United States Office of Pesticides and Toxic Substances
Environmental Protection Office of Pesticide Programs (TS-766C)
Agency Washington, DC 20460
54Q/FS-RR-120 / . ..
• vvEPA Pesticide
Fact Sheet
Name of Chemical: TERBUFOS
Reason for Issuance: Revised Registration Standard
Date issued: September 9, 1988
Fact Sheet Number: 5.2
1. Description of Chemical
Common Name: Terbufos
Chemical Name: S-[[(1,l-dimethylethyl)thio]methyl]0,0-diethyl
phosphorodithioate
Other Chemical Nomenclature: S-([tert-butylthio)methyl]0,0-diethyl
phosphorodithioate (IUPAC); S-(Jb-butylthio) methyl
0,O-diethyl-phosphorodithioate (CA, 8th Collective
Index); j3-tert-butylmercaptomethyl 0,O-diethyl
dithiophosphate
Trade Names: Contraven; Counter; AC 92,100; CL 92,100r and ST-100
Chemical Abstracts Service (CAS) Number: 13071-79-9
EPA Shaughnessy Code: 105001
Year of Initial Registration: 1974
Pesticide Type: Insecticide-Nematicide
Chemical Family: Organophosphate
U.S. Registrant: American Cyanamid Company
2. Use Patterns and Formulations
Application Sites: Terrestrial food crop use on corn; grain
sorghum; and sugar beets.
Formulations: Granular
Methods of Application: Broadcast (nonsoil-incorporated) with air or
ground equipment; and soil-incorporated with ground equipment.
3. Science Findings
Summary Science Statement: Technical terbufos is highly acutely
toxic by the oral, dermal, and inhalation routes of exposure
(Toxicity Category I for all three routes). Terbufos does not
demonstrate an acute neurotoxic, oncogenic, mutagenic,
reproductive, or teratogenic potential. Animal studies have shown
that the chemical is a cholinesterase inhibitor reducing plasma,
brain, and red blood cell cholinesterase activity. The use of
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terbufos poses a potential risk to loaders and applicators and to
persons reentering treated fields following nonsoil-incorporated
broadcast application of the chemical. This is due to the high
'acute toxicity and the cholinesterase inhibiting properties of the
chemical.
Based on the plasma cholinesterase inhibition no-effect-level of
0.00125 mg/kg/day as defined in a 4-week dog study and, using a
safety factor of 10, the acceptable daily dietary intake for
humans in 0.000125 mg/kg/day. The theoretical maximum residue
contribution from the established tolerances is estimated to be
0.000052 mg/kg/day. This is equivalent to 42 percent of the
acceptable daily intake for the average U.S. population. Due to
the numerous gaps in residue chemistry data, the Agency is unable
to complete a tolerance reassessment of terbufos.
Terbufos is highly toxic to birds, fish, and aquatic invertebrates.
The acceptable short-term field study shows significant acute
mortalities of birds, mammals, reptiles, and fish resulting from
broadcast application of terbufos to corn fields at •! pound active
ingredient per acre (1 Ib ai/A). In the same study, the application
of terbufos as a soil-incorporated treatment to corn fields at 2
Ib ai resulted in acute mortalities to birds and reptiles.
The limited environmental fate data are not sufficient to assess
the mobility and leaching properties of tebufos. Terbufos residues
were reported to occur in well water sampling in Iowa and Minnesota.
These reports, however have not been confirmed in the laboratory
and a resampling of the same Iowa wells a year later, in 1986,
showed no detections for terbufos or its degradates. Terbufos is
one of the pesticides the Agency is sampling for in the National
Well Water Survey.
Chemical/Physical Characteristics of the Technical Material:
Color: Clear, brownish
Physical State: Liquid
Odor: Mercaptan-like
Boiling Point: 55 °C at 0.02 mmHg
Stability: Relatively stable in water under neutral or slightly
acidic conditions but is subject to hydrolysis under
alkaline conditions.
Toxicology Characteristics
o Acute Oral: Toxicity Category I (1.6 and 1.3 mg/kg for male and
female rats, respectively).
o Acute Dermal: Toxicity Category I (0.81 and 0.93 mg/kg for male
and female rabbits, respectively).
