indirect Effects of Disulfoton on
the CRLF 12
Table 1.1b Effects Determination Summary for Direct and Indirect Effects of Disulfoton on
the CRLF 13
Table 1.2 Effects Determination Summary for the Critical Habitat Impact Analysis 14
Table 2.1. Physical and Chemical Properties of Disulfoton 20
Table 2.2. Environmental fate and transport data for disulfoton 23
Table 2.3. Environmental fate and transport data for d. sulfoxide and d. sulfone 24
Table 2.4. Maximum degradate amounts in environmental fate studies of disulfoton 25
Table 2.5 Disulfoton Uses Assessed for the CRLF 29
Table 2.6 California Red-legged Frog Recovery Units with Overlapping Core Areas and
Designated Critical Habitat 34
Table 2.7 Assessment Endpoints and Measures of Ecological Effects 46
Table 3.1. Labeled use pattern for each crop, used in assessing disulfoton environmental
exposure 59
Table 3.2. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to Total
Toxic Residues of Disulfoton 61
Table 3.3. PRZM/EXAMS Use-Specific Aquatic Exposure Inputs for Total Toxic
Residues of Disulfoton 62
Table 3.4. PRZM/EXAMS Disulfoton Application Dates 64
Table 3.5. Aquatic EECs (|ig/L) for total toxic residues of disulfoton uses in California..65
Table 3.6 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based Exposures
of the CRLF and its Prey to disulfoton 70
Table 3.7 EECs (ppm) Used to Estimate Indirect Effects to the Terrestrial-Phase CRLF
via Effects to Terrestrial Invertebrate Prey Items 70
Table 3.8. Input Parameters Used to Derive Terrestrial Plant EECs 71
Table 3.9. Terrestrial Plant EECs for Disulfoton. Units in lbs a.i./acre 71
Table 4.1 Freshwater Aquatic Toxicity Profile for Disulfoton 73
Table 4.2 Categories of Acute Toxicity for Aquatic Organisms 74
Table 4.3. Acute Fish Toxicity Values for Disulfoton and Degradates 74
Table 4.4. Range of Acute Fish Toxicity Values for Disulfoton 75
Table 4.5. Freshwater Fish Early Life-Stage Toxicity 76
Table 4.6. Acute Aquatic Invertebrate Toxicity Data for Disulfoton and its Major
Degradates 77
Table 4.7. Freshwater Aquatic Invertebrate Life-Cycle Toxicity 78
Table 4.8 Terrestrial Toxicity Profile for Disulfoton and its Degradates of Concern 79
Table 4.9 Categories of Acute Toxicity for Avian and Mammalian Studies 80
Table 4.10. Toxicity Endpoints Used to Estimate Potential Risk of Direct Effects to
Terrestrial Phase CRLFs 81
Table 4.11. Summary of Available Avian Reproduction Toxicity Studies for Disulfoton... 81
Table 4.12. Summary of Available Mammalian Acute Toxicity Studies for Disulfoton and
its Degradates of Concern 82
Table 4.13. Summary of Available Mammalian Reproduction Toxicity Studies for
Disulfoton 82
Table 4.14. Summary of Available Mammalian Acute Toxicity Studies for Disulfoton 83
Table 4.15. Chronological List of Ecological Incidents 85
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Table 5.1 Summary of Direct Effect RQs for Aquatic-phase CRLFs Based on an LC50 of
37 ug/L and a NOAEC of 4 ug/L in Fish 87
Table 5.2 Summary of Acute and Chronic RQs Used to Estimate Indirect Effects to the
CRLF via Effects on Aquatic Invertebrates as Dietary Food Items (prey of
CRLF juveniles and adults in aquatic habitats) Based on an LC50 of 3.9 ppb
and a NOAEC of 0.011 ppb 87
Table 5.3a. Avian RQs Used To Estimate Potential Risk of Direct Effects to Terrestrial
Phase CRLFs for Spray Applications3 89
Table 5.3b. Avian LD50/Square Foot Analysis Used to Estimate Potential Direct Effects to
the CRLF from Granular and Soil Incorporated Applications3 89
Table 5.4. Reproduction RQs for Birds Used to Estimate Potential Direct Effects to
CRLFs from Spray Uses3 90
Table 5.5. Summary of RQs Used to Estimate Indirect Effects to the Terrestrial-phase
CRLF via Direct Effects on Terrestrial Invertebrates as Dietary Food Items ...90
Table 5.6a. RQs used to Estimate Potential Acute Risks to Mammalian Prey of CRLFs
From Spray Applications 91
Table 5.6b. LD50/Square Foot Analysis Used to Estimate Potential Effects to Mammal
Prey Items of the CRLF (soil incorporated applications)3 91
Table 5.7. Summary of Reproduction RQs used to Estimate Potential Risk to Mammalian
Prey of CRLFs from Spray Applications of Disulfoton 91
Table 5.8. Preliminary Effects Determination Summary for disulfoton - Direct and Indirect
Effects to CRLF 93
Table 5.9. Range of Acute Fish LC50s for Disulfoton 96
Table 5.10. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based Risk
Quotients Based on a Single Application of 1 lb a.i./Acre 98
Table 5.11. Acute RQs for Various Aquatic Invertebrates for Disulfoton 100
Table 5.12. Summary of Direct Effect RQs for Aquatic-phase CRLFs Based on an LC50 of
37 ug/L and a NOAEC of 4 ug/L in Fish 101
Table 5.13. RQ/LOC Ratio for Various Landcover Classes for Aquatic Organisms3 104
Table 5.14. AgDISP predicted Buffer Distance resulting in no Endangered Species LOC
Exceedance for Terrestrial Animals for Disulfoton 105
Table 5.15. Spraydrift Fraction Resulting in no Restricted Use or Acute LOC Exceedance
for Terrestrial Animals for Disulfoton 105
Table 5.16. Spraydrift Fraction Resulting in no LOC Exceedance for Aquatic Animals for
Disulfoton 106
Table 7. la Effects Determination Summary for Direct and Indirect Effects of Disulfoton
on the CRLF 112
Table 7. lb Effects Determination Summary for Direct and Indirect Effects of Disulfoton
on the CRLF 114
Table 7.2 Effects Determination Summary for the Critical Habitat Impact Analysis 115
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List of Figures
Figure 2.1 Chemical Structure of Disulfoton 19
Figure 2.2 Chemical Structure of Disulfoton Sulfoxide 19
Figure 2.3 Chemical Structure of Disulfoton Sulfone 20
Figure 2.4 Summary of Disulfoton Use by Crop in California from 2002 to 2005 30
Figure 2.5 Summary of Disulfoton Use by County in California from 2002 to 2005... 31
Figure 2.6 Recovery Unit, Core Area, Critical Habitat, and Occurrence Designations
for CRI.I 36
Figure 2.7 CRLF Reproductive Events by Month 38
Figure 2.8 Initial area of concern, or "footprint" of potential use, for disulfoton 44
Figure 2.9 Conceptual Model for Disulfoton Effects on Aquatic-Phase of the CRLF.. 50
Figure 2.10 Conceptual Model for Disulfoton Effects on Terrestrial-Phase of the
CRLF 51
Figure 2.11 Conceptual Model for Disulfoton Effects on Aquatic Component of
CRLF Critical Habitat 52
Figure 2.12 Conceptual Model for Disulfoton Effects on Terrestrial Component of
CRLF Critical Habitat 53
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List of Appendices and Attachments
Appendix A Analysis of Products with 2 or More Active Ingredients
Appendix B California PUR Summary Data
Appendix C GIS Analysis
Appendix D Summary of Accepted ECOTOX papers
Appendix E PRZM/EXAMS Model Output
Appendix F T-REX Example Output
Appendix G HED RED Assessment
Appendix H Bibliography of Accepted and Rejected ECOTOX papers
Appendix I Bibliography of Registrant-Submitted Studies
Attachment 1 Life History of the CRLF
Attachment 2 Baseline and Cumulative Effects
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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 disulfoton 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.
Disulfoton is a systemic organophosphate insecticide, acaracide (miticide) registered for
use to control aphids, thrips, mealybugs, other sucking insects, and spider mites on a
variety of commodities including asparagus, beans, broccoli, Brussels sprouts, cabbage,
cauliflower, Christmas trees, cotton, and lettuce. It is also registered for a number of
residential uses including flowers, shrubs, and ornamentals. All registered uses are
considered as part of the federal action evaluated in this assessment.
It is formulated as an emulsifiable concentrate for most agricultural uses, but is also
formulated as a granular product for residential uses, Christmas trees, and cotton.
Applications are generally soil applied: injection, in-furrow spray, or row treatment
followed by soil incorporation. It can also be applied as a foliar treatment by ground or
air. Disulfoton is typically applied 1 time per season, but may be applied multiple times
per year for some crops. Application rates typically range from about 1 to 2 lb ai/A,
although application to Christmas trees is allowed at up to 4.5 lb a.i./A.
Disulfoton itself is moderately mobile and generally non-persistent but its major
degradates are more stable, so total residues are likely to be persistent. Disulfoton
degrades through microbially-mediated degradation in aerobic soil and aquatic
environments but appears to be more stable in anaerobic environments. Disulfoton is
also subject to rapid aquatic and soil photolysis, but it is essentially stable to hydrolysis.
In aerobic soil, disulfoton can be oxidized by chemical reaction and microbial
metabolism to its corresponding disulfoton sulfoxide and disulfoton sulfone, which are
also toxic. The only other major degradate is sulfonic acid. The oxon forms of
disulfoton and d. sulfoxide are formed as minor degradates through hydrolysis and soil
photolysis. The major degradates are more persistent and more mobile than the parent,
and the toxicity of these degradates is similar to or greater than toxicity of the parent for
several of the taxonomic groups included in this assessment. Therefore, a total toxic
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residues approach was used in this assessment. In aerobic soil, the half-life for total
disulfoton residues ranged from 120 to 408 days.
Since CRLFs exist within aquatic and terrestrial habitats, exposure of the CRLF, its prey
and its habitats to disulfoton are assessed separately for the two habitats. Tier-II aquatic
exposure models are used to estimate high-end exposures of disulfoton in aquatic habitats
resulting from runoff and spray drift from different uses. Peak model-estimated
environmental concentrations for total toxic residues in surface water resulting from
different disulfoton uses range from 0.6 |ig/L to 67 |ig/L. California surface water
monitoring data for disulfoton are available from U.S.Geological Survey's National
Water Quality Assessment (NAWQA) program and the California Department of
Pesticide Regulation (CDPR). Most sampling was for disulfoton only, though, so the
data are of limited utility in supplementing modeling analysis of total toxic residues. No
disulfoton above detection limits of 0.02 ug/L to 1 ug/L was reported for any samples
from the NAWQA or CDPR databases, which included 1920 and 2712 samples,
respectively, collected statewide over a period of at least 10 years through 2005.
NAWQA included sampling for d. sulfone as well as disulfoton at 6 sites, and 3 of these
sites were also sampled for d. sulfoxide. D. sulfone was detected at 2 sites at levels up to
0.084 ug/L and there were no detections of d. sulfoxide.
Disulfoton residues also have the potential to reach groundwater. No California
monitoring data report detections of disulfoton, but several targeted studies in other areas
demonstrate that transport of disulfoton to groundwater can occur in some conditions.
Available groundwater monitoring data are primarily for parent only. Given the greater
persistence and mobility of the major degradates of disulfoton, total residues are more
likely to leach; therefore, monitoring for parent only will not likely capture the highest
exposures. Although groundwater per se is not evaluated herein, it could be significant
nonetheless because discharging groundwater may support low-order streams, wetlands,
and intermittent ponds - environments that are favorable to California Red-Legged Frogs
(CRLFs). Long-term chronic concentrations derived from the PRZM-EXAMS model
could reflect background concentrations that might be found in discharged
groundwater/stream baseflow.
The T-REX model was used to estimate disulfoton exposures to the terrestrial-phase
CRLF and its potential prey resulting from uses involving foliar and granular
applications. The AgDRIFT model was also used to estimate deposition of disulfoton on
terrestrial and aquatic habitats from spray drift. Exposure to terrestrial plants was
estimated using Terrplant. The T-HERPS model was used to allow for further
characterization of dietary exposures of terrestrial-phase CRLFs relative to birds.
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
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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.
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 disulfoton 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 a particular type of effect are
below LOCs, the pesticide is determined to have "no effect" on the subject species.
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 disulfoton 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.
Based on the best available information, the Agency makes a Likely to Adversely Affect
determination for the CRLF from the use of disulfoton. Additionally, the Agency has
determined that there is the potential for modification of CRLF designated critical habitat
from the labeled use of the chemical. 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.
Further information on the results of the effects determination is included as part of the
Risk Description in Section 5.2.
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Table 1.1a Effects Determination Summary for Direct and Indirect Effects of Disulfoton on the
CR.LF
Assessment Endpoint
Effects
Determination1
Basis for Determination
Aquatic-Phase CRLF
(Eggs, Larvae, and Adults)
Direct Effects:
Survival, growth, and reproduction of
CRLF individuals via direct effects on
aquatic phases
LAA
Endangered species LOC was exceeded for all uses;
chronic LOC was exceeded for all uses except cotton,
beans, and residential uses. Potential effect was not
considered discountable or insignificant.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via effects to food
supply (i.e., freshwater invertebrates,
non-vascular plants, fish, and frogs)
Freshwater
invertebrates: LAA
Acute and chronic RQs were exceeded for all uses.
Acute RQs ranged from approximately 0.5 to 17 and
chronic RQs ranged from 145 to 5600. The potential
magnitude of effect could be sufficient to result in
indirect effects to the CRLF.
Non-vascular aauatic
olants: NE
No aquatic plant toxicity data have been submitted or
were located in the open literature. Disulfoton is an
insecticide, and EC25s for terrestrial plants were greater
than the maximum application rate.
Fish and froes: LAA
for some uses
Magnitude of potential impacts to fish and aquatic phase
amphibians could be sufficient to indirectly affect the
CRLF for some uses. The highest RQs occurred for the
lettuce, cabbage, and asparagus uses.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via indirect effects on
habitat, cover, and/or primary
productivity (i.e., aquatic plant
community)
Non-vascular
aauatic olants: NE
No aquatic plant toxicity data have been submitted or
were located in the open literature. However,
disulfoton is an insecticide, and EC25s for terrestrial
plants were greater than the maximum application rate.
Vascular aauatic
olants: NE
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.
NE
The EC25 is greater than the highest labeled application
rate for all uses except Christmas trees. The Christmas
tree RQ is <0.5.
1 NE = no effect; NLAA = may affect, but not likely to adversely affect; LAA = likely to adversely affect
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Table 1.1b Effects Determination Summary for Direct and Indirect Effects of Disulfoton on the
CRLF
Assessment Endpoint
Effeets
Determination1
Basis tor Determination
Terrestrial-Phase CRLF
(Juveniles and adults)
Direct Effects:
Survival, growth, and reproduction of
CRLF individuals via direct effects on
terrestrial phase adults and juveniles
LAA
Acute LOC (0.5) was exceeded for all uses for disulfoton
and its degradates. Potential for reproductive effects also
exists for all uses.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via effects on prey (i.e.,
terrestrial invertebrates, small terrestrial
vertebrates, including mammals and
terrestrial phase amphibians)
Terrestrial
invertebrates: LAA
The endangered species LOC of 0.05 was exceeded for
all uses. Also, disulfoton is an insecticide, and the
potential magnitude of effect could be sufficient to result
in indirect effects to the CRLF.
Mammals: LAA
Acute (0.5) and chronic (1.0) LOCs were exceeded for
all uses. The potential magnitude of effect could be
sufficient to result in indirect effects to the CRLF.
Fross: LAA
Acute (0.5) and chronic (1.0) LOCs were exceeded for
all uses. The potential magnitude of effect could be
sufficient to result in indirect effects to the CRLF.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via indirect effects on
habitat (i.e., riparian vegetation)
NE
The EC25 is greater than the highest labeled application
rate for all uses except Christmas trees. The Christmas
tree RQ would be <0.5.
1 NE = no effect; NLAA = may affect, but not likely to adversely affect; LAA = likely to adversely affect
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Table 1.2 Effects Determination Summary for the Critical Habitat Impact Analysis
Assessment Endpoint
Effects
Determination1
Basis for Determination
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.
NE
Effects determination for potential effects related to
impacts on aquatic and terrestrial plants was No
Effect.
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.2
NE
Effects determination for potential effects related to
impacts on aquatic and terrestrial plants was No
Effect.
Alteration of other chemical characteristics necessary
for normal growth and viability of CRLFs and their
food source.
HM
Effects determination for direct and indirect effects
to the CRLF was LAA.
Reduction and/or modification of aquatic-based food
sources for pre-metamorphs (e.g., algae)
NE
Effects determination for potential effects related to
impacts on aquatic plants was No Effect.
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
NE
Effects determination for potential effects related to
impacts on aquatic and terrestrial plants was No
Effect.
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
NE
Reduction and/or modification of food sources for
terrestrial phase juveniles and adults
HM
Effects determination for indirect effects via
reducing available food supply was LAA.
Alteration of chemical characteristics necessary for
normal growth and viability of juvenile and adult
CRLFs and their food source.
HM
Effects determination for direct and indirect effects
was LAA.
1 NE = No effect; HM = Habitat Modification
2 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.
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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.
2. Problem Formulation
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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
disulfoton on all registered agricultural commodities and residential areas. In addition,
this assessment evaluates whether registered uses are 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)) settlement 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. Additional refinements include use of
methodology that refines potential exposures to terrestrial phase CRLFs using food
ingestion levels more specific to amphibians as described in Section 5.2. 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 disulfoton is based on an action area. The action area is the area directly
or indirectly affected by the federal action, as indicated by the exceedance 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 disulfoton 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
16
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"effects determination," one of the following three conclusions will be reached regarding
the potential use of disulfoton in accordance with current labels:
• "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 disulfoton 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 disulfoton.
If a determination is made that use of disulfoton 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 disulfoton use sites) and further evaluation of the potential impact of
disulfoton 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 disulfoton is expected to directly impact living organisms within the action area
(defined in Section 2.7), critical habitat analysis for disulfoton 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 disulfoton that may alter the PCEs of the
CRLF's critical habitat form the basis of the critical habitat impact analysis. Actions that
17
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may affect the CRLF's designated critical habitat have been identified by the Services
and are discussed further in Section 2.6.
2.2 Scope
This assessment includes an evaluation of potential ecological risks of labeled uses of
disulfoton. Labeled uses include asparagus, beans, broccoli, Brussels sprouts, cabbage,
cauliflower, Christmas trees, cotton, and lettuce. It is also registered for a number of
residential uses. Disulfoton is formulated as an emulsifiable concentrate or a granular
product. Application methods include soil treatment via spray, injection, or chemigation,
and foliar treatment via ground spray or air.
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 disulfoton in accordance with the approved product labels for
California is "the action" relevant to this ecological risk assessment.
Although current registrations of disulfoton allow for use nationwide, this ecological risk
assessment and effects determination addresses currently registered uses of disulfoton in
portions of the action area that are reasonably assumed to be biologically relevant to the
CRLF and its designated critical habitat. Labeled disulfoton uses not considered relevant
in this assessment are on coffee, labeled only for Puerto Rico, and Easter lilies, which are
grown in California, but not in areas that overlap the CRLF recovery areas. In California,
Easter lilies are grown only in Del Norte county, which is out of the scope of the CRLF
assessment. Further discussion of the action area for the CRLF and its critical habitat is
provided in Section 2.7.
Chemicals included in this assessment include the parent compound, disulfoton, and a
sulfone and sulfoxide degradate. These degradates have been shown to be of similar
toxicity or more toxic to terrestrial animals compared with disulfoton. The degradates are
also a concern for aquatic organisms; therefore, this assessment considers potential risks
from exposure to disulfoton and the two degradates of concern.
The Agency does not routinely include, in its risk assessments, an evaluation of mixtures
of active ingredients, either those mixtures of multiple active ingredients in product
formulations or those in the applicator's tank. In the case of the product formulations of
active ingredients (that is, a registered product containing more than one active
ingredient), each active ingredient is subject to an individual risk assessment for
regulatory decision regarding the active ingredient on a particular use site. If effects data
are available for a formulated product containing more than one active ingredient, they
may be used qualitatively or quantitatively in accordance with the Agency's Overview
Document and the Services' Evaluation Memorandum (U.S., EPA 2004; USFWS/NMFS
2004).
18
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Disulfoton has registered products that contain multiple active ingredients. Analysis of
the available open literature and acute oral mammalian LD50 data for multiple active
ingredient products relative to the single active ingredient is provided in Appendix A.
The results of this analysis show that an assessment based on the toxicity of the single
active ingredient of disulfoton is appropriate.
2.3 Previous Assessments
Disulfoton has been used for many years; therefore, it has a long regulatory history, and
numerous ecological risk assessments have been conducted. Recently (2002), an interim
Reregi strati on Eligibility Decision (IRED) for disulfoton was issued, which became a
final RED after completion of the OP cumulative assessment in 2006. The following
measures were identified in the IRED as necessary to mitigate ecological risks. These
measures have since been incorporated into the currently approved labels.
• A precautionary bee statement is added to all product labels for liquid formulations of
disulfoton.
• Use is prohibited within a level, well maintained 25 foot vegetative buffer between
treated fields and all permanent water bodies.
• No more than one application of disulfoton per calendar year for all crops, except for
asparagus, barley, coffee, peanuts (North Carolina only), and potatoes, for which no
more than two applications of disulfoton per calendar year are permitted.
• The maximum application rate for Christmas trees is reduced from 78 to 4.5 lbs ai/A
nationally, the use is limited to fir species only, and disulfoton is soil incorporated,
watered in, or applied to areas with permanent groundcover.
• Use on barley, wheat, potatoes, and commercially grown ornamentals (field or
nursery stock) is phased out by June 2005.
Several other uses were phased out and others were not eligible for re-registration.
This assessment incorporates all mitigations that were instituted in the RED, which have
been included in all current labels of disulfoton.
19
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2.4 Stressor Source and Distribution
2.4.1 Physical and Chemical Properties
Disulfoton is a water soluble organophosphate insecticide; some physical and chemical
properties are listed below in Table 2.1. Structures of disulfoton and its major
degradates, disulfoton sulfoxide and disulfoton sulfone, are presented in Figures 2-1 to
2-3. Note that the oxon forms of the compounds in Figures 2-1 to 2-3 would be
represented by replacing the marked (*) sulfur with an oxygen.
