terbufos (Counter) EPA Pesticide Fact Sheet 9/88
EPA 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-[[(l,l-dimethylethyl)thio]methyl]O,O-diethyl
- Other Chemical Nomenclature: S-([tert-butylthio)methyl]O,O-diethyl
phosphorodithioate (IUPAC); S-(t-butylthio) methyl O,O-diethyl-
phosphorodithioate (CA, 8th Collective Index); S-tert-butylmercap-
tomethyl O,O-diethyl dithiophosphate
- Trade Names: Contraven Counter; AC 92,100; CL 92,100 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 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 1 pound active
ingredient per acre (l lb ai/A). In the same study, the application of
terbufos as a soil-incorporated treatment to corn fields at 2 lb 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 terbufos. 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 degrees C at 0.02 mmHg
- Stability: Relatively stable in water under neutral or slightly
acidic conditions but is subject to hydrolysis under
- Acute Oral: Toxicity Category I (1.6 and 1.3 mg/kg for male and
female rats, respectively).
- Acute Dermal: Toxicity Category I (0.81 and 0.93 mg/kg for male and
female rabbits, respectively).
- Acute Inhalation: Toxicity Category I (< 0.2 mg/L).
- Delayed Neurotoxicity: No evidence of acute delayed neurotoxicity
at the 40 mg/kg dosage level tested in hens.
- Subchronic Feeding: The NOEL for both systemic effects and
cholinesterase inhibition in a rat subchronic study is 0.25 ppm.
- Subchronic Dermal: The NOEL for systemic effects in a 30-day rabbit
study is 0.020 mg/kg.
- Mutagenicity: Terbufos did not exhibit mutagenic potential in the
Ames assay, the in vivo cytogenetic assay, and the dominant lethal
- Teratogenicity: The NOEL for developmental toxicity in a rat
teratology study is 0.1 mg/kg/day.
- Reproduction: The NOEL for reproductive effects in a three-generation
rat reproduction study is 0.25 ppm.
- 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.
- 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 l-year dog study is 0.060 mg/kg/day.
The NOEL for plasma and brain ChE from a 1-year rat feeding study is
- 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.
- 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.
Terrestrial Field Study (Level 1): Both soil-incorporated (2 lb
ai/A) and nonsoil-incorporated (1 lb ai/A) resulted in nontarget
mortalities, with the latter application much more severe in its
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 modeling* 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 broadcast 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 characteristics 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,
*The Agency used computer models (SWRRB) and (EXAMS) to simulate runoff
from granular application of terbufos and to predict aquatic concentra-
tions of the chemical. SWRRB is a hydrology model combined with a
pesticide runoff model. EXAMS is a hydrologic model to predict "steady-
state" and "pulseload" behavior of organic toxicants in aquatic
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
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 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.
Results of an acceptable hydrolysis study indicate that terbufos
hydrolyzes at p 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
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 lb 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
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.
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.
In the Terbufos Registration Standard dated June 1983, no outstanding
data gaps were identified for residue chemistry. However, subsequent
amendments to registered uses for terbufos and addenda to the Pesticide
Assessment Guidelines (Subdivision O) 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 sulfone and terbufoxon sulfone
comprised < 7 percent. The major nonphosphorylated metabolite 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 pertaining 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 adequacy 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 will propose
revising 40 CFR 180.352 by changing the wording to read:
". . . terbufos . . . and its phosphorylated (cholinesterase-
- Phosphorothioic acid, S-(t-butyl-thio) methyl O,O-diethyl ester.
- Phosphorothioic acid, S-(t-butyl-sulfinyl) methyl O,O-diethyl
- Phosphorothioic acid, S-(t-butyl-sulfonyl) methyl O,O-diethyl
- Phosphorodithioic acid, S-(t-butyl-sulfinyl) methyl O,O-diethyl
- Phosphorodithioic acid, S-(t-butyl-sulfonyl) metyl O,O-diethyl
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
4. 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
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.
Due to the lack of pertinent environmental fate data, no conclusions
regarding the potential for terbufos to contaminate ground water can be
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
- 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
6. SUMMARY OF OUTSTANDING DATA REQUIREMENTS
Data Due Date*
Special 21-day rat dermal 12 Months
Rabbit teratology 15 Months
Fish & Wildlife
Acute toxicity to estuarine
and marine organisms 12 Months
Fish early life stage 15 Months
Aquatic organism accumulation 12 Months
Terrestrial field study **
Aquatic organism field study 6 Months for protocol
Dislodgeable residue study 27 Months
Photodegradation in water 9 Months
Photodegradation on soil 9 Months
Photodegradation in air 9 Months
Anaerobic soil metabolism 27 Months
Leaching and absorption/desorption 12 Months
Lab volatility 12 Months
Soil field dissipation 27 Months
Rotational crop (field) 50 Months
Fish accumulation 12 Months
Monitoring study (soil, water, 6 Months (protocol)
Crop field trials 18 Months
Processing studies 24 Months
Storage stability 15 Months
Ruminant and poultry metabolism 18 Months
Residue analytical methodology data 15 Months
Majority of data 6-15 Months
*Due date is measured from the date of receipt of the Standard by the
registrant unless otherwise specified.
**1st 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.
7. CONTACT PERSON AT EPA
William H. Miller
Product Manager (16)
Registration Division (TS-767C)
Environmental Protection Agency
Washington, DC 20460
Phone: (703) 557-2600.
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