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o Acute Inhalation: Toxicity Category I (<_ 0.2 mg/L).
o Delayed Neurotoxicity: No evidence of acute delayed neurotoxicity
at the 40 mg/kg dosage level tested in hens.
o Subchronic Feeding: The NOEL for both systemic effects and
cholinesterase inhibition in a rat subchronic study is 0.25 ppm.
o Subchronic Dermal: The NOEL for systemic effects in a 30-day
rabbit study is 0.020 mg/kg.
o Mutagenicity: Terbufos did not exhibit mutagenic potential in the
Ames assay, the _in vivo cytogenetic assay, and the dominant lethal
test.
o Teratogenicity: The NOEL for developmental toxicity in a rat
teratology study is 0.1 mg/kg/day.
o Reproduction: The NOEL for reproductive effects in a three-
generation rat reproduction study is 0.25 ppm.
o Oncogenicity: No oncogenic effects observed in an 18-month mouse
study and a 2-year rat study at doses up to and including 12.0
ppm (1.8 mg/kg/day) and 8.0 ppm (0.4 mg/kg/day), respectively.
o Chronic Toxicity: The NOEL for plasma cholinesterase (ChE)
inhibition from a 4-week dog feeding study is 0.00125 mg/kg/day;
the NOEL for brain/red blood cell ChE from a 1-year dog study
is 0.060 mg/kg/day. The NOEL for plasma and brain ChE from a
1-year rat feeding study is 0.5 ppm.
o Metabolism: Terbufos was rapidly excreted as the diethyl
phosphoric acid and other polar metabolites (83%) in urine
within 168 hours of administration to male rats. Terbufos
and its metabolites were not noted to accumulate in tissues.
Ecological Characteristics:
Based on acceptable laboratory data, technical terbufos is highly
toxic to birds, fish, and aquatic invertebrates.
Acute Avian Toxicity: 28.6 mg/kg (bobwhite).
Dietary Avian Toxicity: 143 and 157 ppm (from two bobwhite studies).
Avian Reproduction: Terbufos was not considered to produce avian
reproductive effects based on results of a bobwhite quail study
and a mallard duck study.
Freshwater Fish Acute Toxicity: Ranges from 0.77 to 20.00 ppb.
Freshwater Invertebrate Acute Toxicity: 0.31 ppb for Daphnia magna.
Marine/Estuarine Fish Acute Toxicity: Data gap.
Marine/Estuarine Invertebrate Toxicity: Data gap.
Marine/Estuarine Mollusk Toxicity: Data gap.
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Terrestrial Field Study (Level 1): Both soil-incorporated (2 Ib
>ai/A) and nonsoil-incorporated (1 Ib ai/A) resulted in nontarget
mortalities, with the latter application much more severe in its
effects.
Based on the adverse effects observed in the level 1 field study
described above, level 2 terrestrial field studies are required to
assess the potential effects on populations of birds, mammals, and
reptiles. Based on the high acute toxicity to aquatic organisms
and results of initial modeling1 conducted by the Agency for the
1983 Terbufos Registration Standard, the estimated environmental
concentration (EEC) of terbufos residues likely to occur in the
aquatic environment may pose an acute hazard for freshwater and
marine/estuarine species. This modeling was conducted for the
soil-incorporated application of terbufos.
Potentially greater hazards are likely for aerial applications of
terbufos granules since soil-incorporated applications typically
provide less exposure than aerial broadpast applications.
Although these theoretical calculations and modeling indicate that
the use of terbufos may result in significant adverse effects to
aquatic species, actual field monitoring data are not available
to support this finding. Moreover, the environmental fate charac-
teristics of terbufos are not accurately defined by available
data. Thus, the models can be used only on a limited basis.
Aquatic residues monitoring studies are required to determine actual
residues in aquatic systems exposed to runoff and spray drift.
Although these studies were previously requested in the 1983
Terbufos Registration Standard, their initiation was delayed
pending the Agency's recalculation of the EECs. Prior to the
completion of this task, reports of fish kill incidents demonstrating
the potential exposure to aquatic organisms under actual field use
conditions became available. These fish kills reportedly resulted
from aerial applications of terbufos to corn fields during conduct
of the level 1 terrestrial field study. In addition, several
environmental fate studies previously found acceptable do not meet
current guideline requirements and need to be repeated.
Terbufos has been identified by the Office of Endangered Species
(OES), U.S. Fish and Wildlife Service (USFWS), as being likely to
jeopardize the continued existence of certain endangered species
Agency used computer models (SWRRB) and EXAMS) to simulate
runoff from granular application of terbufos and to predict aquatic
concentrations of the chemical. SWRRB is a hydrology model combined
with a pesticide runoff model. EXAMS is a hydrologic model to pre-
dict "steady-state" and "pulseload" behavior of organic toxicants in
aquatic ecosystems.