Table 2.1. Physical and Chemical Properties of Disulfoton
Properlj
Value
Chemical Name (common)
Disulfoton
Chemical Name (CAS)
0,0'-diethyl-S-[2-(ethylthio)ethyl]phosphorothioate
CAS Number
298-04-4
Chemical Formula
C8H1802PS3
Molecular Weight
274.39
Chemical Class
Organophosphate
Physical State
Colorless liquid
Specific Gravity (20°C)
1.144
Boiling point (at 0.01 mmHg)
62°C
Aqueous Solubility (25°C)
15 mg/L
Vapor Pressure (20°C)
1.8 xlO-4 mmHg
Henry's Law Constant (*°C)
2.6 x 106 atm-mVmol
S*
h3c
II
p
^ChL
i °
ch3
Figure 2-1. Chemical Structure of Disulfoton
0
*
s
h3c
/ CH
'3
ch3
Figure 2-2. Chemical Structure of Disulfoton Sulfoxide
0,0-Diethyl S-[2-(ethylsulfinyl)ethyl] phosphorodithioate
20
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Figure 2-3. Chemical Structure of Disulfoton Sulfone
0,0-diethyl S-[2-(ethylsulfonyl)ethyl]-phosphorodithioate
2.4.2 Environmental Fate Properties
Disulfoton itself is moderately mobile and generally non-persistent, but its major
degradates are more stable than the parent, so total residues are likely to be persistent.
Disulfoton degrades rapidly through microbially-mediated degradation in aerobic soil and
aquatic environments (ti/2 = 2-17 d) but appears to be more stable in anaerobic
environments (ti/2 = 275 d). Disulfoton is also subject to aquatic and soil photolysis (ti/2
= 3-4 d), but it is essentially stable to hydrolysis (pH 7, ti/2 = 323 d). Based on physical
properties, disulfoton has some potential to volatilize. A field volatility study shows
dissipation to non-detectable levels at 5 ft within 6 hours of application.
Given the rapid transformation of disulfoton, exposure to its degradates is an important
factor in assessing ecological risk. The primary degradates detected in environmental
fate studies were disulfoton sulfoxide and disulfoton sulfone, both of which are of similar
or greater toxicity than the parent compound. D. sulfoxide is formed at maximum levels
of 15% to 95% through all microbial and abiotic processes excluding hydrolysis. D.
sulfone is the major product of aerobic metabolism in soil and aquatic environments,
reaching maximum levels of 19% to 72%. Sulfonic acid is the only other major
degradate, formed at up to 16% through aerobic aquatic metabolism. Carbon dioxide and
bound residues are other end products of metabolism.
Oxygen analogs (oxons) are potential degradates of OP pesticides that are often toxic and
so are important to consider in assessing disulfoton's ecological risk. Transformation of
disulfoton could lead to formation of three oxons: disulfoton oxon, d. sulfoxide oxon, and
d. sulfone oxon. All were tested for in environmental fate studies and found only as
minor degradates: d. sulfone oxon was not detected in any study, disulfoton oxon formed
through hydrolysis at up to 3%, and d. sulfoxide oxon was formed through aquatic and
soil photolysis at up to 0.3% and 4%, respectively. This dataset may not fully represent
potential exposure to oxons, however. In all cases, maximum levels were reached at
study termination, so it is possible that further increases could occur. Additionally, no
study is available that considers photooxidation in air of volatilized disulfoton to its oxon
forms, which could be an important route of transformation. Based on a field volatility
study for disulfoton, if all of the volatilized disulfoton were oxidized, the maximum
amount of disulfoton oxon that could be formed in air at 1 ft above the field would be
20.9 ng/L.
21
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Both major degradates are more persistent and more mobile than the parent. In aerobic
soil environments, half-lives for d. sulfoxide and d. sulfone are 63 to 77 days and 133 to
257 days, respectively. Total toxic residues (TTR) therefore dissipate more slowly and
have a greater potential to leach. Terrestrial field studies indicate that disulfoton
dissipates rapidly following application and does not leach below surface soil layers, but
that d. sulfoxide and d. sulfone are more persistent and have greater leaching potential.
D. sulfone was detected in the 0-6 in layer as late as 271 days following application, and
was found at depths up to 18 inches.
Because of their toxicity and persistence, exposure to the degradates of disulfoton must
be considered as well as exposure to the parent. In this assessment, aquatic exposure is
estimated for the parent and its major toxic degradates as a group. "Total toxic residues"
refers to the sum of three major forms of disulfoton (parent + sulfoxide + sulfone). Oxon
degradates are not considered explicitly in this estimation. Inclusion of oxon data from
available laboratory studies would not affect exposure estimates quantitatively because
the TTR half-lives for the processes for which oxons are found as minor degradates are
already represented as essentially stable. The potential for oxon exposure remains an
uncertainty, however, because not all routes of transformation are accounted for in the
guideline studies.
Tables 2.2 and 2.3 list the environmental fate properties of disulfoton and the degradates
d. sulfoxide and d. sulfone and Table 2.4 lists the major and minor degradates detected in
the submitted environmental fate and transport studies. Results of the fate and transport
studies on which these values are based are briefly described below. TTR half-lives
reported in Table 2.2 were estimated based on studies conducted on the parent
compound. For each study, first order log linear TTR half-lives were calculated using,
for each sampling point, the sum of the percent active ingredient recovered for all three
species. Degradate half-lives reported in Table 2.3 were estimated based on the pattern
of decline observed in studies of the parent compound, rather than measured directly.
22
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Table 2.2. Kiivironincnlsil laic ami Iransporl tin In lor tlisull'olon
Sluclj
Piiivnl
Value
Tul;il Toxic
Residues Value
Source
Hydrolysis Half-life
(at 20° C)
pH 4: 1174 d
pH 7: 323 d
pH 9: 231 d
Stable.
MRID 00143405
Aqueous Photolysis Half-life
3.9 d
141 d
MRID 40471102
Soil Photolysis Half -life
2.8 d
385 d
MRID 40471103
Aerobic Soil Metabolism
Half-life
2.4 d
16.6 d
15.6 d
408 d
120 d
257 d
MRID 40042001
MRID 41585101
MRID 43900101
Anaerobic Soil Metabolism
Half-life
No data
No data
n/a
Aerobic Aquatic Metabolism
Half-life
10.7 d
51 d
MRID 46961201
Anaerobic Aquatic
Metabolism Half-life
275 d
385 d
MRID 46316901
Foliar Dissipation Half-life
No data.
3.3 d
MRID 41201801
Soil Water Partition
Coefficient (Kd)
sand: 4.67 L/kgsoli
sandy loam: 9.66 L/kgsoli
silt loam: 6.85 L/kgsoli
clay loam: 4.47 L/kgsoli
See Table 2.3.
MRID 44373103
Organic Carbon Water
Partition Coefficient (Koc)
sand: 888 L/kgoc
sandy loam: 483 L/kgoc
silt loam: 449 L/kgoc
clay loam: 386 L/kgoc
See Table 2.3.
MRID 44373103
Field Dissipation DT50
4 lb a.i./A x 1: 2d, 3.7 d
4 lb a.i./A x 2: <3 d, <3 d
No detections beneath 6"
Sand/sandy loam soils
TTR persisted up
to 271 days. Half-
lives could not be
calculated.
MRID 43042502
Laboratory Volatility Flux
25% field cap.: 0.026 (ig/cm2/hr
75% field cap.: 0.096 (ig/cm2/hr
No data
MRID 42585802
a TTR half-lives were estimated based on studies conducted on the parent compound. For each study, first order log
linear TTR half-lives were calculated using the sum of all three species detected at each sampling point.
23
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Table 2.3. Kiivironincnlsil laic and Iransporl (InIn lor d. sulfoxide mid d. sull'onc
Slii(l\
1). Sulfoxide
1). SuHoik-
Source;I
Hydrolysis Half-life
(at 20° C)
No data
No data
n/a
Aqueous Photolysis Half-
life
As degradation product,
reached 88% on day 10
No data
MRID 40471102
Soil Photolysis Half -life
As degradation product,
appeared stable between
days 15 and 30
No data
MRID 40471103
Aerobic Soil Metabolism
Half-life
77 d
68 d
63 d
133 d
257 d
MRID 40042001
MRID 41585101
MRID 43900101
Anaerobic Soil Metabolism
Half-life
No data
No data
n/a
Aerobic Aquatic
Metabolism Half-life
46 d
76 d
MRID 46961201
Anaerobic Aquatic
Metabolism Half-life
39 db
120 db
MRID 46766603
MRID 46766604
Foliar Dissipation Half-life
No data
No data
n/a
Soil Water Partition
Coefficient (Kd)
sandy loam: 0.6 L/kgsoli
sandy loam: 1.7 L/kgsoli
silt loam: 0.3 L/kgsoli
loam: 3.5L/kgsoli
sandy loam: 1.36 L/kgsoli
sandy loam: 2.49 L/kgsoli
silt loam: 0.43 L/kgsoli
loam: 5.90 L/kgsoli
MRID 46766601
MRID 46766602
Organic Carbon Water
Partition Coefficient (Koc)
sand: 63 L/kgoc
sandy loam: 94 L/kgoc
silt loam: 61L/kgoc
clay loam: 62 L/kgoc
sand: 136 L/kgoc
sandy loam: 138 L/kgoc
silt loam: 87 L/kgoc
clay loam: 104 L/kgoc
MRID 46766601
MRID 46766602
Field Dissipation
(DT50s could not be calc'd)
In 3 of 4 sites, was
detected up to 90 days,
the final sampling period.
Persisted up to 178 to 364
days. Detected at 6-12".
MRID 43042502
a Unless otherwise noted, degradation rates were estimated based on the pattern of decline observed in studies of the
parent compound, rather than measured directly,
b Measured directly.
24
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Table 2.4. Maximum do»r;ul;ilo :i moil ills in environmental Into studios ol'distil Colon
Moliiholilo
M;i\. Dounidiilc "» of Applied (Time of po;ik)'
M\drol\sis
(pi 1 4.
A(|ii:ilic
Pho(ol\sis
Soil
Pho(ol\sis
Aiiiioi'ohic
A(|ii;ilic
Mol ;i holism
Aerobic
A(|ii;ilic
Mchibolisnr
Aei'ohic Soil
Meliiholism
in ')
Disulfoton Oxon
1-3%
(30 d*)
Disulfoton
Sulfoxide
4-6%
(7 d)
89%
(9.6 d*)
95%
(30 d*)
15%
(364 d*)
63%
(3d)
37-62%
(3-7d)
Disulfoton Oxon
Sulfoxide
0.7%
(9.6*)
4%
(30 d*)
Disulfoton Sulfone
1%
(20 d)
2%
(270 d)
19%
(28)
53-72%
(14-90 d)
Sulfonic Acid
2%
(364 d*)
16%
(59 d)
C02*
0.6%
29%
0-39%
Bound Residues*
34%
32%
24-39%
* Maximum reached at study termination.
1 Unless otherwise noted, unidentified degradates were <3.7% and volatile organic carbon compounds were not
detected.
2 Unidentified degradates reached 11.1% at study termination (168 d).
Abiotic Degradation
Disulfoton is essentially stable to hydrolysis at 20°C, with reported half-lives of 1,174
days, 323 days, and 231 days in sterile aqueous buffered solutions at pH 4, 7, and 9,
respectively. At 40°C, the half-lives were 30, 23.2, and 22.7 days at pH 4, 7, and 9,
respectively. There were no major degradates. Minor degradates included disulfoton
oxon and d. sulfoxide.
In both aqueous and soil environments, disulfoton is photolyzed primarily to d. sulfoxide
with half-lives of 3.9 d in water and 2.8 d on soil (both dark corrected). By study
termination (aqueous, 9.6 d; soil, 30 d), d. sulfoxide had reached 89% and 95% of the
applied respectively. Disulfoton oxon sulfoxide was also formed at low levels (aqueous,
0.7%; soil, 4%). Total toxic residues of disulfoton are therefore essentially stable.
Microbial metabolism in soil
In three aerobic soil metabolism studies in sandy loam soils, disulfoton degraded rapidly,
with DT50s of <3 days in all cases and calculated first-order half-lives of 2.4 to 16.6 days.
In the one study with a sterile control, degradation occurred with a half-life of less than
one month, so the degradation may be due in part to abiotic processes rather than
microbial metabolism.
The primary transformation products are d. sulfoxide and d. sulfone, so total toxic
residues are much more persistent than the parent alone, with 20% to 58% of the applied
compound remaining as total toxic residues by day 270 and TTR half-lives of 120 to 408
days. In all three studies, the parent compound readily oxidizes to d. sulfoxide which
then undergoes further oxidation to the more stable d. sulfone, leading to carbon dioxide
25
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and bound residues as the end products. At study termination, d.sulfone is the primary
compound remaining (17% to 51%), with parent at <1% and d. sulfoxide at 2% to 6%.
In the two studies for which the pattern of decline for d. sulfoxide and d. sulfone could be
determined, estimated first-order half-lives were 63 to 68 d and 133 to 257 d,
respectively. In two of the three studies, transformation of d.sulfone slowed at the end,
remaining nearly stable in the final sampling periods, so the calculated half-lives may
underestimate its persistence.
No acceptable studies of disulfoton metabolism in anaerobic soil conditions have been
submitted.
Microbial metabolism in water-sediment systems
Acceptable aquatic metabolism studies are available for disulfoton in aerobic and
anaerobic conditions and for the degradates d. sulfoxide and d. sulfone in anaerobic
conditions.
In aerobic conditions, disulfoton was observed to degrade rapidly with a total system
first-order half-life of 10.9 d. It was undetectable in water by day 14 and in sediment by
day 90. Toxic residues d. sulfoxide and d. sulfone were formed as major degradates,
reaching total system maximum levels of 63% (day 3) and 19% (day 28), respectively,
and declining with half-lives of 46 d and 76 d. Another major degradate was sulfonic
acid, formed at up to 15.8% (day 59). At study termination (168 d), each major degradate
was at 5-7%), and the remaining residues were present as bound residues (32%) and
carbon dioxide (29%). The total toxic residues half-life was 51 d.
In anaerobic conditions, disulfoton is much more persistent than in aerobic conditions.
With a total system half-life of 227 days, 25.1% remained as parent at day 364. D.
sulfoxide was the only major degradate and reached a maximum of 14.6% in the final
sample. The total toxic residues half-life was 385 d.
A study of d. sulfoxide applied directly in an anaerobic system also demonstrates that
disulfoton is more stable. Over the first 30 days, d. sulfoxide transformed to disulfoton
with a half-life of 12.6 days. Through the final 3 months of the study (to 120 days), the
system remained relatively stable with disulfoton at 75-77%, d. sulfoxide at 10-12%, and
bound residues at 4-6%. During this period, 83-87%) of the residues were found in the
sediment. No d. sulfone was detected throughout the study.
D. sulfone applied directly to an anaerobic water-sediment system partitioned to sediment
at a steady rate without any other degradates formed. By the end of the study (120 days),
20% of the applied sulfone was in the water, 39.1% was extractable as parent in the
sediment, and the remaining was present as bound residue.
Field and Foliar Dissipation
Disulfoton dissipation has been measured in terrestrial field environments and on foliage.
In the field, disulfoton was applied at sites in Fresno, CA and Watsonville, CA as one and
26
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two applications of 4 lbs a.i./A each. In all 4 tests, disulfoton DT50s of 2 to 4 days were
observed in the upper 6 inches of soil and there were no detections below 6 inches. D.
sulfoxide and d. sulfone were detected as well and both were more persistent than the
parent and showed greater tendency to leach. Oxons were not analyzed for. D. sulfoxide
had peak levels within the first week of sampling and in all but one of the sites was still
detected at low levels at 90 days, the final sample date. Two sites had single detections
of d. sulfoxide at low levels in the 6 to 12 inch layer. D. sulfone was more persistent and
showed more tendency to leach. It was detected at low levels for most of the study
period. In the four sites, peak concentrations were recovered in the first two weeks and
the first samples with no detections were found between 178 and 364 days. Both Fresno
plots had detections of d. sulfone at depths of up to 18 inches. In one plot, d. sulfone was
detected in the 6 to 12 inch layer on sampling periods between 14 and 271 days in the
other, there were detections between 28 days and 180 days. In both plots, the final
detection was also the highest concentration. In the 12 to 18 inch layer, several
detections at low levels were found between 90 and 180 days. Application rates were not
confirmed and recoveries at some sampling points exceeded initial recoveries, so
concentrations cannot be reported as percent applied.
A foliar dissipation rate of 3.3 days was calculated based on field monitoring data from a
study in Michigan in which disulfoton was aerially applied to potatoes 3 times at 1 lb
ai/A. Foliar dissipation rate estimates are based on potato foliage samples which were
collected up to 14 days after the third treatment.
2.4.3 Environmental Transport Mechanisms
Potential transport mechanisms include pesticide surface water runoff, spray drift, and
secondary drift of volatilized or soil-bound residues leading to deposition onto nearby or
more distant ecosystems. Surface water runoff and spray drift are expected to be the
major routes of exposure for disulfoton. The toxic degradates (d. sulfoxide and d. sulfone)
have the potential to move vertically down through the soil profile, and potentially into
groundwater, as they form primarily in the shallow subsurface. Groundwater that contains
disulfoton residues may then be discharged into surface waters as baseflow. Soil adsorption
and volatility data are discussed below.
Mobility
Disulfoton is moderately mobile in soil. The major degradates d. sulfoxide and d. sulfone
are more mobile than the parent. Considering the greater persistence of the degradates as
well, their higher mobility indicates that they are more likely to runoff and/or leach.
Based on batch equilibrium studies in four soils, the mean KF for disulfoton was 6.4
mL/g, while for d. sulfoxide and d. sulfone, mean KF values were 1.5 mL/g and 2.5 mL/g,
respectively. The Koc model appears to be appropriate to describe disulfoton adsorption.
Normalized for organic carbon content, Koc values for the parent and the sulfoxide and
sulfone degradates were 551, 70, and 161 mL/g0C, respectively.
Volatility
Disulfoton has been classified as slightly volatile (Burkhard and Guth, 1981, and EPA,
1975). The vapor pressure (1.8 x 10"4mm Hg) and Henry's Law Constant (2.6 x 10"6
27
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atm-m3/mole) values for disulfoton indicate some degree of volatility from both soil and
water. Measured vapor pressure values and Henry's law constants are not available for
the major degradates, but estimates using EPI-SUITE suggest that the degradates are less
likely to volatilize than the parent (D. sulfoxide: v.p. = 3.65 x 10"5, KH = 1.32 x 10"10
atm-m3/mole; D. sulfone: v.p. = 1.12 x 10"5 mm Hg, KH = 1.67 x 10"8 atm-m3/mole).
Given the rapid degradation of the parent compound, lower volatility of the degradates
would lead to lower potential for volatilization of total toxic residues.
Disulfoton was not observed to volatilize in any of the aerobic soil metabolism studies,
but in a laboratory volatilization study, maximum volatilization of 0.026 and 0.096
|ig/cm2/hr was seen in the first 24 hours from sand soil adjusted to 25% and 75% of field
capacity. The study was conducted at 25°C with an air flow of approximately 300
ml/minute (MRID 42585802). Volatility was measured in the field as well (MRID
40471105). The maximum concentration observed in air at 1 foot above ground was 22.2
ng/L. After 6 hours, disulfoton concentrations at the 5 foot level were not detectable.
Photooxidation of OP pesticides in air can be an important route of oxon formation but
no studies have been conducted to test this in disulfoton. Based on the field volatility
study, the maximum amount of oxon that could be formed at the 1 ft level would be 20.9
ng/L, assuming that all volatilized disulfoton were oxidized.
Long Range Atmospheric Transport
Based on the measured physical properties of disulfoton and its degradates, as well as on
field volatility studies, long range atmospheric transport is not expected to be an
important exposure pathway. Available air monitoring studies in the Central Valley and
Sierra Nevada do not include disulfoton as an analyte
(http://www.cdpr.ca.gov/docs/empm/pubs/tac/tacstdvs.htm;
http://www.nature.nps.gov/air/Studies/air toxics/wacap.cfm; Majewski, 1995). One
national study was found that tested for disulfoton at 10 sites, finding it in only 1 out of
123 samples at a concentration of 0.0047 ng/L (Carey and Kutz, 1985).
2.4.4 Mechanism of Action
Organophosphate toxicity is based on the inhibition of the enzyme acetylcholinesterase
which cleaves the neurotransmitter acetylcholine. Inhibition of acetylcholinesterase by
organophosphate insecticides, such as disulfoton, interferes with proper
neurotransmission in cholinergic synapses and neuromuscular junctions (USEPA 2002).
2.4.5 Use Characterization
Analysis of labeled use information is the critical first step in evaluating the federal
action. The current labels for disulfoton represent 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.
28
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Currently approved agricultural uses of disulfoton include asparagus, beans, broccoli,
Brussels sprouts, cabbage, cauliflower, Christmas trees, cotton, and lettuce. Residential
uses are allowed as well, including uses on vegetable gardens and flower beds. All
agricultural uses require a 25 ft vegetated buffer strip around water bodies. Disulfoton is
also registered for some residential uses. Table 2.5 presents the uses and corresponding
application rates and methods of application considered in this assessment.
Table 2.5 Disulfoton Uses Assessed for the CRLF
Use
Application Method
Form
Max. Annual
App Rate'
(lb a.i./A)
Asparagus
Ground spray
Aerial spray
EC
2
(1 lb a.i./A/app, 2 apps/yr)
Beans
Soil injection (side of furrow)
EC
1
Broccoli
Soil injection b
EC
1
Brussels sprouts
Soil injection b
EC
1
Cabbage
Soil injection13
Ground spray (broadcast)0
EC
1 (spray)
2 (soil injection)
Cauliflower
Soil injection b
EC
1
Christmas trees
Broadcast; wetted in
G
4.5
Cotton
Ground spray (in furrow)
Soil injection (side of furrow)
EC
1
Drill planting (in furrow)
G
0.975
Hill-drop planting (in furrow)
G
0.375
Lettuce
Chemigation (drip or trickle)
Soil injection0
EC
2
Residential Uses
(Vegetables, Flowers)
Hand application, broadcast
or per plant
G
1.6 e
a Unless otherwise specified, labels allow only a single application per year.
b Pre-seeding in transplant beds; 2-3 inch incorporation.
0 Side of furrow (at planting) or side-dress (post-emergent).
d Not specified.
e The highest labeled application rate is 0.2 lb a.i./lOOO ft2. Assuming a flower bed/garden size per lot of 200 ft x
10 ft and 4 lots per acre, this is equivalent to 1.6 lb a.i./A.
