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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
  alkaline conditions.

Toxicology Characteristics

- 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
  0.5 ppm.
- 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.

   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.

Environmental Characteristics:

   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 
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 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.

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.

Residue Data:

   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-
     inhibiting) metabolites:

     - 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 


   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.

   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.


- 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


           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

Environmental Fate

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)
  sediment, fish)

Residue Chemistry

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

Product Chemistry

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)
Insecticide-Rodenticide Branch
Registration Division (TS-767C)
Environmental Protection Agency
Washington, 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.