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when used on corn and sorghum. Based on this determination, OES
specified reasonable and prudent alternatives to avoid jeopardizing
the continued existence of the identified species. EPA is working
with USFWS and other Federal and State agencies to implement the
alternatives in a technically sound manner.
Formal consultation will be initiated with OES under Section 7 of the
Endangered Species Act regarding the potential exposure to
endangered species resulting from the registered use of terbufos
on sugar beets.
Environmental Characteristics;
Results of an acceptable hydrolysis study indicate that terbufos
hydrolyzes at pH 5, 7, and 9 with a half-life of 2.2 weeks.
Formaldehyde was the major degradate detected in this study.
Results of an acceptable aerobic soil metabolism study indicate
that terbufos degrades in silt loam soil with a half-life of
26.7 days. The major .degradates detected in this study included
carbon dioxide, terbufos sulfoxide, and terbufos sulfone.
Results of a field dissipation study, classified as supplementary,
indicate that terbufos residues have a half-life of less than
40 days in field plots of loam soil located near Arcola, Illinois,
and sandy loam soil located near Greeley, Colorado treated with
a 15 percent granular formulation at an application rate of 1 Ib
ai/A. The sampling protocol was inadequate to accurately assess
the dissipation of terbufos residues in field soil and a new study
is required.
The available data reviewed by the Agency are not sufficient to
fulfill data requirements nor to assess the environmental fate of
terbufos. Four studies previously reviewed and found acceptable
under the 1983 Terbufos Registration Standard do not meet the
requirements of the Agency's current guidelines and new studies
are required. These are: anaerobic soil metabolism, leaching,
fish accumulation, and field dissipation.
In addition, several new studies are now required due to the
additional method of nonsoil-incorporated, broadcast (air or
ground equipment) application which was not registered at the
time of the 1983 Terbufos Registration Standard.
Tolerance Assessment;
Tolerances for combined residues of terbufos and its ChE-inhibiting
metabolites in or on food commodities are published under Section
180.352 of Title 40 of the Code of Federal Regulations (40 CFR
180.352). These tolerances range from 0.05 to 0.5 ppm.
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Residue Data;
In the Terbufos Registration Standard dated June 1983, no outstanding
data gaps were identified for residue chemistry. However, subse-
quent amendments to registered uses for terbufos and addenda to
the Pesticide Assessment Guidelines (Subdivision 0) for Residue
Chemistry have made it necessary to reevaluate portions of the
data base previously reviewed under the June 1983 Standard. As a
result, some of the original conclusions regarding adequacy of the
data and support for tolerances have been modified in this revised
Registration Standard.
Based on the available plant metabolism studies, the nature of
residues in plants is adequately understood. Of the phosphorylated
metabolites, terbufoxon sulfoxide and terbufos sulfoxide comprised
<_ 30 percent of the residues, and terbufos sulf one and terbuf oxon
sulfone comprised _<_ 7 percent. The major nonphosphorylated metabo-
lite which comprised _<_ 30 percent of the organosoluble residues was
nonphosphorylated terbufoxon sulfone. The available poultry and
ruminant feeding studies do not meet current Guideline requirements
for data depicting the metabolism of terbufos in livestock and new
studies are required. The basic GLC analytical procedure published
as Method I in PAM Vol. II is adequate for collection of data per-
taining to the combined residues of terbufos and its ChE-inhibiting
metabolites on commodities with established tolerances. Method
validation data pertaining to recovery of individual metabolites
from representative plant commodities are being required. The ade-
quacy of the available methods for detection of terbufos residues
of concern in animal products will be evaluated upon receipt of the
required animal metabolism data. Field trial studies are required
for all crops for which there are terbufos tolerances. Processing
studies are also required in addition to storage stability residue
data.
The available poultry and ruminant feeding studies show that no
detectable residues occur in eggs, chicken tissues, milk, or
cattle tissues from animals fed exaggerated dietary levels of
terbufos and its ChE-inhibiting metabolites. However, additional
animal metabolism data are required and a determination regarding
the need for and nature of tolerances for residues in meat, milk,
poultry, and eggs will be made upon receipt and evaluation of
these data.