The Agency's Biological and Economic Analysis Division (BEAD) provides an analysis
of both national- and county-level usage information (Kaul and Jones, 2006) using state-
level usage data obtained from USDA-NASS1, Doane (www.doane.com; the full dataset
is not provided due to its proprietary nature) and the California's Department of Pesticide
1 United States Depart of Agriculture (USDA), National Agricultural Statistics Service (NASS) Chemical
Use Reports provide summary pesticide usage statistics for select agricultural use sites by chemical, crop
and state. See http://www.usda.gov/nass/pubs/estindxl,htm#agchem.
29
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Regulation Pesticide Use Reporting (CDPR PUR) database2. CDPR PUR is considered a
more comprehensive source of usage data than USDA-NASS or proprietary databases,
and thus the usage data reported for disulfoton by county in this California-specific
assessment were generated using CDPR PUR data. Four years (2002-2005) of usage data
were included in this analysis. Data from CDPR PUR were obtained for every pesticide
application made by professional applicators on every use site at the section level
(approximately one square mile) of the public land survey system. BEAD summarized
these data to the county level by site, pesticide, and unit treated. Calculating county-level
usage involved summarizing across all applications made within a section and then across
all sections within a county for each use site and for each pesticide. The county level
usage data that were calculated include: average annual pounds applied, average annual
area treated, and average and maximum application rate across all five years. The units
of area treated are also provided where available. According to the CDPR PUR
database, a total of 54,554 lbs of disulfoton were used in California in 2002, dropping to
31,512 lbs in 2005. Figures 2.4 and 2.5 below show the reported average annual number
of pounds used in each county and for each crop between 2002 and 2005. These data
indicate that the predominant use of disulfoton was on asparagus in San Joaquin county,
representing a quarter of total average annual use over this time period. Another quarter
of total average annual use was in Monterey county, primarily on broccoli, lettuce, and
asparagus. This analysis is not entirely representative of current use patterns because
residential uses are not included in these data, and because labeled uses have changed
since these data were collected. Uses on peppers and wheat, which make up substantial
portions of total use in this analysis, have been cancelled and phased out over the last few
years. Data from the CDPR PUR database are presented in Appendix B.
2 The California Department of Pesticide Regulation's Pesticide Use Reporting database provides a census
of pesticide applications in the state. See http://www.cdpr.ca.gov/docs/pur/purmain.htm.
30
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Total Average lbs Applied Annually by crop
t he sun? A/g I be
17000
A
B
B
B
B
B
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O op
Figure 2.4. Summary of Disulfoton Use by Crop in California from 2002 to 2005
31
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Total Average lbs Applied Annually by county
LUAOOEL RL MNEI AOAEEOOERLI AAAAAAAAAAAHI OOTUUEO
ATLLNL EE PYRNSSRN RDNVAAVCNN NN NNNNN NAS L NAT L NL
MTAUT DSNEONGS I DCOT ANCER TTT SKAONT AT O
EEVSRNONNR SEANOECEDGERABBDFJ LMAAATI N M I ERU
D EAAORO I NN CD RAERSMEEI ROUA AYOASRER
AR RA A G I E I ENREAAI TBCC O L A
ACTD L EN Y DNI NGNQSEALR U A
SOEO LO ETTAOCU ORAU U
S E OORIIOBRZ S
T S DSNBAA
A I C I R
N O S A
O P
County
Figure 2.5. Summary of Disulfoton Use by County in California from 2002 to 2005
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.
32
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2.5.1 Distribution
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 elevation 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 2.6). 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.
Recovery Units
Eight recovery units have been established by USFWS for the CRLF. These areas are
considered essential to the recovery of the species, and the status of the CRLF "may be
considered within the smaller scale of the recovery units, as opposed to the statewide
range" (USFWS 2002). Recovery units reflect areas with similar conservation needs and
population statuses, and therefore, similar recovery goals. The eight units described for
33
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the CRLF are delineated by watershed boundaries defined by US Geological Survey
hydrologic units and are limited to the elevation maximum for the species of 1,500 m
above sea level. The eight recovery units for the CRLF are listed in Table 2.6 and shown
in Figure 2-6.
Core Areas
USFWS has designated 35 core areas across the eight recovery units to focus their
recovery efforts for the CRLF (see Figure 2-6). Table 2.6 summarizes the geographical
relationship among recovery units, core areas, and designated critical habitat. The core
areas, which are distributed throughout portions of the historic and current range of the
species, represent areas that allow for long-term viability of existing populations and
reestablishment of populations within historic range. These areas were selected because
they: 1) contain existing viable populations; or 2) they contribute to the connectivity of
other habitat areas (USFWS 2002). Core area protection and enhancement are vital for
maintenance and expansion of the CRLF's distribution and population throughout its
range.
For purposes of this assessment, designated critical habitat, currently occupied (post-
1985) core areas, and additional known occurrences of the CRLF from the CNDDB are
considered. Historically occupied sections of the core areas are not evaluated as part of
this assessment because the USFWS Recovery Plan (USFWS 2002) indicates that CRLFs
are extirpated from these areas. A summary of currently and historically occupied core
areas is provided in Table 2.6 (currently occupied core areas are bolded). While core
areas are considered essential for recovery of the CRLF, core areas are not federally-
designated critical habitat, although designated critical habitat is generally contained
within these core recovery areas. It should be noted, however, that several critical habitat
units are located outside of the core areas, but within the recovery units. The focus of this
assessment is currently occupied core areas, designated critical habitat, and other known
CNDDB CRLF occurrences within the recovery units. Federally-designated critical
habitat for the CRLF is further explained in Section 2.6.
34
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Table 2.6 California Red-legged Frog Recovery Units with Overlapping Core
Areas and Designated Critical Habitat
Recovery Unit1
(Figure 2.a)
Core Areas 2,1 (Figure 2.a)
Critical Habitat
Units3
Currently
Occupied
(post-1985)
4
Historically
Occupied 4
Sierra Nevada
Foothills and Central
Valley (1)
(eastern boundary is
the 1,500m elevation
line)
Cottonwood Creek (partial)
(8)
--
Feather River (1)
BUT-1A-B
Yuba River-S. Fork Feather
River (2)
YUB-1
--
NEV-16
Traverse Creek/Middle Fork
American River/Rubicon (3)
--
Consumnes River (4)
ELD-1
S. Fork Calaveras River (5)
--
Tuolumne River (6)
--
Piney Creek (7)
—
East San Francisco Bay
(partial)(16)
--
North Coast Range
Foothills and
Western Sacramento
River Valley (2)
Cottonwood Creek (8)
--
Putah Creek-Cache Creek (9)
--
Jameson Canyon - Lower
Napa Valley (partial) (15)
--
Belvedere Lagoon (partial)
(14)
--
Pt. Reyes Peninsula (partial)
(13)
--
North Coast and
North San Francisco
Bay (3)
Putah Creek-Cache Creek
(partial) (9)
--
Lake Berryessa Tributaries
(10)
NAP-1
Upper Sonoma Creek (11)
--
Petaluma Creek-Sonoma
Creek (12)
--
Pt. Reyes Peninsula (13)
MRN-1, MRN-2
Belvedere Lagoon (14)
--
Jameson Canyon-Lower
Napa River (15)
SOL-1
South and East San
Francisco Bay (4)
--
CCS-1A6
East San Francisco Bay
(partial) (16)
ALA-1A, ALA-
IB, STC-1B
--
STC-1A6
South San Francisco Bay
(partial) (18)
SNM-1A
Central Coast (5)
South San Francisco Bay
(partial) (18)
SNM-1A, SNM-
2C, SCZ-1
Watsonville Slough- Elkhorn
Slough (partial) (19)
SCZ-2 5
Carmel River-Santa Lucia
(20)
MNT-2
35
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Estero Bay (22)
--
--
SLO-86
Arroyo Grande Creek (23)
--
Santa Maria River-Santa
Ynez River (24)
--
Diablo Range and
Salinas Valley (6)
East San Francisco Bay
(partial) (16)
MER-1A-B,
STC-1B
--
SNB-16, SNB-26
Santa Clara Valley (17)
--
Watsonville Slough- Elkhorn
Slough (partial)(19)
MNT-1
Carmel River-Santa Lucia
(partial)(20)
--
Gablan Range (21)
SNB-3
Estrella River (28)
SLO-1A-B
Northern Transverse
Ranges and
Tehachapi Mountains
(7)
--
SLO-86
Santa Maria River-Santa
Ynez River (24)
STB-4, STB-5,
STB-7
Sisquoc River (25)
STB-1, STB-3
Ventura River-Santa Clara
River (26)
VEN-1, VEN-2,
VEN-3
--
LOS-16
Southern Transverse
and Peninsular
Ranges (8)
Santa Monica Bay-Ventura
Coastal Streams (27)
--
San Gabriel Mountain (29)
--
Forks of the Mojave (30)
--
Santa Ana Mountain (31)
--
Santa Rosa Plateau (32)
--
San Luis Rey (33)
--
Sweetwater (34)
--
Laguna Mountain (35)
--
1 Recovery units designated by the USFWS (USFWS 2000, pg 49).
2 Core areas designated by the USFWS (USFWS 2000, pg 51).
3 Critical habitat units designated by the USFWS on April 13, 2006 (USFWS 2006, 71 FR 19244-19346).
4 Currently occupied (post-1985) and historically occupied core areas as designated by the USFWS
(USFWS 2002, pg 54).
5 Critical habitat unit where identified threats specifically included pesticides or agricultural runoff
(USFWS 2002).
6 Critical habitat units that are outside of core areas, but within recovery units.
7 Currently occupied core areas that are included in this effects determination are bolded.
36
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Legend
] Recovery Unit Boundaries
Currently Occupied Core Areas
| Critical Habitat
| CNDDB Occurence Sections
County Boundaries q 45
I 1 i_
180 Miles
Figure 2.2 Recovery Unit, Core Area, Critical Habitat, and Occurrence
Designations for CRLF
Recovery Units
Sierra Nevada Foothills and Central Valley
North Coast Range Foothills and Western
Sacramento River Valley
North Coast and North San Francisco Bay
South and East San Francisco Bay
Central Coast
Diablo Range and Salinas Valley
Northern Transverse Ranges and Tehachapi
Mountains
Southern Transverse and Peninsular Ranges
Core Areas
1.
Feather River
2.
Yuba River- S. Fork-Feather River
3.
Traverse Creek/ Middle Fork/ American R. Rubicon
4.
Cosumnes River
5.
South Fork Calaveras River*
6.
Tuolumne River*
7.
Piney Creek*
8.
Cottonwood Creek
9.
Putah Creek - Cache Creek*
10.
Lake Berryessa Tributaries
11.
Upper Sonoma Creek
12.
Petaluma Creek - Sonoma Creek
13.
Pt. Reyes Peninsula
14.
Belvedere Lagoon
15.
Jameson Canyon — Lower Napa River
16.
East San Francisco Bay
17.
Santa Clara Valley
18.
South San Francisco Bay
19.
Watsonville Slough-Elkhorn Slough
20.
Carmel River — Santa Lucia
21.
Gablan Range
22.
Estexo Bay
23.
Arroyo Grange River
24.
Santa Maria River — Santa Ynez River
25.
Sisquoc River
26.
Ventura River — Santa Clara River
27.
Santa Monica Bay — Venura Coastal Streams
28.
Estrella River
29.
San Gabriel Mountain*
30.
Forks of the Mojave*
31.
Santa Ana Mountain*
32.
Santa Rosa Plateau
33.
San Luis Ray*
34.
Sweetwater*
35.
Laguna Mountain*
* Core areas that were historically occupied by the California
red-legged frog are not included in the map
37
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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.
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 2.7 depicts CRLF annual reproductive timing.
Figure 2.7 - CRLF J
reproductive 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,
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
39
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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 (USFWS 2002). According to
Jennings and Hayes (1994), CRLFs also use small mammal burrows and moist leaf litter
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 Table 2.6.
'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 adverse
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 adverse 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;
40
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• 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
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 adverse modification standards for designated critical habitat
(USFWS 2006). Activities that may destroy or adversely modify critical habitat are those
that alter the PCEs and jeopardize the continued existence of the species. Evaluation of
actions related to use of disulfoton 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) 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.
(3) 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.
(4) Elimination of upland foraging and/or aestivating habitat or dispersal habitat.
(5) Introduction, spread, or augmentation of non-native aquatic species in stream
segments or ponds used by the CRLF.
(6) Alteration or elimination of the CRLF's food sources or prey base (also
evaluated as indirect effects to the CRLF).
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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 disulfoton is expected to directly impact living
organisms within the action area, critical habitat analysis for disulfoton 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.
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 disulfoton 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 disulfoton may be expected to have on the
environment, the exposure levels to disulfoton that are associated with those effects, and
the best available information concerning the use of disulfoton 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 disulfoton. 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
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disulfoton, the following agricultural uses are considered as part of the federal action
evaluated in this assessment: asparagus, beans, broccoli, Brussels sprouts, cabbage,
cauliflower, Christmas trees, cotton, and lettuce. In addition, residential uses on flowers,
shrubs, and ornamentals are also considered.
Following a determination of the assessed uses, an evaluation of the potential "footprint"
of disulfoton 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 disulfoton is presented in Figure 2.8. The development of
this map is described in Appendix C.
Once the initial area of concern is defined, the next step is to define the potential
boundaries of the action area by determining the extent of offsite transport via spray drift
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 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 adverse effect concentration (NOAEC) for several
reported effects (see Appendix D, ECOTOX summary), the spatial extent of the action
area (i.e., the boundary where exposures and potential effects to some component of the
ecosystem are less than the Agency's LOC) for disulfoton 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). This does not mean that there is no level below which effects are not expected to
occur, but only that the available data do not allow for a determination of such a
threshold.
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Figure 2.8 Initial area of concern, or "footprint" of potential use, for disulfoton
Disulfoton use - Initial Area of Concern
Compiled from California County boundaries (ESRI, 2002),
US DA G a p An aly si s P rog ra m 0 rch ard/ V in ey ard La ndc ov er (G A F1)
National Land Cover Data base (NLCD) (MRLC, 2001)
Map created Py US Environmental Protection Agency, Office
of Pesticides Programs, Environmental Fate and Effects Division.
Projection: Albers Equal Area Conic USGS, North American
Datum of 1983 (NAD 1 983).
Produced 3C6/2008
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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."3 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
disulfoton (e.g., runoff, spray drift, etc.), and the routes by which ecological receptors are
exposed to disulfoton (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 (Appendix I) 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.
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 disulfoton is provided in Table 2.7.
3 From U.S. EPA (1992). Framework for Ecological Risk Assessment. EPA/630/R-92/001.
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Tsihle 2.7 Assessment l.ndpoinls ;iihI Mesisures of Kcologicsil KITecls
Assessment Endpoint
Measures of Ecological Effects Typically Used for
Risk Assessment4
Aquatic-Phase CRLF
(Eggs, larvae, juveniles, and adults"f
Direct Effects
1. Survival, growth, and reproduction of CRLF
la. Amphibian acute LC50 (ECOTOX) or most sensitive
fish acute LC50 (guideline or ECOTOX) if no suitable
amphibian data are available
lb. Amphibian chronic NOAEC (ECOTOX) or most
sensitive fish chronic NOAEC (guideline or ECOTOX)
lc. Amphibian early-life stage data (ECOTOX) or most
sensitive fish early-life stage NOAEC (guideline or
ECOTOX)
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)
2a. Most sensitive fish, aquatic invertebrate, and aquatic
plant EC50 or LC50 (guideline or ECOTOX)
2b. Most sensitive aquatic invertebrate and fish chronic
NOAEC (guideline or ECOTOX)
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)
3 a. Vascular plant acute EC50 (duckweed guideline test
or ECOTOX vascular plant)
3b. Non-vascular plant acute EC50 (freshwater algae or
diatom, or ECOTOX non-vascular)
4. Survival, growth, and reproduction of CRLF
individuals via effects to riparian vegetation
4a. Distribution of EC25 values for monocots (seedling
emergence, vegetative vigor, or ECOTOX)
4b. Distribution of EC25 values for dicots (seedling
emergence, vegetative vigor, or ECOTOX)
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. Most sensitive birdb or terrestrial-phase amphibian
acute LC50 or LD50 (guideline or ECOTOX)
5b. Most sensitive birdb or terrestrial-phase amphibian
chronic NOAEC (guideline or ECOTOX)
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. Most sensitive terrestrial invertebrate and vertebrate
acute EC50 or LC50 (guideline or ECOTOX)0
6b. Most sensitive terrestrial invertebrate and vertebrate
chronic NOAEC (guideline or ECOTOX)
7. Survival, growth, and reproduction of CRLF
individuals via indirect effects on habitat (i.e.,
riparian and upland vegetation)
7a. Distribution of EC25 for monocots (seedling
emergence, vegetative vigor, or ECOTOX
7b. Distribution of EC25 for dicots (seedling emergence,
vegetative vigor, or ECOTOX)
a 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 Birds are used as surrogates for terrestrial phase amphibians.
4 All registrant-submitted and open literature toxicity data reviewed for this assessment are included in
Appendix A.
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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 disulfoton 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.
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 disulfoton effects data are available.
Adverse modification to the critical habitat of the CRLF includes, but is not limited to,
the following, as specified by USFWS (2006):
• Alteration of water chemistry/quality including temperature, turbidity, and oxygen
content necessary for normal growth and viability of juvenile and adult CRLFs.
• Alteration of chemical characteristics necessary for normal growth and viability
of juvenile and adult CRLFs.
• Significant increase in sediment deposition within the stream channel or pond or
disturbance of upland foraging and dispersal habitat.
• Significant alteration of channel/pond morphology or geometry.
• Elimination of upland foraging and/or aestivating habitat, as well as dispersal
habitat.
• Introduction, spread, or augmentation of non-native aquatic species in stream
segments or ponds used by the CRLF.
• Alteration or elimination of the CRLF's food sources or prey base.
Measures of such possible effects by labeled use of disulfoton on critical habitat of the
CRLF are described in Table 2.8. 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
adverse modification standard established by USFWS (2006).
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Table 2.8 Summary of Assessment Endpoints and Measures of Ecological Effect for
Primary Constituent Elements of Designated Critical Habitat
Assessment Endpoint
Measures of Ecological Effect
Aquatic-Phase CRLF 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.
a. Most sensitive aquatic plant EC50 (guideline or
ECOTOX)
b. Distribution of EC25 values for terrestrial monocots
(seedling emergence, vegetative vigor, or ECOTOX)
c. Distribution of EC25 values for terrestrial dicots
(seedling emergence, vegetative vigor, or ECOTOX)
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 (guideline
or ECOTOX)
b. Distribution of EC25 values for terrestrial monocots
(seedling emergence or vegetative vigor, or ECOTOX)
c. Distribution of EC25 values for terrestrial dicots
(seedling emergence, vegetative vigor, or ECOTOX)
Alteration of other chemical characteristics necessary
for normal growth and viability of CRLFs and their
food source.
a. Most sensitive EC50 or LC50 values for fish or aquatic-
phase amphibians and aquatic invertebrates (guideline or
ECOTOX)
b. Most sensitive NOAEC values for fish or aquatic-phase
amphibians and aquatic invertebrates (guideline or
ECOTOX)
Reduction and/or modification of aquatic-based food
sources for pre-metamorphs (e.g., algae)
a. Most sensitive aquatic plant EC50 (guideline or
ECOTOX)
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 (seedling
emergence, vegetative vigor, or ECOTOX)
b. Distribution of EC25 values for dicots (seedling
emergence, vegetative vigor, or ECOTOX)
c. Most sensitive food source acute EC50/LC50 and NOAEC
values for terrestrial vertebrates (mammals) and
invertebrates, birds or terrestrial-phase amphibians, and
freshwater fish.
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.
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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 disulfoton to the environment.
The following risk hypotheses are presumed for this endangered species assessment:
The labeled use of disulfoton 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 disulfoton release mechanisms, biological receptor types, and effects
endpoints of potential concern. The conceptual models for aquatic and terrestrial phases
of the CRLF are shown in Figures 2.9 and 2.10, respectively, and the conceptual models
for the aquatic and terrestrial PCE components of critical habitat are shown in Figures
49
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2.11 and 2.12, respectively. 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
Receptors
Attribute
Change
Disulfoton applied to use site,
and d. sulfoxide and d. sulfone, formed through transformation of the parent
compound in water and soil.
I
| Spray drift | | Runoff |
Surface water/
Sediment
Soil
-W Groundwater
.T
Long range
atmospheric
transport
¦ Wet/dry depo sition ¦
J
T
Uptake/gills
or integument
Uptake/gills
or integument
Uptake/cell,
roots^leaves
Aquatic Animals
Invertebrates
Vertebrates
Ingestion
Red-legged Frog
Eggs Juveniles
Larvae Adult
Tadpoles
I
Individual organisms
Reduced survival
Reduced growth
Reduced reproduction
Aquatic Plants
Non-vascular
Vascular
T
Ingestion
Food chain
Reduction in algae
Reduction in prey
Riparian plant
terrestrial
exposure
pathways see
Figure 2.11
Habitat integrity
Reduction in primary productivity
Reduced cover
Community change
Figure 2.9. Conceptual model for disulfoton effects on aquatic phase of the red-
legged frog.
50
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Stressor
Source
Exposure
Media
Disulfoton applied to use site,
and d. sulfoxide and d. sulfone, formed through transformation of the parent
compound in water and soil.
Runoff
Dermal uptake/
" Ingestion
Root
.*
Long range
atmospheric
transport
Soil
Terrestrial/riparian plants
grasses/forbs, fruit, seeds
(trees, shrubs)
uptake
¦ Wet/dry deposition
-~Ingestion
Ingestion
-~ Ingestion
i : i
Ingestion
Mammals
i
Receptors
Attribute
Change
Red-legged Frog
Juvenile
Adult
Individual organisms
Reduced survival
Reduced growth
Reduced reproduction
Food chain
Reduction in prey
Habitat integrity
Reduction in primary productivity
Reduced cover
Community change
Figure 2.10. Conceptual model for disulfoton effects on terrestrial phase of the red-
legged frog.
51
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Figure 2.11. Conceptual Model for disulfoton Effects on Aquatic Component of
Red-Legged Frog Critical Habitat.
52
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Stressor
Source
Exposure
Media and
Receptors
Disulfoton applied to use site,
and d. sulfoxide and d. sulfone, formed through transformation of the parent
compound in water and soil.
Spray
drift
Dermal uptake/
" Ingestion ¦*"
Terrestrial plants
grasses/forbs, fruit, seeds
(trees, shrubs)
1
Runoff
Soil
Root uptake
Wet/diy deposition
7
*.
