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phorate (Thimet) EPA Pesticide Fact Sheet 12/88

EPA Pesticide Fact Sheet

Name of Chemical: Phorate
Reason for Issuance: Registration Standard
Date Issued: December 1988
Fact Sheet Number: 34.2


                      1. DESCRIPTION OF CHEMICAL

- Generic name:  O,O-diethyl S-[(ethylthio)methyl] phosphorodithioate
- Common name:  Phorate
- Trade Names:  Thimet, AAstar and Rampart
- Other Chemical Nomenclature:  O,O-diethyl-S-(ethylthio) methyl
  phosphorodithioate and O,O-diethyl S[(ethylthio)methyl] ester
- EPA Pesticide Chemical (Shaughnessy) Number:  057201
- Chemical Abstracts Service (CAS) number:  298-02-2
- Year of initial registration:  1959
- Pesticide type:  Insecticide
- Chemical family:  Systemic Organophosphate
- U.S. Registrants:  American Cyanamid Company, Uniroyal Chemical
  Company Inc., Aceto Chemical Company Inc., Wilbur Ellis Co.,
  Riverside Terra Corp., Farm Bureau Cooperative, and Platte Chemical
  Co.

                 2. USE PATTERNS AND FORMULATIONS

- Application sites
  - Terrestrial food crop use on beans, corn (field and sweet), cotton,
    hops, peanuts, potatoes, sorghum, soybeans, sugar beets, sugarcane,
    barley and wheat.
  - Terrestrial non-food crop use on lilies (bulb production).
- Formulations:  Granular
- Pests Controlled:  Various leaf-feeding insects, mites, and soil
  insects
- Methods of application:  Soil and foliar applications (band,
  broadcast, in-furrow and drilling) using conventional ground and
  aerial equipment

                          3. SCIENCE FINDINGS

Summary Science Statement

     Technical phorate is in Toxicity Category I by the oral, dermal and 
inhalation routes. The acute oral administration of phorate to hens did 
not cause a delayed neurotoxic effect. Based on results of acceptable 
subchronic and chronic feeding studies with rats and dogs, 
cholinesterase (plasma, blood or brain) is the primary target for 
phorate. Phorate does not produce oncogenic effects, based on results of 
acceptable chronic studies in rats or dogs. Phorate does not induce 
teratogenic effects, based on results of acceptable teratology and 
reproduction studies. Phorate did not cause a mutagenic response in 
several in vitro (microbial and mammalian cells) studies or in an in 
vivo dominant lethal study. Results of an acceptable metabolism study 
using male rats indicated that a large proportion of phorate labeled 
metabolites were excreted in urine and feces within 24 hours of dosing. 
The total radioactivity levels in tissues was low. The oxidative, 
phosphorylated products (metabolites of phorate which may be more potent 
anticholinesterase compounds through oxidative desulfuration and/or 
sulfide oxidation) represented a minor proportion of the phorate 
metabolites measured.

     Based on acceptable laboratory data, technical phorate is 
characterized as very highly toxic to birds on an acute oral basis; 
highly toxic to birds on a dietary basis; very highly toxic to mammals 
on a dietary basis and very highly toxic to freshwater fish and aquatic 
invertebrates and estuarine and marine organisms on an acute toxicity 
basis. Results of the terrestrial field studies (Level l) showed 
mortalities to avian and mammalian species.  An aquatic field study 
(required in the 1984 Standard) is still in progress and is due in 1991.

     Many of the tolerances for phorate are still not adequately 
supported. Additional data (residue studies, residue analytical methods, 
processing and cooking studies, poultry metabolism study and storage 
stability data) are needed before the Agency can determine the adequacy 
of current tolerance levels and perform a tolerance reassessment. Based 
on the NOEL for brain cholinesterase in a one year dog study (0.05 
mg/kg) and applying an uncertainty factor of 100, the Agency has 
calculated the Anticipated Residue Contribution (ARC) for the U.S. 
population average to be 0.000491 mg/kg/day, corresponding to 98% of the 
RfD. The ARC assumes residues are present at tolerance levels, but takes 
into account percent of crop treated, where possible. For children 1 to 
6 years of age, the ARC occupies 235% of the RfD, and for non-nursing 
infants, the ARC occupies 331% of the RfD. The Agency is requiring 
processing and cooking studies to assess anticipated residue levels in 
meat and milk. The Agency expects that cooking and processing of meat 
and milk will reduce residues of phorate to levels which will be of 
little or no concern.


Chemical/Physical Characteristics of the Technical Material

- Chemical/Physical Characteristics
  - Color: pale straw to light brown (TGAI, 2749-106); colorless to very
    light yellow (TGAI, 241-212 and 241-213)
  - Physical state: liquid
  - Odor: characteristic of mercaptan containing compounds
  - Boiling Point: 118-120 degrees C, 0.8 mm Hg
  - Specific Gravity: 1.15 at 20 degrees C (TGAI, 2749-106); 1.17 at
    25 degrees C (TGAI for 241212 and 241-213)
  - Solubility: 50 mg/l in water; miscible with carbon tetrachloride,
    vegetable oils (unspecified), xylene, and unspecified alcohols,
    ethers and esters
  - pH: 5-7 (TGAI, 2749-106); 3.56-3.81 (TGAI, 241-212 and 241-213)
  - Viscosity:  80 cps at 21 degrees C
  - Corrosion:  non-corrosive to steel, aluminum, porcelain, fiberglass,
    and phenolic resins