The established tolerances for terbufos are presently expressed in
terms of terbufos and its ChE-inhibiting metabolites without
specifying the latter as phosphorylated metabolites. The Agency
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will propose revising 40 CFR 180.352 by changing the wording to
read:
"... terbufos . . . and its phosphorylated (cholinesterase-
inhibiting) metabolites:
o Phosphorothioic acid, S-(t-butyl-thio) methyl 0,0-diethyl ester.
o Phosphorothioic acid, S-( t-butyl-sulf inyl) methyl C),_0-diethyl
ester.
o Phosphorothioic acid, S-(_t-butyl-sulfonyl) methyl _0,^D-diethyl
ester.
o Phosphorodithioic acid, S-(t-butyl-sulfinyl) methyl 0^,0-diethyl
ester.
o Phosphorodithioic acid, S-( jt-butyl-sulfonyl) methyl £,0-diethyl
ester."
Acceptable Daily Intake;
Based on the plasma ChE inhibition NOEL as defined in a 4-week dog
study (0.00125 mg/kg/day) and using a safety factor of 10, the
acceptable daily intake or reference dose (RfD) for humans is
0.000125 mg/kg/day.
Summary of Regulatory Positions and Rationales
Terbufos is not being placed in special review at this time. Field
studies are needed to completely assess the potential risk to
wildlife, including endangered species. The Agency is conducting
a comparative avian risk assessment of various granular pesticides,
including terbufos. When this assessment is completed, further
regulatory action may be taken.
The restricted use classification of the 15 percent granular end-use
product based on the high acute oral and dermal toxicity to humans
is being retained.
A level II terrestrial field study; monitoring studies in soil,
water, sediment, and fish; and aquatic organism field studies are
being required for the completion of the Agency's assessment of
the potential risk to both avian and aquatic species.
A special 21-day dermal study in rats and a dislodgeable residue
study are being required for the completion of the Agency's
assessment of the potential risk to workers reentering corn fields
following a nonsoil-incorporated application of terbufos.
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Due to the lack of pertinent environmental fate data, no conclusions
regarding the potential for terbufos to contaminate ground water
can be made.
EPA is developing a program to reduce or eliminate exposure to endangered
species to a point where use does not result in jeopardy, and will
issue notice of any necessary labeling revisions when the program
is developed. No additional labeling is required at this time.
Labeling requirements issued in PR Notices 87-4 and 87-5 have been
withdrawn pending reissuance.
5. Summary of Required Label Modifications
An updated Environmental Hazard statement is required.
Updated worker safety rules and protective clothing statements are
required.
A 7-day reentry interval statement is required for use of terbufos as
a broadcast application to corn.
A label statement prohibiting use of terbufos as a broadcast
application to seed corn prior to detasseling activities is
required.
6. Summary of Outstanding Data Requirements
Data Due Date1/
Toxicology
Special 21-day rat dermal 12 Months
Rabbit teratology 15 Months
Fish s Wildlife
Acute toxicity to estuarine
and marine organisms 12 Months
Fish early life stage 15 Months
Aquatic organism accumulation 12 Months
_VDue date is measured from the date of receipt of the Standard by
the registrant unless otherwise specified.
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Data
Fish & Wildlife (cont'd)
Terrestrial field study
Aquatic organism field study
Reentry
Dislodgeable residue study
Environmental Fate
Photodegradation in water
Photodegradation on soil
Photodegradation in air
Anaerobic soil metabolism
Leaching and absorption/desorption
Lab volatility
Soil field dissipation
Rotational crop (field)
Fish accumulation
Monitoring study (soil, water,
sediment, fish)
Residue Chemistry
Crop field trials
Processing studies
Storage stability
Ruminant and poultry metabolism
Residue analytical methodology data
Product Chemistry
Majority of data
Due Date^/
6 Months for protocol
27 Months
9 Months
9 Months
9 Months
27 Months
12 Months
12 Months
27 Months
50 Months
12 Months
6 Months (protocol!
18 Months
24 Months
15 Months
18 Months
15 Months
6-15 Months
_1_/Due date is measured from the date of receipt of the Standard by
the registrant unless otherwise specified.
2/1 st Annual Report December 31 , 1989
2nd Annual Report* December 31, 1990
3rd Annual Report* December 31, 1991
Final Report** December 31, 1992
*A determination may be made at this time to conclude the study, in
which case a final report will be due 3 months after notification.
**The due date applies if the study has not been determined to be
concluded by earlier reviews.
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7. Contact person at EPA
William H. Miller
Product Manager (16)•
Insecticide-Rodenticide Branch
Registration Division (TS-767C)
Environmental Protection Agency
VJashington, DC 20460
Phone: (703) 557-2600.
DISCLAIMER: The information presented in this Chemical Information Fact
Sheet is for informational purposes only and may not be used to fulfill
data requirements for pesticide registration and reregistration.
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