Long range
atmospheric
transport
Attribute
Change
Habitat
PCEs
Ingestion
Red-legged Frog
Juvenile
Adult
3
Individual organisms
Reduced survival
Reduced growth
Reduced reproduction
^•Ingestion
Ingestion
i ; [
fMammals"]
t
Other chemical
characteristics
Adversely modified
chemical characteristics
Population
Reduced survival
Reduced growth
Reduced reproduction
Food resources
Reduction in food
Community
Reduced seedling emergence or
vegetative vigor (Distribution)
Elimination and/or disturbance of
upland or dispersal habitat
Reduction in primary productivity
Reduced shelter
Restrict movement
Figure 2.12. Conceptual Model for disulfoton Effects on Terrestrial Component of
the Red-Legged Frog Critical Habitat.
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 disulfoton and its toxic major degradates are
characterized and integrated to assess the risks. This is accomplished using a risk
quotient (ratio of exposure concentration to effects concentration) approach. Exposure
concentrations used in calculating risk quotients account for toxic major degradates of
disulfoton as well as to the parent, either as "total toxic residues" (parent + sulfoxide +
sulfone) or as individual species. 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 disulfoton is estimated using the
probit dose-response slope and either the level of concern (discussed below) or actual
calculated risk quotient value.
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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 disulfoton along with available monitoring data
indicate that runoff and spray drift are the principle potential transport mechanisms of
disulfoton to the aquatic and terrestrial habitats of the CRLF. Disulfoton has a limited
potential for long-range transport. In this assessment, transport of disulfoton through
runoff and spray drift is considered in deriving quantitative estimates of disulfoton
exposure to CRLF, its prey and its habitats.
Measures of exposure are based on aquatic and terrestrial models that predict estimated
environmental concentrations (EECs) of disulfoton and/or its degradates 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 (Appendix I).
Exposure to disulfoton and two degradates were quantified (sulfone and sulfoxide
degradates). The aquatic assessment utilized a total toxic residue approach to quantify
potential risks from exposure to degradates. Potential risks to terrestrial organisms from
exposure to each degradate were quantified. Oxon degradates of disulfoton and its
sulfone and sulfoxide degradates are also a concern, but exposure to these degradates is
considered qualitatively because transformation pathways and routes of exposure are not
accounted for in guideline environmental fate data and because toxicity data are not
available for quantitative comparison.
Aquatic Exposures
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 disulfoton total toxic residues that may occur in
surface water bodies adjacent to application sites receiving disulfoton 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
disulfoton.
PRZM/EXAMS modeling was conducted using a total toxic residues approach to account
for exposure to disulfoton as well as its toxic degradates d. sulfoxide and d. sulfone.
54
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Model inputs were selected based on available environmental fate studies. Half-lives
were calculated from these data to represent the rate of degradation of the total toxic
residues of disulfoton, including the toxic degradates d. sulfoxide and d. sulfone as well
as the parent compound and adsorption inputs were based on the most mobile species.
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 1-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.
Terrestrial Exposures
Terrestrial exposure to disulfoton and two degradates were quantified (sulfone and
sulfoxide degradates). Toxicity data are not available for other degradates to allow for a
risk estimation. However, oxon degradates of disulfoton and its sulfone and sulfoxide
degradates are also a concern. The oxon degradates form at low levels (<4%).
Nonetheless, if the oxon degradates are considerably more toxic than disulfoton, then the
EECs used in this assessment could underestimate potential risks.
Exposure to each degradate with available toxicity data was estimated separately by
considering the highest amount of degradate that formed in available laboratory studies.
The sulfoxide degradate has been shown to form up to 95% of parent and the sulfone
degradate has been shown to form up to 72% of parent. It is acknowledged that exposure
will likely occur to parent and degradates concurrently. The RQs for parent and each
degradate are expected to encompass the potential risks for the total residues that may be
found on food items of the CRLF and its prey.
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. For modeling purposes, direct
exposures of the CRLF to disulfoton 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 disulfoton are bound by using the dietary based EECs for small insects and
large insects.
55
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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 were derived for Christmas
trees using Terrplant (v. 1.2.2.). EECs were only derived for the Christmas tree use
because the toxicity data indicate that the EC25 is higher than the highest labeled
application rate for all disulfoton uses except Christmas trees.
AgDRIFT, a spray drift model was used to assess exposures of terrestrial phase CRLF
and its prey to disulfoton deposited on terrestrial habitats by spray drift. AGDISP
(version 8.13; dated 12/14/2004) (Teske and Curbishley, 2003) is used to simulate aerial
and ground applications. In addition to the buffered area from the spray drift analysis,
the downstream extent of disulfoton 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.
The assessment of risk for direct effects to the terrestrial-phase CRLF makes the
assumption that potential risks to birds is similar to or less than potential risks to
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
LD50, LC50 and ECso- LD stands for "Lethal Dose", and LD50 is the amount of a material,
56
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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.
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
disulfoton, 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 disulfoton 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).
This assessment estimated potential risk from exposure to total toxic residues (TTR) or to
parent disulfoton and specific degradates for which toxicity data exists. Toxicity data are
not available for other degradates to allow for a risk estimation. Of particular concern are
oxon degradates of disulfoton and its sulfone and sulfoxide degradates. The oxon
degradates form at low levels (<4%); however, if the oxon degradates are considerably
more toxic than disulfoton, then the EECs used in this assessment could underestimate
potential risks.
For this endangered species assessment, listed species LOCs are used for comparing RQ
values for acute and chronic exposures of disulfoton directly to the CRLF. If estimated
exposures directly to the CRLF of disulfoton 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
57
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(aquatic and terrestrial invertebrates, fish, frogs, and mice), the listed species LOCs are
also used. If estimated exposures to CRLF prey of disulfoton 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".
3. Exposure Assessment
Disulfoton is formulated as an emulsifiable concentrate and granular formulation.
For most crops, it is applied directly to and incorporated in soil. However, for others,
it may be applied by air or ground spray.
Risks from ground boom and aerial applications are expected to result in the highest
off-target levels of disulfoton 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. Disulfoton is
also labeled for application through soil injection. With soil injection, runoff is the
primary route of off-target transport.
Exposure to major toxic degradates of disulfoton is considered in this assessment as
well as exposure to the parent. The aquatic exposure assessment estimates
concentrations of total toxic residues, or parent + d. sulfoxide + d. sulfone. The
terrestrial exposure assessment estimates exposure to each compound individually.
3.1 Label Application Rates and Intervals
Disulfoton labels may be categorized into two types: labels for manufacturing uses
(including technical grade disulfoton and its formulated products) and end-use
products. While technical products, which contain disulfoton 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 aphids, mites, thrips, and other insect pests.
The formulated product labels legally limit disulfoton's potential use to only those
sites that are specified on the labels.
Currently registered agricultural uses of disulfoton relevant to CRLF critical habitat
in California include use on asparagus, beans, broccoli, Brussels sprouts, cabbage,
cauliflower, Christmas trees, and cotton. Non-agricultural uses include application to
58
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residential flower beds and vegetable gardens. The uses being assessed are
summarized in Table 3.1. More detail about use patterns is provided in Section 2.4.
Presence of a 25-foot well maintained vegetative buffer strip between application
sites and all permanent water bodies is also specified on the labels for all agricultural
uses. For residential uses, determining application rates in lb a.i./A, as necessary for
modeling purposes, required making assumptions about treated area. The highest
residential application rate of 0.02 lb a.i./lOOO ft2, labeled for broadcast granular
application to beds prior to planting, was estimated to be equivalent to 1.6 lb a.i./A,
based on the assumption of 0.25 acre lots with 20 x 100 ft gardens on each lot,
leading to 8,000 ft2 treated per acre.
Table 3.1. Labeled use pattern for each crop, used in assessing disulfoton
environmental exposure.
Uses
Application
Methods
Application
Rate
(lb a.i./A)
No. of Apps.
Application
Interval
(days)
PHI
(days)
Asparagus
Ground spray,
Aerial spray
1
2
71
180
Beans
Soil injection
1
1
n/a
60
Broccoli
Soil injection
1
1
n/a
14
Brussels
sprouts
Soil injection
1
1
n/a
30
Cabbage
Soil injection
2
1
n/a
42
Ground spray
1
1
n/a
40
Cauliflower
Soil injection
1
1
n/a
NSd
Christmas
trees
Broadcast granular
4.5
1
n/a
NS
Cotton
Soil injection,
Ground spray (in
furrow)
1
1
n/a
NS
Drill planting
0.975
1
n/a
NS
Hill drop planting
0.325
1
n/a
60
Lettuce
Soil injection,
Chemigation (drip
or trickle)
2
1
n/a
NS
Residential
Uses
Broadcast granular
1.62
i3
n/a
NS
1 Label does not specify application interval. Modeling assumed that a 7 day interval would be conservative.
2 The highest labeled application rate is 0.2 lb a.i./lOOO ft2 (Reg. No. 432-1286, BEAD LUIS report 8/22/07)
Based on an assumption of 4 lots per acre, each lot with a flower bed/garden of 200 ft x 10 ft, this is equivalent to
1.6 lb a.i./A.
3Label does not specify the number of applications. The broadcast applications are intended for planting time, so
modeling assumed a single application.
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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 disulfoton use. Each of these sites represents a 10
hectare field that drains into a 1-hectare pond that is 2 meters deep and has no outlet.
Exposure estimates generated using the standard 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 first-order streams. As a group, there are factors that make these water
bodies more or less vulnerable than the standard surrogate pond. Static water bodies that
have larger ratios of drainage area to water body volume would be expected to have
higher peak EECs than the standard pond. These water bodies will be either shallower or
have large drainage areas (or both). Shallow water bodies tend to have limited additional
storage capacity, and thus, tend to overflow and carry pesticide in the discharge whereas
the standard pond has no discharge. As watershed size increases beyond 10 hectares, at
some point, it becomes unlikely that the entire watershed is planted to a single crop,
which is all treated with the pesticide. Headwater streams can also have peak
concentrations higher than the standard pond, but they tend to persist for only short
periods of time and are then carried downstream.
Additionally, PRZM/EXAMS modeling does not account for transport to groundwater
followed by discharge to surface water as a possible route of aquatic exposure.
Discharging groundwater is likely to support low-order streams, wetlands, and
intermittent ponds, environments that are favorable to California Red-Legged Frogs
(CRLFs). Groundwater specific modeling is not conducted in this assessment. Long-term
chronic concentrations derived from the PRZM-EXAMS model should reflect
background concentrations that might be found in discharged groundwater/stream
baseflow.
Because the disulfoton degradates d. sulfoxide and d. sulfone are also toxic, modeling
was conducted to estimate exposure to total toxic residues (TTR) of all three compounds.
Crop-specific management practices for all of the assessed uses of disulfoton were used
for modeling, including application rates, number of applications per year, application
intervals, buffer widths and resulting spray drift values modeled from AgDRIFT, and the
first application date for each crop.
3.2.2 Model Inputs
3.2.2.1 Physical Properties and Environmental Fate Inputs
Disulfoton environmental fate data were discussed previously and are listed in Table 2.2.
Chemical-specific model input parameters for PRZM and EXAMS are based on these
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data and are listed in Table 3.2. Environmental fate inputs represent properties of the
total toxic residues (TTR) of disulfoton, including the toxic degradates d. sulfoxide and d.
sulfone as well as the parent compound. Oxon degradates are not considered explicitly in
this estimation. TTR transformation rates were estimated based on studies conducted on
the parent compound. For each study, first order log linear TTR half-lives were
calculated using the sum of all three species detected at each sampling point. Soil-water
partitioning coefficients (Kd) were measured directly for each toxic species. In order to
provide a conservative exposure estimate, the Kd from the most mobile compound, d.
sulfoxide, was chosen as a modeling input.
Table 3.2. PRZM/EXAMS Environmental Fate Inputs for Aquatic Exposure to Total
Toxic Residues of Disulfoton
Fate Property Value1 Comment Source
Molecular Weight
274.39
MRID 150088
Henry's constant
2.6 x 10"6 atm m3/mol
EFED One-liner
5/21/97
Vapor Pressure
1.8 x 10"4 mm Hg
MRID 150088
Solubility in Water
150 mg/L
Measured value x 10
EFED One-liner
5/21/97
Photolysis in Water
141 days
MRID 40471102
Aerobic Soil Metabolism
Half-lives
418 days
Upper 90% confidence bound
on the mean of three values
MRID 40042201,
41585101,43800101
Hydrolysis at pH 7
Stable
TTR remain at 97% at end of
study.
MRID 00143405
Aerobic Aquatic
Metabolism
(water column)
181 days
Single value x 3,
corrected for hydrolysis
MRID 49691201
Anaerobic Aquatic
Metabolism (benthic)
Stable
Single value x 3 = 1100 days
MRID 46316901
Koc
70 mL/goc
Mean Koc for d. sulfoxide, the
most mobile component
MRID 46766601
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
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3.2.2.2 Use-specific Management Practices Inputs
Use specific management practices for all of the assessed uses are also considered in
modeling. Application rates, intervals, and methods were previously listed in Table 3.1.
Scenarios and other crop-specific model input parameters for PRZM and EXAMS are
included in Table 3.3, with justification for these inputs discussed below.
Table 3.3. P
Residues of
RZM/EXAMS Use-Specific Aquatic Exposure Inputs for Total Toxic
)isulfoton
Use
PRZM Scenario
Ap|). date
Application
Method
CAM11
A pp.
Eft'icicncv
(%)
Drift
(%)
Asparagus
CA Row Crop
Sep. 15
Ground spray
2
99
2.7
Aerial spray
2
95
9
Beans
CA Row Crop
Jan. 15
Soil injection
5
100
0
Broccoli,
Cauliflower
CA Cole Crop
Feb. 8
Soil injection
5
100
0
Brussels
sprouts
CA Lettuce
Feb. 1
Soil injection
5
100
0
Cabbage
CA Cole Crop
Feb. 8
Soil injection
5
100
0
Ground spray
1
99
2.7
Christmas
trees
CA Forestry
Aug. 28
Broadcast
(granular)
1
100
0
Cotton
CA Cotton
Apr. 25
Soil injection
5
100
0
Ground spray
(in furrow)
1
99
1
Drill planting
5
100
0
Hill-drop planting
5
100
0
Lettuce
CA Lettuce
Feb. 1
Ground spray
1
99
2.7
Chemigation
(drip or trickle)
1
100
0
Residential
CA Residential
Apr. 1
Broadcast
(granular)
1
100
0
a CAM 1 = Soil applied, default incorporation depth of 4 cm, linearly decreasing with depth
CAM 2 = Foliar applied
CAM 5 = Soil applied, incorporation depth of 4 cm, linearly increasing with depth.
3.2.2.3 PRZM/EXAMS Scenarios and Application Dates
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. A group of scenarios are available to represent crops grown in California,
including standard scenarios as well as scenarios developed specifically for CRLF
assessments. For most disulfoton uses, scenarios are available which were developed
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specifically for that use. In cases where a scenario does not exist for a specific use, it is
necessary to assign a surrogate scenario. Asparagus and beans are both included as crops
for which the California Row Crop scenario was developed. The California Cole Crop
scenario was developed based on broccoli and is appropriate for cauliflower and cabbage
as well, because these are also cole crops with similar cultivation requirements as
broccoli. The California Lettuce scenario was developed for lettuce and is also an
appropriate surrogate for Brussels sprouts, which are leafy vegetables with similar
cultural practices as lettuce. The California Forestry scenario was developed to represent
Northern California forests, which include Christmas tree farms. The California
Residential scenario was developed specifically for residential uses on lawns and
gardens.
Application dates are not specified on product labels. For modeling, the dates of first
application were developed based on label instructions, crop profiles maintained by the
USD A and University of California
(http://www.ipm.ucdavis.edu/PDF/PESTNOTES/index.html
http://www.ipmcenters.org/cropprofiles/GetCropProfiles.cfm; accessed 5/08), historical
use data from the California PUR dataset, and planting dates and precipitation data
specific to each scenario (scenario metadata:
http://www.epa.gov/oppefedl/models/water/state crop.htm). Several of the labeled use
patterns specify application at time of planting while others also allow application after
plants are established, as necessary. Crop profiles indicate that, due to the long pre-
harvest intervals required by labels, disulfoton is typically applied early in the growing
season. Many of these crops can be planted and harvested year-round, so for modeling
purposes the early season was defined by the crop emergence, maturity and harvest dates
specified in each scenario. Modeled application dates were selected to be between the
planting date and the limit of the pre-harvest interval, as defined in the scenario. Within
this window, dates were selected to have a high potential for runoff, as indicated by
precipitation data from local weather stations, in order to provide conservative exposure
estimates. Historical data were considered as well to verify that the chosen application
date adequately represented typical use pattern. For residential uses and application to
Christmas trees, emergence and harvesting dates are less relevant and historical use data
were not available, so other assumptions had to be made. For each disulfoton use, the
selected application date and its justification are defined in Table 3.4.
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Table 3.4. PRZM/EXAM
S Disulfoton Application Dates
Use
App. date
Application date comments
Asparagus
Sep. 15
PUR data 2001-2005 indicate that most applications to asparagus occur in August -
October. Within this time period, Sep. 15 had the highest precipitation.
Beans
Jan. 15
CA Row Crop Scenario {Crop emergence: Jan 1, Crop harvest Apr 8).
Label specifies apply at seeding. Near the scenario planting time, Jan 15 had the highest
precipitation.
Broccoli,
Cabbage,
Cauliflower
Feb. 8
CA Cole Crop Scenario {Crop emergence: Janl, Crop harvest. Marl).
Between planting and the limit of the PHI, Feb. 8 had the highest precipitation.
Brussels sprouts,
Lettuce
Feb. 1
CA Lettuce Scenario {Crop emergence: Feb 16, Crop harvest: May 12).
Between planting and the limit of the PHI, Feb. 1 had the highest precipitation.
Cotton
Apr. 25
CA Cotton Scenario {Crop emergence: May 1, Crop harvest: Nov. 11)
Label specifies apply at seeding. Near the scenario planting time, Apr 25 had the highest
precipitation.
Christmas Trees
Jun. 28
No PUR historical use data are available for Christmas trees. UC crop profiles indicate
that pest pressure from spider mites and aphids, the primary targets of disulfoton on
Christmas trees, is highest between June and September. Multi-run modeling found that
within this season, Jun 28 provided conservative EECs.
Residential
Apr. 1
Most residential uses intend that disulfoton be applied at planting. For modeling, EFED
assumed that residential planting occurs in the spring. Apr. 1 was the highest
precipitation date in this season.
3.2.2.5 Application Methods
Modeling parameters for which inputs are based on the application method include
Chemical Application Method (CAM) as well as application efficiency and spray drift.
CAM 1 represents application directly to soil and was used for ground spray prior to crop
emergence, chemigation, and broadcast granular application methods. This CAM
assumes maximum active ingredient on the soil surface with concentrations decreasing
linearly with depth to a default incorporation depth of 4 cm. CAM 2 represents foliar
applications and was input for the asparagus use for both ground and aerial spray
methods because the asparagus label indicates the application is made after emergence.
CAM 5 represents application beneath the soil, with concentrations increasing linearly
with depth to a maximum concentrations at a user defined depth. In this case, the
incorporation depth was defined as 4 cm. Disulfoton is intended to be applied near plant
roots, and crop profiles indicate that most of these crops are planted to depths of less than
1 inch. Therefore a 4 cm injection depth was used in order to be consistent with the
default incorporation assumptions for CAM 1.
Application efficiency inputs were set to the default values corresponding to each
application method: 100% efficiency for all soil injection, granular, and drip irrigation
applications, 99% for ground spray applications, and 95% for aerial spray applications.
Spray drift is set to 0% for granular and soil injection application methods, because it is
assumed that using these methods, all active ingredient remains on the treated field and
that drift is not a factor. For spray applications, spray drift inputs were estimated using
the AgDRIFT model in order to account for the effect of the label requirement for 25-ft
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buffer zones between cropped areas and water bodies. For aerial applications, the
AgDRIFT default is to assume a droplet size distribution that is fine to medium. This
leads to a spray drift estimate of 9% of the applied active ingredient. For broadcast
ground spray applications, the estimated drift was 2.7% of the applied, based on the
conservative assumptions that a high boom height is used and that the droplet size
distribution is very fine to fine. Some ground spray applications of disulfoton are
specified as in-furrow spray which has a lower potential for drift than broadcast
applications. In these cases, use of a low boom was assumed to be more appropriate,
leading to an estimated drift of 1% of the applied. The low boom height considered by
AgDRIFT is 4 ft, which is likely conservative for an in-furrow spray application.
3.2.3 Results
PRZM/EXAMS EECs representing l-in-10 year peak, 21-day, and 60-day concentrations
of total toxic residues of disulfoton in the aquatic environment are located in Table 3.5.
All model output are included in Appendix E. Estimated aquatic exposures are highest
for disulfoton use on lettuce with a peak EEC of 67 ug/L. Peak EECs for other
agricultural crops ranged from 1.8 ug/L for beans to 24 ug/L for cabbage. Use on
residential gardens and flowerbeds resulted in a peak EEC of 3.7 ug/L.
Table 3.5. Aquatic EECs (jig/L) for total toxic residues of disulfoton uses in California
Crops Represented
Applieation Method
Peak EEC
2.1-day average
EEC
60-day average
EEC
Asparagus
Ground spray
18.8
17.3
14.6
Aerial spray
22.6
20.7
17.7
Beans
Soil injection
1.8
1.6
1.3
Broccoli
Soil injection
3.6
3.3
2.8
Brussels sprouts
Soil injection
5.8
5.4
4.6
Cabbage
Ground spray
23.6
23.2
21.8
Soil injection
7.1
6.6
5.5
Cauliflower
Soil injection
3.6
3.3
2.8
Christmas trees
Broadcast granular
15.1
14.1
12.3
Cotton
Soil injection
0.6
0.5
0.4
Hilldrop
0.8
0.7
0.6
Drill Planting
2.3
2.1
1.7
Ground spray
(in furrow)
4.8
4.3
3.5
Lettuce
Soil injection
11.6
10.8
9.2
Chemigation
66.7
61.8
53.7
Residential Uses
Broadcast granular
3.7
3.5
3.2
65
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3.2.4 Existing Monitoring Data
A critical step in the process of characterizing EECs is comparing the modeled estimates
with available surface water monitoring data. In the case of disulfoton, however,
available monitoring data are primarily for parent alone and so are of limited utility in
validating model estimates for total toxic residues. Disulfoton is less persistent than its
other toxic degradates, so impacts from disulfoton use may not be identified through
disulfoton sampling alone. Included in this assessment are California-specific disulfoton
data for both surface and groundwater from the USGS NAWQA program
(http://water.usgs.gov/nawqa). which include limited sampling for degradates d. sulfoxide
and d. sulfone, and surface water data from the California Department of Pesticide
Regulation (CDPR). Additionally, this discussion includes national surface water and
groundwater data presented in the previous RED, from NAWQA, from the STORET
database, and from several individual local studies.
3.2.4.1 Previous Assessment Surface Water Data
The disulfoton RED published in 2002 considered national monitoring data available as
of that time. Although these data do not represent California-specific environmental
conditions, they are useful to provide insight into the potential for transport of disulfoton
to surface water and groundwater. On a national scale, NAWQA data collected through
1998 had detections in 0.27% of all samples at levels from 0.01 to 0.06 ug/L, with
detections in 0.20% of samples from agricultural streams and 0.61% of samples from
urban streams. A separate monitoring study in Virginia targeted to one watershed (50%
agricultural/50%) forested) detected disulfoton in 3 samples at 2 sites at concentrations
from 0.37 to 6.11 ug/L. The low detection was at the same site as the high, but collected
three hours later.
3.2.4.2 Previous Assessment Ground Water Data
The previous RED discusses groundwater data from studies found in the Pesticides in
Ground Water database. Disulfoton was tested for with no detections in a number of
studies nationally, including 974 wells in California. No details are reported about the
studies, although the RED notes that detection limits as high as 6 ug/L reduce the
certainty of the data. The RED also discusses 3 groundwater monitoring studies which
targeted vulnerable wells in agricultural areas. One study in North Carolina had no
detections while two studies, in Virginia and Wisconsin, had detections from 0.01 ug/L to
100 ug/L. In Virginia, monthly sampling for 4 years detected disulfoton at 5 of 8 wells.
Six detections ranged from 0.04 to 2.87 ug/L. In Wisconsin, individual samples from 29
wells were tested for disulfoton with detections ranging from 4.0 to 100 ug/L. The
samples were from the Central Sands area of Wisconsin where environmental conditions
are conducive to preferential flow, and at least 14 other pesticides were detected in the
samples, so the observed leaching may have been due to local environmental conditions
that may not be typical in California. These studies demonstrate that in some conditions,
disulfoton has the potential to reach groundwater, but Application practices in these
66
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watersheds are not reported and the environmental conditions may differ from those
typical in the region of concern for the CRLF.
3.2.4.3 USGS NAWQA Surface Water Data
NAWQA monitoring data are available for disulfoton in California surface waters,
although this monitoring does not target specific chemicals or uses. Between 1993 and
2006, 1920 samples were taken at 74 sites with no detections of disulfoton above the
detection limit of 0.02 ug/L. Sites included those with land cover classified as
agriculture, urban, mixed, and other, and the majority were in Stanislaus, San Bernardino,
and Merced counties. Six sites were sampled for d. sulfone as well as disulfoton and 3 of
these were also sampled for d. sulfoxide. D. sulfone was detected at 2 sites, one
agricultural site in Stanislaus county and one urban site in Sacramento county (DL =
0.006 - 0.016 ug/L). In Stanislaus county, d. sulfone was detected in 1 out of 28 samples
at 0.01 ug/L. At the Sacramento county site, d. sulfone was detected in 14 out of 14
samples taken over 15 months. The peak level of 0.084 ug/L was detected in June and a
steady decline to 0.018 ug/L was observed in biweekly samples through October.
Bimonthly samples through the following August remained below 0.036 ug/L except for
one spike to 0.069 ug/L in June. There were no detections for d. sulfoxide.
3.2.4.4 USGS NAWQA Groundwater Data
The NAWQA groundwater California dataset included 672 samples from 374 wells
analyzed for disulfoton between 1993 and 2006. 90 of these samples were also tested for
d. sulfoxide and 171 for d. sulfone. There were no detections of either parent or
degradates in any of the analyzed samples. Detection limits for all species were < 0.02
ug/L. Samples represented all NAWQA study areas in California. 47% of the parent
samples were from sites with agricultural landcover, 40% with mixed and/or other, and
13%) with urban land cover.
3.2.4.5 California Department of Pesticide Regulation (CDPR) Data
CDPR maintains a database of monitoring data of pesticides in CA surface waters. The
sampled water bodies include rivers, creeks, urban streams, agricultural drains, the San
Francisco Bay delta region and storm water runoff from urban areas. The database
contains data from 51 different studies by federal state and local agencies as well as
groups from private industry and environmental interests. Some data reported in this
database are also reported by USGS in NAWQA; therefore, there is some overlap
between these two data sets (http://www.cdpr.ca.gov/docs/emon/surfwtr/surfdes.htm).
From 1991-2005, 2712 samples from 173 CA surface water sites were analyzed for
disulfoton. About 60%> of the samples are from the San Joaquin Valley region and 25%
from the Sacramento Valley, with the remaining samples dispersed throughout the state.
There were no disulfoton detections above the detection limits of 0.01 to 1 ug/L. D.
sulfoxide and d. sulfone were not included in the database.
67
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3.2.4.6 Atmospheric Monitoring Data
Available studies monitoring atmospheric transport in the Central Valley and Sierra
Nevada do not include disulfoton as an analyte
(http://www.cdpr.ca.gov/docs/empm/pubs/tac/tacstdvs.htm;
http://www.nature.nps.gov/air/Studies/air toxics/wacap.cfm; Majewski, 1995). One
national study is available which tested for disulfoton at 10 sites, finding it in only 1 out
of 123 samples at a concentration of 0.0047 ng/L, suggesting that disulfoton is not likely
to be present in ambient air (Carey and Kutz, 1985).
3.3. Terrestrial Animal Exposure Assessment
T-REX (Version 1.3.1) is used to calculate dietary and dose-based EECs of disulfoton 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 assessing potential risk to the terrestrial-phase CRLF and its prey (e.g. terrestrial
insects, small mammals and terrestrial-phase frogs), exposures to disulfoton resulting
from spray applications were modeled, which include applications to asparagus via
ground and aerial spray. Also, ground broadcast spray is allowed for additional crops
including broccoli, Brussels sprouts, cabbage, cauliflower, Christmas trees, cotton, and
Easter lilies. These are pre-plant applications followed by incorporation into soil. Foliar
residues on the field from these uses are expected to be lower than those resulting from
direct foliar applications from the asparagus use. However, drift potential off the field
remains equivalent to foliar ground spray applications, and insect EECs are presumed to
be comparable for soil and foliar sprays. Therefore, the T-REX estimates of pesticide
residues on insects will be used for all uses with ground spray, regardless of foliar or soil
application.
Disulfoton may also be applied to soil in furrow (incorporated), via injection, or as a
granular that is subsequently wetted into the soil. Potential risks from these applications
were estimated using the LD50 per square foot method. In addition, disulfoton is a
systemic insecticide that is taken up into the plant, and insects are killed when they
consume contaminated plants. EFED's current methodologies do not allow for
quantification of potential exposures and risks from consumption of systemic pesticides
that have been taken up through the root system and distributed throughout the plant.
Therefore, potential risks from this exposure route will be qualitatively discussed.
Given that no data on interception and subsequent dissipation from foliar surfaces
suitable for estimating foliar dissipation half-lives is available for disulfoton, a foliar
dissipation half-life of 35 days (default) was used. However, foliar dissipation half life
was only used in the derivation of EECs for asparagus because multiple applications are
not allowed for crops other than asparagus.
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T-REX is also used to calculate EECs for terrestrial insects exposed to disulfoton.
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 bees. Available acute contact toxicity data for
bees exposed to disulfoton (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 disulfoton 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 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 3.6). EECs used to estimate potential exposures to
insects are presented in Table 3.7. An example output from T-REX v. 1.3.1 is available
in Appendix F.
EECs for the sulfoxide and sulfone degradates were estimated by multiplying the
maximum amount of degradate formed in available laboratory studies by the disulfoton
EEC. For example, the sulfoxide degradate has been shown to form up to 95% of parent
disulfoton. Therefore, the sulfoxide EEC would equal the disulfoton EEC x 0.95 x (MW
degradate / MW disulfoton).
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Table 3.6 Upper-bound Kenega Nomogram EECs for Dietary- and Dose-based
Exposures of the CRLF and its
*rey to disulfoton
Use
Application
Rate
(lbs a.i./Acre)
EECs for CRLF
EECs for Prey
(small mammals)
Dose-based EEC
Dietary-
based
EEC
Dose-based
EEC
Dietary-based
EEC
Cabbage, cotton
1
Single
application
154 mg/kg-bw
135
mg/kg-
diet
228 mg/kg-bw
240 mg/kg-diet
Asparagus
1
(2 apps, 14-
day interval)
270 mg/kg-bw
240
mg/kg-
diet
402 mg/kg-bw
420 mg/kg-diet
LD50/square foot analysis3
Use
Application
Rate
(lbs a.i./Acre)
Application
Method
EEC Assumptions
EEC (mg a.i./f2)
Beans, broccoli, Brussels
sprouts, cabbage,
cauliflower, cotton
1
Soil injection;
Incorporated
ground spray
(cotton)
99% incorporation;
assumed a 6-inch
furrow and 12-inch
spacing between
furrows/rows
0.33
Christmas trees
4.5
Granular
broadcast, wetted
in
85% Incorporated
7.03
Cabbage and Lettuce
2
Soil injection
99% incorporation;
assumed injection
occurs within a 6-
inch space and 12-
inch spacing
between rows
0.42
Residential
1.6
Granular
broadcast, wetted
in
85% Incorporated
2.5
a LD50 per square foot analysis does not include potential exposures from consumption of treated plants or
contaminated insects that have consumed treated plants.
Table 3.7 EECs (ppm) Used to Estimate Indirect Effects to the Terrestrial-Phase
Use
Small Insect
Large Insect
Asparagus (1 lb a.i./Acre, 2 applications, 14-day
interval)
240
26
Cabbage , cotton (1 lb a.i./Acre, 1 application)
135
15
70
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3.4. Terrestrial Plant Exposure Assessment
Potential risks to terrestrial plants were quantified for Christmas trees (granular
formulation). The EC25 and NOAEC in plants was greater than the maximum
application rate for spray applications for all uses except Christmas trees. Inputs use for
Terrplant (v. 1.2.2) are in Table 3.8, and results are listed in Table 3.9.
Table 3.8. Input Parameters Used to Derive Terrestrial Plant EECs.
Input Parameter
Symbol
Value
Units
Application Rate
A
4.5
lbs a.i./acre
Incorporation
I
1
none
Runoff Fraction
R
0.02
none
Drift Fraction
D
0
none
Seedling Emergence EC25
—
1.9
lbs a.i./acre
Vegetative Vigor EC25
--
2.4
lbs a.i./acre
Table 3.9. Terrestrial Plant EECs for Disulfoton. Units in lbs a.i./acre.
Description
EEC
Runoff to dry areas
0.09
Runoff to semi-aquatic areas
0.9
Spray drift
0 (granular formulation)
Total for dry areas
0.09
Total for semi-aquatic areas
0.9
4. Effects Assessment
This assessment evaluates the potential for disulfoton 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 disulfoton.
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
71
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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 on October, 2007. 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
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 disulfoton.
Additional information on the sublethal effects available in the open literature and
evaluated by the Health Effects Division (HED) is included in Appendix G.
Citations of all open literature, including those 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 D.
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
72
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System (EIIS), are conducted to further refine the characterization of potential ecological
effects associated with exposure to disulfoton. A summary of the available aquatic and
terrestrial ecotoxicity information, use of the probit dose response relationship, and the
incident information for disulfoton are provided in Sections 4.1 through 4.4, respectively.
This assessment evaluates the potential for disulfoton to adversely affect the CRLF. Two
degradates are also included in this assessment. The available data suggests that the
sulfone and sulfoxide degradates may also be a concern to both aquatic and terrestrial
phase CRLFs. Toxicity data for disulfoton and the two degradates of concern are also
discussed in Sections 4.1 to 4.4.
4.1 Toxicity of Disulfoton to Aquatic Organisms
Table 4.1 summarizes the most sensitive aquatic toxicity endpoints used for this
assessment, based on an evaluation of both the submitted studies and the open literature,
as previously discussed. A brief summary of submitted and open literature data
considered relevant to this ecological risk assessment for the CRLF is presented below.
Additional discussion of the data was presented in the Interim Reregi strati on Eligibility
Decision (IRED, 2002), which may be referenced for additional information and may be
found at the following url:
http://www.epa.gov/pesticides/reregistration/REDs/disulfoton_ired.pdf
Ta
)le 4.1 Freshwater Aquatic Toxicity Profile for Disulfoton
Assessment Endpoint
Test
Chemical
Species
Toxicity Value
Used in Risk
Assessment
Citation MRID #
(Author & Date)
Comment
Acute Direct Toxicity to
Aquatic-Phase CRLF
Disulfoton
Bluegill
39 ppb
MRID 00068268
Very highly toxic.
Sulfone
Bluegill
112 ppb
MRID 42585108
Highly toxic
Sulfoxide
Bluegill
188 ppb
MRID 42585107
Highly toxic
Chronic Direct Toxicity
to Aquatic-Phase CRLF
Disulfoton
(degradates
not tested)
Bluegill
4 ppb
MRID 41935801
Value extrapolated using
acute to chronic ratio
derived from other fish
species and applied to the
acute bluegill LC50
(IRED: U.S. EPA, 2002).
Indirect Toxicity to
Aquatic-Phase CRLF via
Acute Toxicity to
Freshwater Invertebrates
(i.e. prey items)
Disulfoton
Glass
shrimp
3.9 ppb
MRID 40094602
Very highly toxic.
Sulfone
de gradate
Daphnid
35 ppb
MRID 42585112
Very highly toxic
Sulfoxide
de gradate
Daphnid
64 ppb
MRID 42585109
Very highly toxic
Indirect Toxicity to
Aquatic-Phase CRLF via
Chronic Toxicity to
Freshwater Invertebrates
(i.e. prey items)
Disulfoton
Glass
shrimp
0.01 ppb
MRID 41935802
Value extrapolated using
acute to chronic ratio
derived from daphnids
and applied to the acute
glass shrimp LC50.
Sulfone
Daphnids
0.14 ppb
MRID 43738001
--
Sulfoxide
Daphnids
1.5 ppb
MRID 43738002
--
Indirect Toxicity to
Disulfoton
No data
73
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Aquatic-Phase CRLF via
Acute Toxicity to Non-
vascular Aquatic Plants
and
degradates
Indirect Toxicity to
Aquatic-Phase CRLF via
Acute Toxicity to
Vascular Aquatic Plants
Disulfoton
and
degradates
No data
Toxicity to fish and aquatic 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
LCS0 (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 disulfoton 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 disulfoton to the CRLF. Effects to freshwater fish resulting from exposure to
disulfoton may 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).
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
The most sensitive acute freshwater LC50s are summarized in Table 4.3. Disulfoton is
very highly toxic to fish on an acute exposure basis. The most sensitive available LC50
is 39 ug/L (MRID 00068268) in bluegill sunfish. The sulfone and sulfoxide degradates
are less toxic than disulfoton to fish, but are highly toxic to fish. The bluegill LC50 for
the sulfone and sulfoxide degradates are 112 ppb and 188 ppb, respectively.
Table 4.3. Acute Fish Toxicity Values for I
Freshwater
Species
Results (ppb ai)
Toxicity Category
Source of Data
MRID
Bluegill
LC50=39
Probit slope: 4.5 (2 - 9)a
very highly toxic
00068268
Bluegill
LC50 (sulfone metabolite)
highly toxic
42585108
isulfoton and Degradates
74
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=112
Probit slope: 5.4(3.4-7.5)
Bluegill
LC50 (sulfoxide metabolite)
=188
Probit slope: 4.5 (2 - 9)a
highly toxic
42585107
a 4.5 is the default slope with 2 and 9 representing reasonable lower and upper bounds (U.S. EPA, 2004).
Bluegill sunfish was the most sensitive species tested as shown in Table 4.3. LC50s for
other species are summarized in Table 4.4.
Table 4.4. Range of Acute Fish Toxicity Values for Disulfoton
Freshwater
Species
Results (ppb ai)
Toxicity Category
Source of Data (MRID)
Bluegill
39 -300
Highly to very highly toxic
40098001,0068268
Rainbow trout
1850 to 3000
Moderately toxic
40098001 and 68268
60,000
(sulfoxide)
Probit slope: 11
(6.4 - 16)
Slightly toxic
42585110
>9,200
(sulfone)
Moderately toxic
42565111
Channel
Catfish
4700
Moderately toxic
40098001
Goldfish
7200
Moderately toxic
229299
Largemouth
Bass
60 - 120
Very highly toxic
0003503,40098001,
Fathead
minnow
59 -4300
Very highly toxic
0003503
Guppy
280
Highly Toxic
229299
4.1.1.2 Freshwater Fish: Chronic Exposure (Growth/Reproduction)
Studies
Available early life stage toxicity studies are summarized in Table 4.5. The NOAEC in
rainbow trout was 220 ppb, which is approximately 8.4 fold lower than the most sensitive
acute LC50. Rainbow trout were considerably less sensitive to disulfoton than bluegill
sunfish, and no chronic studies in bluegill have been submitted. The most sensitive
NOAEC in rainbow trout from an early life stage study (MRID 41935801) was 220 ppb,
which is considerably higher than the most sensitive acute LC50 reported in bluegill of
37 ppb. No chronic study in bluegill has been submitted or was located in the open
literature. Therefore, an acute to chronic ratio was used to estimate a chronic NOAEC in
bluegill using the following equation.
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Bluegill NOAEC = Bluegill LC50 / (Trout LC50 / Trout NOAEC)
= 37 ppb / (1850 ppb / 220 ppb)
= 37 ppb / 8.4
= 4 ppb
Table 4.5. Freshwater Fish Early Life-Stage Toxicity
Species
NOAEC/LOAEC
(ppb ai)
Endpoints
Affected
MRU) No.
Author/Year
Comments
Rainbow trout
(Oncorhynchus
mykiss)
220/420
Growth
41935801
1991
Acceptable study.
Bluegill
4 ppb
N/A
N/A
NOAEC in bluegill
estimated using acute to
chronic ratio based on
rainbow trout data.
4.1.1.3 Freshwater Fish: Sublethal Effects and Additional Open Literature
Information
In the available submitted acute toxicity studies, sublethal effects were not observed at
levels that did not also induce mortality. In the submitted chronic studies, sublethal
effects were not observed at levels below the NOAEC.
In the open literature, Arnold et al. (1996) reported cytologic effects in the liver at levels
as low as 0.1 ug/L, which is below the acute and chronic toxicity values used in this
assessment. These effects were not chosen for use in this effects determination because
they could not be directly linked to the assessment endpoints of survival, growth, and
reproduction. No other sublethal effects were reported at levels lower than the NOAEC
used to calculate RQs for this assessment in the open literature.
4.1.1.4 Aquatic-phase Amphibian: Acute and Chronic Studies
No useful studies in amphibians were located in the open literature or were submitted to
the Agency.
4.1.2 Toxicity to Freshwater Invertebrates
Freshwater aquatic invertebrate toxicity data were used to assess potential indirect effects
of disulfoton to the CRLF. Effects to freshwater invertebrates resulting from exposure to
disulfoton may 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.
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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
The available data indicates that disulfoton and its degradates of concern are very highly
toxic to aquatic invertebrates. Results of the available freshwater invertebrate toxicity
data are summarized in Table 4.6.
Table 4.6. Acute Aquatic Invertebrate Toxicity Data for Disulfoton and its Major
Degradates.
Freshwater
Species
Test Material
Results
(ppb ai)
Toxicity Category
Source of Data
Daphnia
Disulfoton
13
Probit slope:
reliable slope
not available
Very highly toxic
MRID 00143401
Sulfone
metabolite
35
Probit slope:
3.5 (2.3-4.7)
Very highly toxic
MRID 42585112
Sulfoxide
metabolite
64
Probit slope:
4.6 (3.1-6.1)
Very highly toxic
MRID 42585109
Scud
Disulfoton
27 to 52
Very highly toxic
MRID 05017538; 40098001
Glass shrimp
Disulfoton
3.9
Very highly toxic
MRID 40094602
Stonefly
Disulfoton
5 to <8.2
Very highly toxic
MRID 229299, 40098001
4.1.2.2 Freshwater Invertebrates: Chronic Exposure Studies
Freshwater invertebrate life-cycle tests are summarized in Table 4.7. The most sensitive
NOAEC in daphnids was 0.037 ug/L for disulfoton. The available NOAECs for the
sulfone and sulfoxide degradates were 0.14 and 1.5 ug/L, respectively. The most
sensitive species in acute studies was the glass shrimp. No chronic studies in glass
shrimp were available. Therefore, an acute to chronic ratio was used to estimate a
NOAEC for glass shrimp using the following equation:
Glass shrimp NOAEC = Glass shrimp LC50 / (Daphnid EC50 / Daphnid NOAEC)
= 3.9 ppb / (13 ppb / 0.037 ppb)
= 3.9 ppb / 351
= 0.01 ppb
77
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Table 4.7. Freshwater Aquatic Invertebrate Life-Cyc
e Toxicity
Species
NOAEC/ LOAEC
(PPb)
Endpoints Affected
MRU) No.
Author/Year
Study
Classification
Waterflea
(Daphnia magna)
Disulfoton:
0.037/0.070
survival, length, and #
young/adult
41935802
Blakemore/1991
core
Waterflea
(Daphnia magna)
Sulfone degradate:
0.14/0.27
length
43738001
Bowers/1995
core
Waterflea
(Daphnia magna)
Sulfoxide degradate:
1.53/2.97
Weight & length
43738002
Bowers/1995
core
Glass shrimp
0.01
N/A - Estimated value
4.1.2.3 Freshwater Invertebrates: Open Literature Data
No studies were located in the open literature that reported toxicity values that were more
sensitive than studies used to calculate RQs in this assessment.
4.1.3 Toxicity to Aquatic Plants
Aquatic plant toxicity studies may be used as one of the measures of effect to evaluate
whether disulfoton may affect primary production and the availability of aquatic plants as
food for CRLF tadpoles. Primary productivity is essential for indirectly supporting the
growth and abundance of the CRLF.
Two types of studies may be used to evaluate the potential of disulfoton to affect aquatic
plants. Laboratory and field studies were used to determine whether disulfoton may
cause direct effects to aquatic plants. However, no freshwater aquatic plant studies have
been submitted to the Agency or were located in the open literature. Therefore, the
potential toxicity of disulfoton to aquatic plants was not quantified.
4.2 Toxicity of Disulfoton to Terrestrial Organisms
Table 4.8 summarizes the most sensitive terrestrial toxicity endpoints used to assess
potential risks to 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. Additional
information is presented in Sections 4.2.1. to 4.2.4.
78
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Table 4.8 Terrestrial Toxicity Profile for Disulfoton and its Degrat
Endpoint
Test
Material
Species
Toxicity Value
Used in Risk
Assessment
Citation
mrii)#
(Author &
Date)
Comment
Acute Direct Effects
to Terrestrial-Phase
CRLF (LD50)
Disulfoton
Mallard
Duck
LD50: 6.5
mg/kg-bw
MRID
00160000
—
Sulfone
Degradate
Bobwhite
Quail
LD50 = 18 mg/kg-
bw
Probit slope: not
calculated
42585103
Sulfoxide
Degradate
Bobwhite
Quail
LD50 = 9.2 mg/kg-
bw
Probit slope: 6.2
(2.8-9.5
42585102
Acute Direct Toxicity
to Terrestrial-Phase
CRLF (LC5o)
Disulfoton
Japanese quail
LC50: 333 ppm
0034769
LC50 in bobwhite
quail was 544
mg/kg-diet (MRID
0094233).
Sulfone
Degradate
Bobwhite
quail
LC50: 558 ppm
42585106
Data in other species
have not been
submitted.
Sulfoxide
Degradate
Bobwhite
quail
LC50: 456 ppm
42585105
Data in other species
have not been
submitted.
Chronic Direct
Effects to Terrestrial-
Phase CRLF
Disulfoton
Mallard
duck
NOAEC=37
LOAEC=80
(decreased adult
and hatchling body
weight)
43032502
--
Indirect Effects to
Terrestrial-Phase
CRLF (via acute
toxicity to
mammalian prey
items)
Disulfoton
Laboratory
Rat
LD50=1.9mg
ai/kg
072293
--
Sulfone
Degradate
Laboratory
Rat
LD50 (sulfone
metabolite) =11.24
mg/kg
0071873
--
Indirect Effects to
Terrestrial-Phase
CRLF (via chronic
toxicity to
mammalian prey
items)
Disulfoton
Laboratory
Rat
NOAEL=0.04
mg/gk-bw
LOAEL= 1.2
mg/kg-bw
(decreased litter
size and pup
survival)
00157511
--
Indirect Effects to
Terrestrial-Phase
CRLF (via acute
Disulfoton
Honey bee
LD50: 4.1 ug
ai/bee
05004151
—
Sulfone
42582902
ates of Concern
79
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Endpoint
Test
Material
Species
Toxicity Value
Used in Risk
Assessment
Citation
mrii)#
(Author &
Date)
Comment
toxicity to terrestrial
metabolite
Honey bee
LD50: 0.96 ug/bee
invertebrate prey
items)
Sulfoxide
metabolite
Honey bee
LD50: 1.1 ug /bee
42582901
--
Indirect Effects to
Terrestrial- and
Aquatic-Phase CRLF
(via toxicity to
terrestrial plants)
Disulfoton
Seedlins
Emersence
Monocots
EC25: >1.9 lbs
a.i./Acre
46526601
--
Seedlins
Emersence
Dicots
EC25: >1.9 lbs
a.i./Acre
46526601
--
Vesetative
Visor
Monocots
EC25: >2.4 lbs
a.i./Acre
46526602
--
Vesetative
Visor
Dicots
EC25: >2.4 lbs
a.i./Acre
46526602
--
Acute toxicity to terrestrial animals is categorized using the classification system shown
in Table 4.9 (U.S. EPA, 2004). Toxicity categories for terrestrial plants have not been
defined.
Table 4.9 Categories of Acute Toxicity for Avian and Mammalian Studies
Toxicity Category
Oral LDS0
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 disulfoton; therefore, acute
and chronic avian toxicity data are used to assess the potential direct effects of disulfoton
to terrestrial-phase CRLFs.
4.2.1.1 Birds: Acute Exposure (Mortality) Studies
Available acute oral and subacute dietary studies are summarized in Table 4.10 below.
Disulfoton is very highly toxic to birds on an acute oral basis and moderately toxic on a
subacute dietary basis. In addition, the sulfone and sulfoxide degradates were shown to
80
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be approximately as toxic to birds as disulfoton on a subacute dietary basis and more
toxic than disulfoton on an acute oral basis.
Table 4.10. Toxicity Endpoints Used to Estimate Potential Risk of Direct Effects to
Terrestrial Phase CRLFs
Species
Test Type / chemical
Results (ppm ai)
Toxicity
Classification
Source of Data
Northern bobwhite quail
Subacute dietary
Disulfoton
LC50 = 544
moderately toxic
0094233
Subacute dietary
Sulfone degradate
LC50 = 558
Probit slope: 5.4 (2.8 - 7.9)
moderately toxic
42585106
Sub acute dietary
Sulfoxide degradate
LC50 = 456 mg/kg
Probit slope: 3.0 (1.7-4.3)
highly toxic
42585105
Mallard Duck
Subacute dietary
Sulfoxide degradate
LC50 = 823 ppm
Probit slope 6.2 (2.6 - 9.7)
moderately toxic
42585104
Subacute dietary
Sulfone degradate
LC50 = 622 ppm
Probit slope: 5.8 (2.8 - 8.9)
moderately toxic
42585101
Japanese quail
Subacute dietary
Disulfoton
LC50=333
highly toxic
0034769
Mallard duck
Acute oral
Disulfoton
LD50=6.54 mg ai/kg
very highly toxic
00160000
Bobwhite quail
Acute oral
Disulfoton
LD50: 39 mg/gk-bw
Probit slope: 4.8 (0.9 - 8.6)
highly toxic
42585803
Acute oral
Sulfoxide degradate
LD50 = 9.2 mg/kg-bw
Probit slope: 6.2 (2.8 - 9.5)
very highly toxic
42585102
Acute oral
Sulfone degradate
LD50 = 18 mg/kg-bw
Probit slope: not calculated
highly toxic
42585103
4.2.1.2 Birds: Chronic Exposure (Growth, Reproduction) Studies
Available reproduction toxicity studies in birds are summarized in Table 4.11 below.
Data were not submitted on the degradates of concern. The NOAEC in both mallard
ducks and bobwhite quail was 37 ppm based on reduced body weight.
Table 4.11. Summary of Available Avian Reproduction Toxicity Studies for
Disulfoton.
Species
Endpoint Tested
Results
Source of Data
Mallard duck
reproduction
NOAEC=37 mg/kg-diet
LOAEC=80 mg/kg-diet
(decreased adult and
hatchling body weight)
43032502
Bobwhite quail
reproduction
NOAEC=37 mg/kg-diet
LOAEC=74 mg/kg-diet
(decreased adult body
43032501
81
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weight)
4.2.2 Toxicity to Mammals
Mammalian toxicity data are used to assess potential indirect effects of disulfoton to the
terrestrial-phase CRLF. Effects to small mammals resulting from exposure to disulfoton
may 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).
4.2.2.1 Mammals: Acute Exposure (Mortality) Studies
Available acute oral toxicity studies are summarized in Table 4.12 below. Disulfoton is
very highly toxic to mammals on an acute oral basis. In addition, the sulfone metabolite
is also highly toxic to mammals on an acute oral basis, although it was approximately 10-
fold less toxic than disulfoton to mammals on an acute basis. Data on the sulfoxide
degradate have not been submitted.
Table 4.12. Summary of Available Mammalian Acute Toxicity Studies for
Disulfoton and its Degradates of Concern
Test Species
Study Type
Toxicity Value
Toxicity
Category
MRID
Laboratory rat
acute oral
LD50=1.9mg ai/kg
Adj. LD50: 4.2 mg/kg-bw
very highly
toxic
072293
Laboratory rat
sulfone metabolite
acute oral
LD50 =11.24 mg/kg
Adj. LD50: 25 mg/kg-bw
highly toxic
0071873
4.2.2.2 Mammals: Chronic Exposure (Growth, Reproduction) Studies
Available reproduction toxicity studies in mammals are summarized in Table 4.13
below. Disulfoton affected reproductive success (defined as decreased litter size and pup
survival) at 2.4 mg/kg-bw with a NOAEC of 0.8 mg/kg-bw. Neither the sulfone nor the
sulfoxide degradate have been tested for reproductive effects to mammals.
Table 4.13. Summary of Available Mammalian Reproduction Toxicity Studies for
Disulfoton
Test Species
Study Type
Toxicity Value
Toxicity
Category
MRID
Laboratory rat
2-generation
reproduction
NOAEL=0.04 mg/gk-bw
LOAEL= 1.2 mg/kg-bw
(decreased litter size and pup
survival)
N/A
00157511
82
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4.2.3 Toxicity to Terrestrial Invertebrates
Terrestrial invertebrate toxicity data are used to assess potential indirect effects of
disulfoton to the terrestrial-phase CRLF. Effects to terrestrial invertebrates resulting
from exposure to disulfoton may also indirectly affect the CRLF via reduction in
available food.
4.2.3.1 Terrestrial Invertebrates: Acute Exposure (Mortality) Studies
Submitted acute exposure studies in terrestrial invertebrates are summarized in Table
4.14. A number of studies have also been conducted that evaluated the efficacy of
disulfoton with respect to insecticidal activity. Although these studies evaluated effects
to terrestrial invertebrates, the study designs do not allow for an estimate of a dose or
application rate associated with a toxicity value that can be used in risk assessment.
Table 4.14. Summary of Available Mammalian Acute Toxicity Studies for
Disulfoton
Test Species
Test Type
Toxicity Value
Reference (MRID) / Comment
Honey bee
acute contact
LD50: 4.1ugai/bee
05004151
Honey bee
acute contact
LD50 (sulfone metabolite):
0.96 ug/bee
42582902
Honey bee
acute contact
LD50 (sulfoxide
metabolite): 1.11 ug/bee
42582901
Honey bee
acute foliar
residue
RT25 (8 EC) < 3hrs at 1.0
lbai/A
0163423 / RT 25 is the residual time required
to reduce mortality of caged bees to field
weathered spray deposits.
4.2.4 Toxicity to Terrestrial Plants
Terrestrial plant toxicity data are used to evaluate the potential for disulfoton to affect
riparian zone and upland vegetation within the action area for the CRLF. Impacts to
riparian and upland (i.e., grassland, woodland) vegetation may result in indirect effects 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.
Plant toxicity data from both registrant-submitted studies and studies in the scientific
literature were reviewed for this assessment. Registrant-submitted studies are conducted
under conditions and with species defined in EPA toxicity test guidelines. Sub-lethal
endpoints such as plant growth, dry weight, and biomass are evaluated for both monocots
and dicots, and effects are evaluated at both seedling emergence and vegetative life
stages. Guideline studies generally evaluate toxicity to ten crop species. A drawback to
83
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these tests is that they are conducted on herbaceous crop species only, and extrapolation
of effects to other species, such as the woody shrubs and trees and wild herbaceous
species, contributes uncertainty to risk conclusions.
Commercial crop species have been selectively bred, and may be more or less resistant to
particular stressors than wild herbs and forbs. The direction of this uncertainty for
specific plants and stressors, including disulfoton, is largely unknown. Homogenous test
plant seed lots also lack the genetic variation that occurs in natural populations, so the
range of effects seen from tests is likely to be smaller than would be expected from wild
populations.
A Tier I seedling emergence test and a Tier II vegetative vigor test was submitted. The
EC25 for seedling emergence and vegetative vigor was >1.9 lbs a.i./Acre and >2.4 lbs
a.i./Acre, respectively.
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 disulfoton 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. Results of this analysis are presented in the Risk Description
(Section 5.2).
4.4 Incident Database Review
A review of the EIIS database for ecological incidents involving disulfoton was
completed on March 10, 2008. Several reports of wildlife poisonings are associated with
disulfoton. These poisoning incidents are summarized in Table 4.15 below. Some of
84
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these incident reports support EPA's concerns for acute risk. In particular, one incident
reported that birds consuming insects that fed on plants treated with disulfoton were
killed. This incident further emphasizes the potential importance of an exposure pathway
that is not quantified in this assessment. Consumption of insects that have consumed
plant material contaminated with disulfoton via systemic uptake and translocation in the
plant resulted in effects to higher trophic level organisms that fed on such insects.
Table 4.15. Chronological List of Ecological Incidents
Start
Date
Misuse?
(yes/no/un
known)
Incident Description
6/12/95
unknown
Johnston County, NC: Fish kill occurred in commercial fish pond. Crop fields nearby
treated with pesticides. Water, soil and vegetation samples analyzed for a variety of
pesticides. Disulfoton, as well as several other pesticides, was found at 0.2-2.5 ppm in
vegetation samples. Possible certainty index for disulfoton. (Incident Report No.
1003826-002).
1/24/94
unknown
Puerto Rico: 6 grackles fell dead from tree in yard of private residence. Dead heron and
owl also found in vicinity. Use site and method not reported. Birds had depressed acetyl
cholinesterase. Analysis of GI contents of a grackles showed disulfoton at 2.37 ppm
wet weight. Highly probable certainty index for disulfoton. (Incident Report No.
1003966-004).
6/11/94
unknown
Arapahoe CO: Fish kill following application of Di-Syston EC. to wheat just before
heavy rain. Water samples contained disulfoton sulfoxide at 29.5-48.7 ppb and
disulfoton sulfone at 0.0199-0.214 ppb. (Incident Report No. 1001167-001).
6/18/93
No
Young County,TX: 18 Swainson's hawks dead, 1 severely disabled in a cotton field.
Cotton seed had been treated with disulfoton prior to planting, -10 days before the birds
were discovered. No additional applications of OP or carbamate pesticides made in
vicinity of field. Autopsies showed no trauma or disease. Lab analysis showed insect
material in GI tracts; this material contained disulfoton (~7 ppm); no other OP or
carbamate insecticides were present. Hawks fed on insects, which had been feeding on
the young cotton plants, which contained disulfoton residues. (L.Lyon, Div. of
Environmental Contaminants, U.S. Fish and Wildlife Service, Arlington, VA.)
6/22/91
unknown
Onslow County, NC: Fish kill in pond at private residence. Pond received runoff from
neighboring tobacco field; pondwater analysis showed disulfoton and several other
pesticides, including endosulfan. Disulfoton sulfoxide found in water at 0.32 ppb.
Endosulfan had highest concentration (1.2 (ig/L). and is toxic to fish, but disulfoton
cannot be ruled out as a possible cause of death. No tissue analysis. Possible certainty
index for disulfoton. (Incident Report No. B0000216-025).
4/26/91
unknown
Sussex County, DE: 9 American robins dead following application of granular
disulfoton at tree nursery. Corn and soybeans also in vicinity. No laboratory analysis.
Probable certainty index for disulfoton. (Incident Report No. 1000116-003).
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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 disulfoton
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
concern (LOCs) for each category evaluated (U.S. EPA, 2004). 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 disulfoton usage
scenarios summarized in Section 3 and the appropriate aquatic toxicity endpoint reported
in Section 4. 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 disulfoton (Section 3) and the appropriate toxicity endpoint from
Section 4.
5.1.1 Exposures in the Aquatic Habitat
5.1.1.1 Direct Effects to Aquatic-Phase CRLF
RQs used to estimate potential risks to aquatic phase CRLFs are summarized in Table
5.1. Direct effects to the aquatic-phase CRLF are based on peak EECs and the lowest
acute toxicity value for freshwater fish. In order to assess direct chronic risks to the
CRLF, 60-day EECs and the lowest chronic toxicity value for freshwater fish are used.
RQs exceeded the LOC for either acute or chronic effects for all uses. Therefore,
disulfoton may directly affect the CRLF. Additional analysis on the potential for
disulfoton to adversely affect the CRLF is in Section 5.2.
86
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Table 5.1 Summary of Direct Effect RQs for Aquatic-phase CRLFs Based on an
Use
Peak
Aeutc
Probability
60-Dav EEC
RQ
LOC Excccdancc and
EEC
RQ
of Individual
(l*g/L)
(NOAEC: 4
Risk Interpretation
(jig/L)
(LC50:
37 ug/L)
Effect (acute
effects only)
ug/L)
Beans, Broccoli,
1.8-3.6
0.05-
1 in 4E8 to 1
1.3-2.8
0.33-0.70
The endangered species
Cauliflower
0.097
in 4E5
acute LOC is exceeded.
Residential,
3.7 - 15
0.10-
1 in 3E5 to
3.2-12
0.8
Restricted use LOC is
Cotton, Brussels
0.41
1 in 25
(residential),
exceeded for these uses,
sprouts,
0.88
and the chronic LOC is
Christmas trees,
(cotton), 1.1
exceeded for Brussels
lettuce
- 3 (other
uses)
sprouts, Christmas
trees, and lettuce.
Asparagus,
23-24
0.62-
1 in 6 to
18-22
3.5-8.8
Acute and chronic
cabbage
0.65
1 in 5
LOCs are exceeded for
these uses.
Lettuce (drip
67
1.8
1 in 1.1
54
14
Acute and chronic LOC
irrigation)
exceeded
5.1.1.2 Indirect Effects to Aquatic-Phase CRLF via Reduction in Prey
(non-vascular aquatic plants, aquatic invertebrates, fish, and frogs)
Aquatic Invertebrates
Indirect acute effects to the aquatic-phase CRLF via effects to prey (invertebrates) in
aquatic habitats are based on peak EECs in the standard pond and the lowest acute
toxicity value for freshwater invertebrates. For chronic risks, 21-day EECs and the lowest
chronic toxicity value for invertebrates are used to derive RQs. A summary of the acute
and chronic RQ values for exposure to aquatic invertebrates (as prey items of aquatic-
phase CRLFs) is provided in Table 5.2. RQs exceeded the LOC for acute and chronic
effects for all uses. Therefore, disulfoton may affect the CRLF. Additional analysis on
the potential for disulfoton to adversely affect the CRLF by reducing aquatic invertebrate
prey base is in Section 5.2.
Table 5.2 Summary of Acute and Chronic RQs Used to Estimate Indirect Effects to
the CRLF via Effects on Aquatic Invertebrates as Dietary Food Items (prey of
CRLF juveniles and adults in aquatic habitats) Based on an LC50 of 3.9 ppb and a
Use
Peak
EEC
(Ug/L)
Acute RQ
Probability of
Individual
Effect (acute
effects only)
21-Day EEC
(Ug/L)
Chronic RQ
LOC Exceedance and
Risk Interpretation
All uses
1.8-67
0.46 to 17
1 in 16 to 1 in 1
1.6-23
145 - 5600
Acute and chronic
LOCs were exceeded
for all uses.
87
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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 (Table 5.1) are used to
assess potential indirect effects to the CRLF based on a reduction in freshwater fish and
frogs as food items. RQs exceeded the LOC for acute and chronic effects for most uses.
Therefore, disulfoton may affect the CRLF. Additional analysis on the potential for
disulfoton to adversely affect the CRLF is in Section 5.2.
5.1.1.3 Indirect Effects to CRLF via Reduction in Habitat and/or
Primary Productivity (Freshwater Aquatic Plants)
No aquatic plant toxicity data are available for derivation of RQs. The effects
determination for potential indirect effects to the CRLF by affecting aquatic plants is
presented in Section 5.2.
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 spray applications of disulfoton either to soil or foliage. 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 and
acute oral and subacute dietary toxicity endpoints for avian species. RQs used to
estimate potential risks to terrestrial phase CRLFs are in Tables 5.3a and 5.3.b, Based
on exceedance of the acute and reproduction EECs, a preliminary "may effect"
determination is made. Additional analysis and the effects determination is presented in
Section 5.2.
88
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Table 5.3a. Avian RQs Used To Estimate Potential Risk of Direct Effects to
Terrestrial Phase CRLFs
'or Spray Applications3
Use
Assessed Effect
and Species
Chemical
EEC3
Toxicity Value
RQ
Asparagus
Direct Acute Effect
Disulfoton
270
LD50: 6.5 mg/kg-bw
80
(1 lbs
(dose)
Adj LD50: 3.4 mg/kg-bw
a.i./Acre, 2
apps, 14-day
Sulfoxide Degradate
217
LD50: 9.2 mg/kg-bw
LD50Adj: 6.6 mg/kg-bw
33
invert)
Sulfone Degradate
216
LD50: 18 mg/kg-bw
LD50Adj: 13 mg/kg-bw
16
Direct Acute Effect
Disulfoton
240
LC50: 330 ppm
0.72
(dietary)
Sulfone Degradate
192
LC50: 558 ppm
0.42
Sulfoxide Degradate
240
LC50: 456 ppm
0.43
Cabbage and
Direct Acute Effect
Disulfoton
150
LD50: 6.5 mg/kg-bw
44
cotton (1 lb
(dose)
Adj LD50: 3.4 mg/kg-bw
a.i./Acre,
single
Sulfoxide Degradate
150
LD50: 9.2 mg/kg-bw
LD50Adj: 6.6 mg/kg-bw
23
application)
(1 lb
Sulfone Degradate
120
LD50: 18 mg/kg-bw
LD50Adj: 13 mg/kg-bw
6.7
a.i./Acre,
Direct Acute Effect
Disulfoton
140
LC50: 330 ppm
0.4
single
(dietary)
Sulfone Degradate
110
LC50: 558 ppm
0.20
application)
Sulfoxide Degradate
140
LC50: 456 ppm
0.31
a EECs for the sulfoxide and sulfone degradates were estimated assuming a 95% and 72% formation rate
from parent, respectively (Section 2). RQs are based on small insect EECs
Table 5.3b. Avian LD50/Square Foot Analysis Used to Estimate Potential Direct
Effects to the C
*LF from Granular and Soil Incorporated Applications3
Use
Application Rate
(lbs a.i./Acre)
Application Method
(% incorporated)
EEC (mg a.i./f12)
LD50/ft2
Beans, broccoli,
Brussels sprouts,
cabbage,
cauliflower, cotton
1
Soil incorporated, spray
or injection (99%)
0.33
3.1
Christmas trees
4.5
Granular broadcast,
wetted in (85%)
7.0
104
Cabbage and
Lettuce
2
Soil injection (99%)
0.42
6.2
Residential
1.6
Granular broadcast,
wetted in (85%)
2.5
37
LD50 per square foot analysis does not include exposures from consumption of contaminated plants that
have taken up and translocated the chemical throughout the plant. However, risk to CRLFs that consume
insects that have fed on treated foliage presumably exceed LOCs based on the incident data.
Potential direct reproduction effects from exposure to disulfoton to the terrestrial-phase
CRLF are derived by considering dietary-based exposures modeled in T-REX for a small
bird (20g) consuming small invertebrates. Reproduction effects are estimated using the
lowest available toxicity data for birds. EECs are divided by toxicity values to estimate
chronic dietary-based RQs. RQs used to estimate potential direct reproduction effects are
summarized in Table 5.4. RQs were only estimated for chronic exposures for spray
applications. Although potential risks to reproduction of CRLFs and their prey from soil
89
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injection and in-furrow applications was not quantified, risks may be above concern
levels for these types of applications as discussed in Section 5.2 (Risk Description).
Table 5.4. Reproduction RQs for Birds Used to Estimate Potential Direct Effects to
CRLFs from Spray Uses" i
Use
Assessed Effect
and Species
Chemical
Toxicity
Value
EEC
(ppm)
RQ
Asparagus
2 applications of 1 lb
a.i./acre, 14-day interval
Direct Reproduction
Effect
Disulfoton
NOAEC: 37
ppm
240
6.4
Other uses
Single application of 1 lb
a.i./Acre
211
5.7
a RQs are based on small insect EECs
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 potential risks of disulfoton 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 4 |ig a.i./bee by 1 bee/0.128g,
which is based on the weight of an adult honey bee. EECs (|ig a.i./g of bee) calculated by
T-REX for small and large insects are divided by the calculated toxicity value for
terrestrial invertebrates, which is 31 |ig a.i./g of bee. The resulting RQs were 7.7 for
asparagus and 4.4 for other uses. Based on LOC exceedances for the surrogate terrestrial
invertebrate, a preliminary "may effect" determination was made. Additional analysis is
presented in Section 5.2.
Table 5.5. Summary of RQs Used to Estimate Indirect Effects to the Terrestrial-
phase CRLF via Direct Effects on Terrestrial Invertebrates as Dietary Food Items
Use
Small Insect EEC
Small Insect RQ
Large Insect EEC
Large Insect RQ
Asparagus
240
7.7
26
0.84
All other uses
135
4.4
15
0.48
5.1.2.2.2 Mammals
Risk quotients used to evaluate potential indirect effects resulting from impacts to
mammalian prey are presented in Table 5.6. RQs were 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
90
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toxicity data. EECs are divided by the toxicity value to estimate acute and reproduction
dose-based RQs as well as reproduction dietary-based RQs.
Table 5.6a. RQs used to Estimate Potential Acute Risks to Mammalian Prey of
Use
Chemical
EEC
(mg/kg-bw)
Toxicity
Value
(mg/kg-bw)
RQ
Asparagus
2 applications of 1
lb a.i./acre, 14-day
interval
Disulfoton
400
LD50: 1.9
Ad). LD50: 4.2
96
Sulfone Degradate
320
LD50: 11
Ad). LD50: 25
13
Sulfoxide
Degradate
Not calculated due to lack of toxicity data
Other uses
Single application
of 1 lb a.i./Acre
Disulfoton
229
LD50: 1.9
Ad). LD50: 4.2
55
Sulfone Degradate
180
LD50: 11
Ad). LD50: 25
7.3
Sulfoxide
Degradate
Not calculated due to lack of toxicity data
Table 5.6b. LD50/Square Foot Analysis Used to Estimate Potential Effects to
Mammal Prey Items of the CRLF (soil incorporated applications)"
Use
Application Rate
(lbs a.i./Acre)
Application Method
(% incorporated)
EEC (mg a.i./f2)
LD50/ft2
Beans, broccoli,
Brussels sprouts,
cabbage,
cauliflower, cotton
1
Soil incorporated, spray
or injection (99%)
0.33
3.3
Christmas trees
4.5
Granular broadcast,
wetted in (85%)
7.0
104
Cabbage and
Lettuce
2
Soil injection (99%)
0.42
6.6
Residential
1.6
Granular broadcast,
wetted in (85%)
2.5
37
a LD50 per square foot analysis does not specifically evaluate exposures from consumption of plants that
have taken up the material through the roots and translocated the material throughout the plant. However,
risk to CRLFs that consume insects that have fed on treated foliage presumably exceed LOCs based on the
incident data.
Table 5.7. Summary of Reproduction RQs used to Estimate Potential Risk to
Mammalian Prey of CRLFs from Spray Applications of Disulfoton
Use
EEC
Toxicity Value
RQ
Asparagus
2 applications of 1 lb
a.i./acre, 14-day
interval
400 mg/kg-bw
AdjNOAEL: 0.09 mg/kg-bw
4600
420 mg/kg-diet
NOAEC: 0.8 mg/kg-diet
530
Other uses
Single application of
1 lb a.i./Acre
228 mg/kg-bw
Ad)NOAEL: 0.09 mg/kg-bw
2600
240 ppm
NOAEC: 0.8 ppm
300
91
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5.1.2.2.3
Terrestrial Amphibians
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.
No amphibian toxicity data were located that evaluated potential effects from exposure to
disulfoton or its degradates of concern. Avian RQs used to evaluate potential effects to
amphibians were summarized in Table 5.3. Acute and reproduction RQs exceeded LOCs
for birds. Because birds serve as a surrogate for terrestrial phase amphibians and reptiles,
LOC exceedances for birds suggest that amphibian prey could be impacted. Additional
analysis of the potential impacts to terrestrial amphibians as they relate to the effects
determination is in Section 5.2 (Risk Description).
5.1.2.3 Indirect Effects to CRLF via Reduction in Terrestrial Plant
Community (Riparian and Upland Habitat)
Potential indirect effects to the CRLF resulting from direct effects on riparian and upland
vegetation were not quantified for any use except Christmas trees because the EC25 was
higher than the highest labeled application rate for all uses except Christmas trees. Non-
endangered terrestrial plant RQs were <0.5 for Christmas trees, which is below the
terrestrial plant LOC that is used for indirect effects determinations. The effects
determination is presented in Section 5.2.
5.1.3 Primary Constituent Elements of Designated Critical Habitat
For disulfoton 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.
5.2 Risk Description
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 disulfoton's use within the action
area. However, if direct or indirect effect LOCs are exceeded or effects may modify the
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PCEs of the CRLF's critical habitat, the Agency concludes a preliminary "may affect"
determination for the FIFRA regulatory action regarding disulfoton. A summary of the
results of the risk estimation (i.e., "no effect" or "may affect" finding) is provided in
Table 5.8 for direct and indirect effects to the CRLF.
Table 5.8. Preliminary Effects Determination Summary for disulfoton - Direct and
Indirect Effects to CRLF
Assessment Endpoint
Preliminary
Effects
Determination
Basis For Preliminary Determination
Aquatic Phase
(eggs, larvae, tadpoles, juveniles, and adults)
Survival, growth, and reproduction of
CRLF individuals via direct effects on
aquatic phases
May affect
Acute LOC exceedance for all uses.
Chronic RQs exceeded the LOC for all
uses except beans, broccoli, cotton,
residential, and cauliflower.
Survival, growth, and reproduction of
CRLF individuals via effects to food
supply (i.e., freshwater invertebrates,
non-vascular plants)
May affect
Acute and chronic LOCs were exceeded
for all uses. Acute RQs were 0.5 to 12 and
chronic RQs were 150 to 3800.
Survival, growth, and reproduction of
CRLF individuals via indirect effects
on habitat, cover, and/or primary
productivity (i.e., aquatic plant
community)
No effect
No LOC exceedance for terrestrial or
aquatic plants
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.
No effect
No LOC exceedance for terrestrial or
aquatic plants
Terrestrial Phase
(Juveniles and adults)
Survival, growth, and reproduction of
CRLF individuals via direct effects on
terrestrial phase adults and juveniles
May affect
LOCs were exceeded for disulfoton and
two degradates of concern.
Survival, growth, and reproduction of
CRLF individuals via effects on prey
(i.e., terrestrial invertebrates, small
terrestrial mammals and terrestrial
phase amphibians)
May affect
LOCs were exceeded for all taxonomic
groups of prey items.
Survival, growth, and reproduction of
CRLF individuals via indirect effects
on habitat (i.e., riparian vegetation)
No effect
Potential risks to terrestrial plants were
lower than the concern level.
For disulfoton 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
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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. The following PCEs may be adversely impacted by disulfoton; other
PCEs are related to potential adverse impacts to aquatic or terrestrial plants, which are
not expected to be adversely impacted by labeled use of disulfoton to an extent that is
expected to indirectly affect the CRLF.
• Alteration of chemical characteristics necessary for normal growth and viability
of CRLFs and their food source.
• Reduction and/or modification of food sources for terrestrial phase juveniles and
adults
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:
• 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,
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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 disulfoton.
Acute RQs exceeded the endangered species LOC for all uses and ranged from 0.05 to
1.8. Based on the assumptions of a probit slope and a default slope of 4.5 (lower and
upper bounds of 2 to 9) the probability of an individual mortality ranges from 1 in 4E8
(lowest RQ, beans) to approximately 1 in 1 (lettuce). Although the magnitude of some of
the estimated probabilities of individual effects may be considered discountable, the slope
of the dose-response curve is uncertain. The default slope of 4.5 was used for this
analysis because slopes could not be obtained from the available studies. A more shallow
or steep dose-response curve would result in a higher or lower estimated probability of an
effect at RQs lower than 1. For example, use of lower and upper reasonable bounds for
probit slopes of 2 to 9 (U.S. EPA, 2004) results in an estimated probability of an
individual mortality of 1 in 26 to 1 in 1E15. Based on the exceedance of the endangered
species LOC for all uses and uncertainty in the dose-response curve, it was concluded
that disulfoton is likely to adversely affect the CRLF for all uses.
For the residential use, this conclusion of acute adverse effects to the CRLF is based on
assumptions about application rates that are conservative and may lead to overestimates
of potential exposure. The labels for residential uses do not define maximum application
rates in terms of lb a.i/A and depend instead on the size of a garden or the number of
plants treated. Extrapolation of these rates to a lb a.i./A value, as is necessary for the
aquatic models used in this assessment, requires assumptions about the area that will be
treated. The EEC of 3.7 ppb, which leads to an LOC exceedance with an RQ of 0.10,
assumes that there are 4 lots per acre and that each one has a flower bed/garden of 2000
ft2. If less area is treated, EECs would be lower and RQs may not exceed the LOC. With
an endpoint of 37 ug/L, then any EEC less than 1.85 ug/L would not exceed the
endangered species LOC. If actual application rates are half of those assumed, i.e. if the
garden sizes are only 1000 ft2 or if only 2 lots out of 4 have gardens treated with
disulfoton, then the LOC would not be exceeded. Additionally, some of the residential
labels define application rates by number of plants, rather than by area treated. At a
labeled application rate of 0.0013 lb a.i./plant, 650 plants per acre could be treated
without exceeding the LOC. Many typical residential applications, then, would not lead
to LOC exceedances and conclusions of risk.
The chronic RQs also exceed the LOC of 1 for all uses except for beans, residential, and
cotton. Chronic RQs for uses that did not exceed LOCs ranged from 0.33 (beans) to 0.88
(cotton). RQs for other uses exceeded the chronic LOC of 1.0 and ranged from 1.1 to 14
(lettuce).
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Disulfoton is not expected to remain in the terrestrial environment for very long with a
half life of several days. Therefore, after several days post application, runoff to aquatic
systems will consist primarily of degradates. The EECs used in this assessment included
total toxic residues (parent disulfoton and its degradates of concern). The most toxic
degradate was the sulfone degradate, which is approximately 3-fold less toxic than
disulfoton to fish. Therefore, assuming the toxicity of the residue in water is similar to
the most toxic degradate tested, then RQs would be approximately 3-fold lower after
several days post application. Therefore, assuming toxicity of the sulfone would still
result in LOC exceedance for several uses.
The effects determination was based on the most sensitive species tested (bluegill).
However, a number of fish species have been tested including rainbow trout, catfish,
goldfish, largemouth bass, fathead minnows, and guppies. Some species have shown
similar sensitivity to bluegill; however, other species tested have shown lower sensitivity
(Table 5.9). RQs would remain above LOCs for several uses if the CRLF is as sensitive
as largemouth bass, fathead minnows, guppies, or bluegill. However, RQs would be
lower than the endangered species LOC of 0.05 for rainbow trout, catfish, and goldfish
for all uses.
"able 5.9. Range of Acute Fish LC5
Os for Disulfoton.
Freshwater Species
Results (ppb ai)
Toxicity Category
Source of Data
Bluegill
39-300
Highly to very highly toxic
40098001,0068268
Rainbow trout
1850 to 3000
Moderately toxic
MRIDs 40098001 and
68268
Channel Catfish
4700
Moderately toxic
40098001
Goldfish
7200
Moderately toxic
229299
Largemouth Bass
60 - 120
Very highly toxic
0003503,40098001
Fathead minnow
59 -4300
Very highly toxic
0003503
Guppy
280
Highly Toxic
229299
In addition, several incidents involving freshwater fish have been reported as summarized
below:
6/12/95 Johnston County, NC: Fish kill occurred in commercial fish pond. Crop fields
nearby treated with pesticides. Water, soil and vegetation samples analyzed for a
variety of pesticides. Disulfoton, as well as several other pesticides, was found at
0.2-2.5 ppm in vegetation samples. Possible certainty index for disulfoton.
(Incident Report No. 1003826-002).
6/11/94 Arapahoe CO: Fish kill following application of Di-Syston EC. to wheat just
before heavy rain. Water samples contained disulfoton sulfoxide at 29.5-48.7
ppb and disulfoton sulfone at 0.0199-0.214 ppb. (Incident Report No. 1001167-
001).
6/22/91 Onslow County, NC: Fish kill in pond at private residence. Pond received
runoff from neighboring tobacco field; pondwater analysis showed disulfoton
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and several other pesticides, including endosulfan. Disulfoton sulfoxide found in
water at 0.32 ppb. Endosulfan had highest concentration (1.2 (ig/L). and is toxic
to fish, but disulfoton cannot be ruled out as a possible cause of death. No tissue
analysis. Possible certainty index for disulfoton. (Incident Report No. B0000216-
025).
The incidences support the conclusion that freshwater fish (and aquatic phase amphibians
as a surrogate) may be affected by labeled uses of disulfoton. Therefore, the RQ analysis
together with the presence of several incidences that associated fish mortality with
disulfoton use support the conclusion that the labeled uses of disulfoton are likely to
adversely affect aquatic phase CRLFs.
5.2.1.2 Terrestrial-Phase CRLF, Direct Effects
Acute and chronic RQs exceeded the LOC for endangered birds for all uses. The highest
dose-based acute RQ for disulfoton was 80 based on an adjusted LD50 of 3.4 mg/kg-bw
and EEC derived assuming 2 applications of 1 lb a.i./Acre with a 14 day application
interval. Assuming a single application of 1 lb a.i./Acre results in an RQ of 46 for
disulfoton. The associated probability of an individual effect at an RQ of 46 or 80
approaches 100% for reasonable lower and upper bound probit slopes of 2 to 9 (U.S.
EPA, 2004).
LD50s for the degradates are similar (within a factor of 3) to those of disulfoton, and
EECs were similar to those of disulfoton. RQs for the sulfoxide and sulfone degradates
also exceeded the endangered species LOC and the LD50 (RQ >1). The amount of
degradate that may form in the environment is likely variable. This assessment assumed
that the amount of degradate that formed was equivalent to the highest observed
degradate level from the available degradation studies, which was 94% of parent for the
sulfoxide (photolysis) and 72% of parent for the sulfone degradate (aerobic metabolism).
The RQ analysis was based on an evaluation of potential risks to birds. However,
terrestrial amphibians are poikilotherms (body temperature varies with environmental
temperature) while birds are homeotherms (temperature is regulated, constant, and
largely independent of environmental temperatures). As a consequence, the caloric
requirements of amphibians are markedly lower than birds. Therefore, on a daily dietary
intake basis, birds consume more food than amphibians. This can be seen when
comparing the caloric requirements for free living iguanid lizards to Passeriformes (song
birds) (U.S. EPA, 1993):
iguanid FMR (kcal/day)= 0.0535 (bw g)A0.799
passerine FMR (kcal/day) = 2.123 (bw g)A0.749
With relatively comparable slopes to the allometric functions, one can see that, given a
comparable body weight, the free living metabolic rate of birds can be 40 times higher
than reptiles, though the requirement differences narrow with high body weights.
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To quantify the potential differences in food intake and resulting potential differences in
pesticide exposure between birds and terrestrial amphibians, RQs were calculated based
on food intake estimates considered to be more representative of CRLFs. These results
are in Table 5.10. Consideration of the different dietary behaviors of the CRLF
compared with birds does not alter conclusions of this assessment. The highest acute RQ
was 23 based on a single application of 1 lb a.i./Acre, and the acute RQ was exceeded for
all food items except other terrestrial phase amphibians. Modeling of two applications
would also result in RQs that exceed LOCs.
Table 5.10. Upper Bound Kenaga, Acute Terrestrial Herpetofauna Dose-Based
Risk Quotients Based on a Single Application of 1 lb a.i./Acre
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
RQ
EEC
RQ
EEC
RQ
EEC
RQ
EEC
RQ
1.4
6.50
5.24
0.81
0.58
0.09
N/A
N/A
N/A
N/A
N/A
N/A
37
6.50
5.15
0.79
0.57
0.09
N/A
N/A
N/A
N/A
0.18
0.03
238
6.50
3.38
0.52
0.38
0.06
23.26
3.58
1.45
0.22
0.12
0.02
In addition to LOC exceedances, several incidences have associated disulfoton exposure
to bird mortality as summarized below. No incidences involving terrestrial amphibians
have been reported.
One incident (L.Lyon, Div. of Environmental Contaminants, U.S. Fish and Wildlife Service,
Arlington, VA) reported that birds consuming insects that fed on plants treated with
disulfoton were killed. This incident further emphasizes the potential importance of an
exposure pathway that is not quantified in this assessment. Consumption of insects that
have consumed plant material contaminated with disulfoton via system uptake and
translocation in the plant resulted in effects to higher trophic level organisms that fed on
insects.
1/24/94 Puerto Rico: 6 grackles fell dead from tree in yard of private residence. Dead
heron and owl also found in vicinity. Use site and method not reported. Birds had
depressed acetyl cholinesterase. Analysis of GI contents of a grackles showed
disulfoton at 2.37 ppm wet weight. Highly probable certainty index for
disulfoton. (Incident Report No. 1003966-004).
6/18/93 Young County,TX: 18 Swainson's hawks dead, lseverely disabled in a cotton
field. Cotton seed had been treated with disulfoton prior to planting, -10 days
before the birds were discovered. No additional applications of OP or carbamate
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pesticides made in vicinity of field. Autopsies showed no trauma or disease. Lab
analysis showed insect material in GI tracts; this material contained disulfoton
(~7 ppm); no other OP or carbamate insecticides were present. Hawks fed on
insects, which had been feeding on the young cotton plants, which contained
disulfoton residues. (L.Lyon, Div. of Environmental Contaminants, U.S. Fish
and Wildlife Service, Arlington, VA.)
4/26/91 Sussex County, DE: 9 American robins dead following application of granular
disulfoton at tree nursery. Corn and soybeans also in vicinity. No laboratory
analysis. Probable certainty index for disulfoton. (Incident Report No. 1000116-
003).
The incidences support the conclusion that birds (and terrestrial phase amphibians as a
surrogate) may be affected by labeled uses of disulfoton. Therefore, the RQ analysis
together with the presence of several incidences that associated bird mortality with
disulfoton exposure support the conclusion that the labeled uses of disulfoton are likely to
adversely affect terrestrial phase CRLFs.
5.2.2 Indirect Effects (via Reductions in Prey Base), Aquatic Phase CRLFs
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. No to toxicity data are
currently available for aquatic plants; therefore, EC50s cannot be derived for use in risk
assessment. However, disulfoton is an insecticide with low toxicity to terrestrial plants.
No terrestrial plant incidents have been reported for disulfoton with a certainty index of
"probably" or higher. Therefore, there is no compelling evidence that aquatic or
terrestrial plants will be impacted to a degree that would affect CRLFs by labeled uses.
5.2.2.2 Aquatic Invertebrates
The potential for disulfoton 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 ranged from 0.5 - 17 based on the most sensitive
species tested (glass shrimp). Therefore, the acute LOCs were exceeded for all uses. The
associated estimated probability of an individual mortality at these RQs is greater than 1
in 10 based on a probit dose-response slope of 4.5 (default). The most sensitive species
tested was the glass shrimp. Toxicity of disulfoton to other aquatic invertebrate species is
in Table 5.11.
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Table 5.11. Acute RQs for Various Aquatic Invertebrates for Disulfoton.
Freshwater
Species
Test Material
Most Sensitive
Toxicity Value
(ppb ai)
RQ
Source of Data
Daphnia
Disulfoton
13
0.14-5.2
MRID 00143401
Scud
Disulfoton
27
0.07-2.5
MRID 05017538; 40098001
Glass shrimp
Disulfoton
3.9
0.46 - 17
MRID 40094602
Stonefly
Disulfoton
5
0.36-13
MRID 229299, 40098001
These data suggest that multiple aquatic invertebrate species may be affected by
disulfoton exposure at levels estimated in this assessment.
Disulfoton degrades to more stable degradates somewhat rapidly in the environment with
a half-life of approximately 3 days. Therefore, aquatic prey of the CRLF may also be
exposed to disulfoton degradates. EC50s for the sulfone and sulfoxy degradates were
approximately 3 and 5 times, respectively, less toxic to daphnids than disulfoton.
However, it is uncertain if other invertebrate species are more or less sensitive to
degradates because studies evaluating the toxicity of disulfoton degradates in species
other than daphnids are not available.
Because the acute LOC of 0.5 is approached or exceeded for all uses based on the most
sensitive aquatic invertebrate tested, and the potential magnitude of effects to aquatic
invertebrate species tested could result in indirect effects to the CRLF, the effect
determination for potential indirect effects to the CRLF via reduction in available food
supply is "likely to adversely affect."
5.2.2.3 Fish and Aquatic-Phase Frogs
Potential risk to freshwater fish were described in Section 5.2.1 (direct effects). It was
concluded that labeled disulfoton uses are likely to adversely affect CRLFs. However,
this does not necessarily correlate with potential indirect effects to CRLFs that consume
fish because a LAA determination for a direct effect is made on the individual level
(might a single individual be affected?). The potential for indirect effects is evaluated
based on the potential magnitude of effects to the food item. RQs based on the most
sensitive species tested are in Table 5.12.
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Table 5.12.
LC50 of 37
Summary of Direct Effect RQs for Aquatic-phase CRLFs Based on an
Use
Peak
EEC
(jig/L)
Aeutc
RQ
(LC50:
37 ug/L)
Probability
of Individual
Effect (acute
effects only)
60-Dav EEC
(l*g/L)
RQ
(NOAEC: 4
ug/L)
LOC Excccdancc
and Risk
Interpretation
Beans, Broccoli,
Cauliflower
1.8-3.6
0.05-
0.097
1 in 4E8 to 1
in 4E5
1.3-2.8
0.33-0.70
The endangered
species acute LOC is
exceeded.
Residential,
Cotton, Brussels
sprouts,
Christmas trees,
lettuce
3.7 - 15
0.10-
0.41
1 in 3E5 to
1 in 25
3.2-12
0.8
(residential),
0.88 (cotton),
1.1 - 3 (other
uses)
Restricted use LOC is
exceeded for these
uses, and the chronic
LOC is exceeded for
Brussels sprouts,
Christmas trees, and
lettuce.
Asparagus,
cabbage
23-24
0.62-
0.65
1 in 6 to
1 in 5
18-22
3.5-8.8
Acute and chronic
LOCs are exceeded
for these uses.
Lettuce (drip
irrigation)
67
1.8
1 in 1
54
14
Acute and chronic
LOC exceeded
Freshwater fish RQs ranged from 0.05 (beans) to 1.8 (lettuce) for the most sensitive
species tested (bluegill). Based on a default probit slope of 4.5, the estimated probability
of an individual effect to the most sensitive fish species was approximately 1 in 3E5 or
less for beans, broccoli, cauliflower, and residential However, this analysis was based on
the default slope of 4.5. Based on a reasonable lower bound probit slope, the probability
of an individual mortality would range between approximately 1 in 200 to 1 in 50. A
probability of an individual mortality of this magnitude would result in an undetectable
reduction in available prey of the CRLF and would, therefore, be an insignificant effect
(an effect that may occur, but would not harm or harass the assessed species). Therefore,
the effects determination for these uses is not likely to adversely affect.
However, for all other uses (broccoli, asparagus, Brussels sprouts, cauliflower, and
cabbage), the magnitude of effect would not be considered discountable and was 1 in 25
or greater based on a default probit slope of 4.5. Therefore, the effects determination is
likely to adversely affect the CRLF for these uses.
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 RQ used to estimate potential effects to
terrestrial invertebrates was 7.7. Based on the default probit slope of 4.5, the probability
of an individual mortality approaches 100%. Two degradates have been shown to be
more toxic to bees than disulfoton. The LD50s for the sulfoxide and sulfone degradates
are 1.1 and 0.96 ug/bee, respectively, compared with an LD50 of 4 ug/bee for disulfoton.
These degradates have been shown to form up to 94% of parent in degradation studies.
Therefore, exposure to these degradates could also impact terrestrial invertebrates and
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indirectly affect the CRLF. AgDrift analysis indicates that the non-endangered species
LOC of 0.5 could be exceeded for up to approximately 100 feet from the treated site.
This analysis shows that terrestrial invertebrates could be impacted at a level that could
indirectly affect the CRLF by reducing available food. Therefore, a finding of likely to
adversely affect (LAA) was made for the potential for disulfoton and its degradates to
potentially impact terrestrial invertebrate prey base as available food.
5.2.2.5 Mammals
Dietary information for terrestrial-phase CRLFs indicate that large adult frogs consume
terrestrial vertebrates, including mice. Acute mammalian RQs were up to approximately
100 for disulfoton, and LOCs were also exceeded for the sulfone degradate (sulfoxide
degradate was not assessed due to lack of toxicity data). At these RQs, the probability of
an individual mortality would approach 100%. In addition, the reproduction RQs were as
high as 4600 for disulfoton. The RQ analysis suggests that exposed mammals could be
impacted to a level that could adversely affect individual CRLFs that depend on them for
food. AgDisp analysis indicates that LOCs would be exceeded for >1000 feet from the
application site (see Table 5.14). Therefore, it was concluded that disulfoton and its
degradates are likely to adversely affect the CRLF.
5.2.2.6 Terrestrial-phase Amphibians
Terrestrial-phase adult CRLFs also consume other frogs. RQ values representing direct
exposures of disulfoton to terrestrial-phase CRLFs are used to represent exposures of
disulfoton to frogs in terrestrial habitats. As demonstrated in Table 5.10, acute RQs that
incorporated herptile food intake levels exceeded the acute LOC of 0.5 for frogs that
consume several potential prey items of CRLFs at an application rate of 1 lb a.i./acre
(single application). Therefore, it was concluded that disulfoton and its degradates are
likely to adversely affect the CRLF.
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, rather than energy, to the system,
as attachment sites for many aquatic invertebrates, and refugia for juvenile organisms,
such as fish and frogs. Emergent 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 are typically assessed using RQs from freshwater aquatic vascular and non-
vascular plant data. However, no aquatic plant studies were submitted. Therefore, an
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evaluation of potential effects to aquatic plants could not be quantified. Based on the low
toxicity and risk to terrestrial plants, potential magnitude of impacts to aquatic plants are
not expected to be such that the CRLF may be indirectly affected.
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. 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.
Concern levels for terrestrial plants were not exceeded. Therefore, it was concluded that
use of disulfoton is expected to have "no effect" on the CRLF by affected terrestrial
plants.
5.2.4 Modification to Designated Critical Habitat
Based on the lack of potential effects to the CRLF resulting from impacts to terrestrial or
aquatic plants, labeled uses of disulfoton are not expected to impact critical habitat based
on PCEs that are related to presence and maintenance of aquatic or terrestrial vegetation.
However, two PCEs may be impacted by use of disulfoton:
1. Reduction and/or modification of food sources for terrestrial phase juveniles and
adults
2. Alteration of other chemical characteristics necessary for normal growth and
viability of CRLFs and their food source.
5.2.5 Distance From Treated Site Effects May Occur
This assessment concluded that labeled uses of disulfoton could adversely affect the
CRLF by direct and/or indirect effects when used according to the label on asparagus,
beans, broccoli, Brussels sprouts, cabbage, cauliflower, Christmas trees, cotton, lettuce,
and residential areas (all uses). Therefore, the CRLF could be affected in its habitat that
overlaps with areas that produce these commodities and residential areas. Potential
effects are not limited to the treated field. The environmental fate properties indicate that
runoff and spray drift represent significant potential transport mechanisms of disulfoton
to the aquatic and terrestrial habitats of the CRLF. Therefore, there is potential for
disulfoton to be transported outside of the area where it is directly applied. Two transport
pathways were evaluated to determine the potential distance from treated sites that could
be impacted. Spray drift deposition was evaluated to determine the distance from treated
sites that spray drift deposition would no longer be expected to affect CRLFs. The only
uses with spray application methods for which spray drift is a potential transport pathway
are asparagus, cabbage, and cotton. Also, for aquatic phase frogs, the distance
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downstream from use sites needed to dilute disulfoton concentrations to an extent that
would not longer result in direct or indirect effects to the CRLF was evaluated.
Downstream dilution analysis is relevant for all crops. This analysis is described briefly
below and in more detail in Appendix C.
Since this screening level risk assessment defines taxa that are predicted to be exposed
through runoff and drift to disulfoton 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 down stream dilution
assessment for determining the extent of the affected lotic aquatic habitats (flowing
water); and (2) the spray drift assessment for determining the extent of potentially
affected terrestrial habitats.
5.2.5.1 Downstream Dilution
In order to determine the extent of potential effects to lotic (flowing) aquatic habitats, the
agricultural uses resulting in the greatest ratios of the RQ to the LOC for any endpoint for
aquatic organisms is used to determine the distance downstream for concentrations to be
diluted below levels that would be of concern {i.e. result in RQs above the LOC). This
analysis is in Table 5.13 below. For this assessment, the greatest ratio was 5600 (the
highest aquatic invertebrate RQ = 5600; LOC = 1; 5600 / 1= 5600; see Table 5.13) for
indirect effects to the CRLF through reproductive effects to aquatic invertebrates exposed
to disulfoton (lettuce chemigation use). Using methods described in Appendix C,
downstream analysis using this RQ determined that 257 km is the maximum distance
from the edge of any potential use area to a point where it falls below the LOC.
Table 5.13. RQ/LC
>C Ratio for Various Landcover Classes for Aquatic Organisms3
Direct/Indirect Effects
to CRLF
Exposure
Cropland
Highest RQa
RQ/LOC Ratio
Direct and Indirect -
Fish and Aquatic
Amphibians
Acute
1.8
36
Chronic
54
54
Indirect-Aquatic
Invertebrates
Acute
17
340
Chronic
5600
5600
a RQ Calculations are presented in Section 5.1; LOC for acute and chronic effects is 0.05 and 1.0,
respectively.
5.2.5.1 Spray Drift
Table 5.14 indicates that at distances greater than approximately 8336 feet from the
treated site, RQs for terrestrial organisms will be below LOCs. This evaluation was
based on potential indirect effects to the CRLF from potential reduction in prey. The
endangered species LOC was used for this analysis for acute effects; however, the
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probability of an individual effect at the endangered species LOC is approximately 1 in
30,000 assuming a probit slope of 4.5. The resulting potential impact to animal
abundance would not be detectable in the environment. Therefore, distances associated
with alternative levels of risk were calculated and are presented in Table 5.15. The
restricted use LOC (0.2) and acute LOC (0.5) was used for this analysis. Based on a
probit slope of 4.5, the probability of an individual mortality at these LOCs are
approximately 1 in 100 and 1 in 10, respectively.
Table 5.14. AgDISP predicted Buffer Distance resulting in no Endangered Species
LOC Exceec
ance for Terrestrial Animals for Disulfoton
Effect/
Taxonomic
Group
Acute or
Chronic
Effect
Highest
RQa
Spray drift
Fraction
Needed to
Reduce RQs to
Below LOCsb
Distance from Treated Site Fraction is
Achieved
Aerial spray
(Asparagus;
1 lb a.i./A)
Ground spray
(Asparagus,
Cabbage;
1 lb a.i./A)
Direct
(avian RQs)
Acute
44
0.23 %
4091 ft
3123 ft
Chronic
5.7
17.5 %
223 ft
256 ftc
Indirect-
mammals
Acute
55
0.18%
4452 ft
3218 ft
Chronic
2600
0.04 %
8336 ft
4258 ft
Indirect-
Terrestrial
Invertebrates
Acute
Contact
Exposures
(small insect)
4.4
1.14%
2404 ft
2670 ftc
a RQ Calculations are presented in Section 5.
b Spray drift fraction = l/(RQ/LOC); Acute LOC = 0.1 (end. species), Chronic LOC = 1, Terrestrial Invertebrate LOC
= 0.05
c Drift levels from aerial applications are generally expected to be higher than similar ground boom applications. As a
result, if the aerial model suggests a smaller action area, that would be protective of ground applications, even though
in some cases, uncertainties in the AgDisp model can lead to results for ground applications which are greater than for
aerial applications.
Table 5.15. Spraydrift Fraction Resulting in no Restricted Use or Acute LOC
Exceedance for Terrestrial Animals for Disulfoton
Effect/Taxono
mic Group
Acute or
Chronic
Effect
Highest
RQa
Spraydrift
Fraction Needed
to Reduce RQs
to Below
Restricted Use
LOC b
Distance from
Treated Site
Fraction is
Achieved
(aerial)
Spraydrift
Fraction Needed
to Reduce RQs
to Below Acute
LOCb
Distance from
Treated Site
Fraction is
Achieved
(aerial)
Direct
(avian RQs)
Acute
44
0.45 %
3228
1.14%
2404
Indirect-
mammals
Acute
55
0.36 %
3474
0.91 %
2582
a RQ Calculations are presented in Section 5.
b Restricted Use LOC = 0.2, Acute LOC = 0.5.
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Similar to the analysis described above, the buffer distance needed to get below the most
sensitive aquatic LOC was determined. This distance identifies those locations where
water bodies can be impacted by spray drift deposition alone (no runoff considered)
resulting in concentrations above the LOC. As with the terrestrial assessment, for each
aquatic taxa of concern, the fraction of the application rate needed to reduce exposures to
levels below LOCs is calculated. Based on this fraction and estimation of spray drift
patterns, AgDISP determines the buffer distance required between the treated field and
the water body to result in exposure estimates below the level of concern. Distances were
based on the highest RQs for aerial applications to asparagus at 1 lb a.i./A. Drift levels
from aerial applications are generally expected to be higher than similar ground boom
applications and so results of the aerial model would be protective of ground applications
as well. The analysis yields much lower buffer distances than the terrestrial buffer, as
presented in Table 5.16.
Table 5.16. Spraydrift Fraction Resulting in no LOC Exceedance for Aquatic
Animals for Disulfoton
Effect/
Taxonomic
Group
Acute or
Chronic
Effect
Highest
RQa
LOC
Classification
Spray drift
Fraction Needed
to Reduce RQs
to Below LOCsb
Distance from Treated Site
Fraction is Achieved
(aerial)
Direct &
Indirect
(Fish and
Frogs)
Acute
1.8
End. Species
2.78 %
1151ft
Restricted
5.56 %
387 ft
Acute
27.78 %
Oft
Chronic
54
NA
1.85 %
2057 ft
Indirect
(Aquatic
Invertebrates)
Acute
17
End. Species
0.29 %
3415 ft
a RQ Calculations are presented in Section 5.
b Spray drift fraction = l/(RQ/LOC); Acute LOC = 0.05 (end. species), 0.1 (restricted), 0.5 (acute); Chronic LOC = 1
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. Additionally, for residential uses, labels do not express application
rates on a per acre basis. Therefore, for purposes of aquatic modeling, conservative
assumptions were made about the area that would be treated, leading to uncertainty in
estimating exposure.
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6.1.2 Aquatic Exposure Modeling of Disulfoton
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.
Surface water modeling using PRZM/EXAMS does not consider discharge of
contaminated groundwater to stream baseflow as a potential route of aquatic exposure.
Modeling results may therefore underestimate exposure in some surface waters relevant
to CRLF habitat 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 are assumed to reflect
background concentrations that might be found in discharged groundwater/stream
baseflow. Groundwater monitoring data available from California sites have no
detections of disulfoton or its transformation products. One study conducted in
Wisconsin, though, found levels of up to 100 ug/L, indicating that disulfoton does have
the potential to reach groundwater and suggesting that the assumption of lower disulfoton
levels in discharging groundwater may not be conservative.
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
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
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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. Except for the asparagus use, all aquatic modeling assumed that
disulfoton was applied to bare soil which may overestimate exposure from foliar
applications because foliar dissipation is not accounted for. Additionally, application
dates were chosen to represent times within the potential application window when
precipitation was highest, which leads to conservative exposure estimations. 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.
Aquatic modeling was conducted for total toxic residues, including disulfoton + d.
sulfoxide + d. sulfone. This adds additional uncertainty to the modeling inputs because
degradation half-lives were calculated from studies intended to investigate degradation
patterns for the parent compound and may not include sufficient data to accurately
represent transformation of the degradates as well. Koc inputs were based on the most
mobile component of the total residues and so may overestimate mobility of the group.
Disulfoton labels require a 25 foot buffer around any surface water bodies. Tools are not
currently available to evaluate the effectiveness of a vegetative setback on preventing
runoff and loadings. The effectiveness of vegetative setbacks 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
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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.
Exposure estimates are based on modeling alone because limited monitoring data for total
residues are available to compare to the modeled estimates. Available monitoring data
are primarily for parent disulfoton and do not include the more persistent and more
mobile degradates and so are not likely to capture all exposure. Both the NAWQA and
CDPR include monitoring data in California for disulfoton in surface and ground water
with no detections. The specific use patterns (e.g. application rates and timing, crops)
associated with the agricultural areas are unknown, but they are assumed to be
representative of potential disulfoton use areas. Both d. sulfoxide and d. sulfone have
been detected in a subset of these data despite limited sampling. This demonstrates that
total toxic residues are more likely to reach water bodies than parent alone and indicates
that the lack of detections of parent disulfoton is insufficient to conclude that exposure is
unlikely. The assessment discussion of disulfoton detections in surface and ground water
monitoring data from sites outside of California. There is uncertainty in these data
because the environmental conditions may not be representative of conditions in the area
of concern.
6.1.3 Usage Uncertainties
County-level usage data were obtained from California's Department of Pesticide
Regulation Pesticide Use Reporting (CDPR PUR) database. Four years of data (2002 -
2005) were included in this analysis because statistical methodology for identifying
outliers, in terms of area treated and pounds applied, was provided by CDPR for these
years only. No methodology for removing outliers was provided by CDPR for 2001 and
earlier pesticide data; therefore, this information was not included in the analysis because
it may misrepresent actual usage patterns. 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 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.
6.1.4 Terrestrial Exposure Modeling of disulfoton
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
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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.
6.1.5 Spray Drift Modeling
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 disulfoton from multiple applications, each application of disulfoton
would have to occur under identical atmospheric conditions (e.g., same wind speed and
same wind direction) and (if it is an animal) the animal being exposed would have to be
located in the same location (which receives the maximum amount of spray drift) after
each application. 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, especially as the distance increases from the site of application,
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since the model does not account for potential obstructions (e.g., large hills, berms,
buildings, trees, etc.).
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.
6.2.2 Use of Surrogate Species Effects Data
Guideline toxicity tests and open literature data on disulfoton 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 t 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.
As discussed in Section 4, cytolologic effects were observed in the liver of fish exposed
to disulfoton at levels as low as 0.1 ug/L (Arnold et al. (1996). These effects were not
used in the current risk assessment for the purpose of this effects determination because
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the level of effects observed in the available studies have not been directly correlated
with the assessment endpoints of survival or reproduction.
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 disulfoton 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 disulfoton. Additionally, the Agency has
determined that there is the potential for modification of CRLF designated critical habitat
from the use of the chemical. This determination applies to all currently labeled uses.
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.
Table 7.1a Effects Determination Summary for Direct and Indirect Effects of Disulfoton on the
CRLF
Assessment Endpoint
Effects
Determination1
Basis for Determination
Aquatic-Phase CRLF
(Eggs, Larvae, and Adults)
Direct Effects:
Survival, growth, and reproduction of
CRLF individuals via direct effects on
aquatic phases
LAA
Endangered species LOC was exceeded for all uses;
chronic LOC was exceeded for all uses except cotton,
beans, and residential uses. Potential effect was not
considered discountable or insignificant.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via effects to food
supply (i.e., freshwater invertebrates,
non-vascular plants, fish, and frogs)
Freshwater
invertebrates: LAA
Acute and chronic RQs were exceeded for all uses.
Acute RQs ranged from approximately 0.5 to 17 and
chronic RQs ranged from 145 to 5600. The potential
magnitude of effect could be sufficient to result in
indirect effects to the CRLF.
Non-vascular aauatic
olants: NE
No aquatic plant toxicity data have been submitted or
were located in the open literature. Disulfoton is an
insecticide, and EC25s for terrestrial plants were greater
than the maximum application rate.
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Table 7.1a Effects Determination Summary for Direct and Indirect Effects of Disulfoton on the
CRLF
Assessment Endpoint
Effeets
Determination1
Basis for Determination
Fish and froes: LAA
for some uses
Magnitude of potential impacts to fish and aquatic phase
amphibians could be sufficient to indirectly affect the
CRLF for some uses. The highest RQs occurred for the
lettuce, cabbage, and asparagus uses.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via indirect effects on
habitat, cover, and/or primary
productivity (i.e., aquatic plant
community)
Non-vascular
aciuatic olants: NE
No aquatic plant toxicity data have been submitted or
were located in the open literature. However,
disulfoton is an insecticide, and EC25s for terrestrial
plants were greater than the maximum application rate.
Vascular aauatic
olants: NE
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.
NE
The EC25 is greater than the highest labeled application
rate for all uses except Christmas trees. The Christmas
tree RQ would be <0.5.
1 NE = no effect; NLAA = may affect, but not likely to adversely affect; LAA = likely to adversely affect
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Table 7.1b Effects Determination Summary for Direct and Indirect Effects of Disulfoton on the
CR.LF
Assessment Endpoint
Effects
Determination1
Basis for Determination
Terrestrial-Phase CRLF
(Juveniles and adults)
Direct Effects:
Survival, growth, and reproduction of
CRLF individuals via direct effects on
terrestrial phase adults and juveniles
LAA
Acute LOC (0.5) was exceeded for all uses for disulfoton
and its degradates. Potential for reproductive effects also
exists for all uses.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via effects on prey (i.e.,
terrestrial invertebrates, small terrestrial
vertebrates, including mammals and
terrestrial phase amphibians)
Terrestrial
invertebrates: LAA
The endangered species LOC of 0.05 was exceeded for
all uses. Also, disulfoton is an insecticide, and the
potential magnitude of effect could be sufficient to result
in indirect effects to the CRLF.
Mammals: LAA
Acute (0.5) and chronic (1.0) LOCs were exceeded for
all uses. The potential magnitude of effect could be
sufficient to result in indirect effects to the CRLF.
Fross: LAA
Acute (0.5) and chronic (1.0) LOCs were exceeded for
all uses. The potential magnitude of effect could be
sufficient to result in indirect effects to the CRLF.
Indirect Effects:
Survival, growth, and reproduction of
CRLF individuals via indirect effects on
habitat (i.e., riparian vegetation)
NE
The EC25 is greater than the highest labeled application
rate for all uses except Christmas trees. The Christmas
tree RQ would be <0.5.
1 NE = no effect; NLAA = may affect, but not likely to adversely affect; LAA = likely to adversely affect
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Table 7.2 Effects Determination Summary for the Critical Habitat Impact Analysis
Assessment Endpoint
Effects
Determination1
Basis for Determination
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.
NE
Effects determination for potential effects related to
impacts on aquatic and terrestrial plants was No
Effect.
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.2
NE
Effects determination for potential effects related to
impacts on aquatic and terrestrial plants was No
Effect.
Alteration of other chemical characteristics necessary
for normal growth and viability of CRLFs and their
food source.
HM
Effects determination for direct and indirect effects
to the CRLF was LAA.
Reduction and/or modification of aquatic-based food
sources for pre-metamorphs (e.g., algae)
NE
Effects determination for potential effects related to
impacts on aquatic plants was No Effect.
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
NE
Effects determination for potential effects related to
impacts on aquatic and terrestrial plants was No
Effect.
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
NE
Reduction and/or modification of food sources for
terrestrial phase juveniles and adults
HM
Effects determination for indirect effects via
reducing available food supply was LAA.
Alteration of chemical characteristics necessary for
normal growth and viability of juvenile and adult
CRLFs and their food source.
HM
Effects determination for direct and indirect effects
was LAA.
1 NE = No effect; HM = Habitat Modification
2 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.
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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
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
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
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complete prediction of effects to individual frogs and potential modification to
critical habitat.
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