Toxicology Characteristics

- Acute Oral:  Toxicity Category I (LD50 of 3.7 and 1.4 mg/kg in male
  and female rats, respectively)
- Acute dermal:  Toxicity Category I (LD50 of 9.3 and 3.9 mg/kg in male
  and female rats, respectively)
- Acute inhalation:  Toxicity Category I (LD50 of 60 and 11 mg/m3 for
  male and female rats, respectively)
- Primary dermal irritation:  None Available.  Not required since the
  toxicity of phorate prohibits the administration of appropriate dosage
  levels.
- Primary eye irritation:  None Available.  Not required since the
  toxicity of phorate prohibits the administration of appropriate dosage
  levels.
- Skin sensitization:  None available. Not required due to the high
  acute toxicity of the chemical.
- Delayed Neurotoxicity:  Did not induce delayed neurotoxicity in an
  acceptable study in hens.
- Subchronic non-rodent study:  None available. Not required since
  acceptable chronic data for the non-rodent are available.
- Subchronic rodent study:  A rat study is available. The LEL in this
  study was 2.0 ppm (0.1 mg/kg/day); the NOEL was 0.66 ppm (0.033
  mg/kg/day).
- Chronic toxicity:  Dog study is available (NOEL and LEL for systemic
  toxicity were 50 and 250 ug/kg/day, respectively). Mouse study is
  available (NOEL and LEL were .45 and .9 ug/kg/day, respectively). Rat
  study is available (LEL was 0.05 mg/kg/day, NOEL was not determined)
- Oncogenicity:  The rat combined chronic toxicity and oncogenicity
  study did not reveal any evidence that phorate was oncogenic under the
  condition of that study. Based on a reevaluation of the mouse study,
  the evidence does not show that an MTD was attained. Confirmatory data
  are required.
- Mutagenicity:  Phorate was negative in all areas of mutagenicity
  testing (gene mutation, structural chromosome aberration and tests for
  other genotoxic effects)
- Teratogenicity:  A rat study is available (LEL for developmental
  toxicity, based on embryotoxicity, and maternal toxicity was 0.50
  mg/kg and the corresponding NOEL for each was 0.25 mg/kg). A rabbit
  study is available (LOEL and NOEL for maternal toxicity was 0.5 and
  0.15 mg/kg, respectively. The NOEL for developmental toxicity was 1.2
  mg/kg, the highest dosage administered).
- Reproduction:  A mouse study is available (LEL was .45 mg/kg/day and
  the NOEL was .23 mg/kg/day).
- Metabolism: A study in male rats is available. Results indicated that
  a large proportion of the administered 14C was recovered in urine and
  feces.  Oxidative, phosphorylated products only represented a minor
  proportion of the metabolites measured.


Environmental Characteristics

     Based on the results of an acceptable leaching study, 14C phorate 
was reported to be very mobile to mobile in loamy sand, sandy loam, silt 
loam, and loam soils.  The 1984 Registration Standard indicated that 
phorate has some potential to leach through the soil and contaminate 
groundwater.  Based on recently submitted data, phorate does not appear 
to be a potential leacher.  However, its sulfone and sulfoxide 
degradates show greater persistence and mobility in soil, and therefore 
may have a greater leaching potential.  Since data are still 
outstanding, the Agency cannot fully assess phorate's potential for 
contaminating groundwater.


Ecological Characteristics

     Based on acceptable acute data, technical phorate is characterized 
as very highly toxic to birds on an acute oral basis, highly toxic to 
birds on a dietary basis, very highly toxic to mammals on a dietary 
basis, and very highly toxic to freshwater fish and aquatic 
invertebrates and estuarine and marine organisms on an acute toxicity 
basis.

- Acute LD50 (mallard):  0.62 mg/kg
- Acute LD50 (chukar):  12.8 mg/kg
- Dietary LC50:  248 ppm (waterfowl); 441 ppm (upland gamebirds) 28 ppm
  (small mammals)
- Freshwater invertebrates toxicity (96-hr LC50) for amphipods: 0.68 ppb
  to 9 ppb
- Fish acute toxicity (96-hr LC50) for rainbow trout:  6 to 13 ppb
- Fish acute toxicity (96-hr LC50) for bluegill sunfish:  2 ppb; 5 ppb
  for bass
- Estuarine fish and invertebrates (LC50) 0.11 to 1.9 ppb for shrimp and
  1.3 to 5.0 ppb for spot and sheepshead minnow; for mollusks (900 ppb)


Tolerance Assessment

     Tolerances for residues of phorate in or on food and feed 
commodities are published in 40 CFR 180.206.  A tolerance for residues 
of phorate on the processed feed commodity, dried sugarbeet pulp, is 
published in 21 CFR 180.590.  Tolerances are expressed in terms of 
phorate and its cholinesterase-inhibiting metabolites.

     Based on data submitted in response to the 1984 Standard, the 
nature of the residue in plants is adequately understood.  The nature of 
the residue in animals not adequately understood. A poultry metabolism 
study is required.  The available data support the established 
tolerances for the combined residues of phorate and its cholinesterase-
inhibiting metabolites in or on potatoes, sugar beets, sugar beet tops, 
sugar beet pulp, and soybeans.  Additional data (residue studies, 
processing and cooking studies, residue analytical methods, poultry 
metabolism study and storage stability data) are needed before the 
Agency can determine the adequacy of current tolerance levels and 
perform a tolerance reassessment.

     The Agency has performed a preliminary dietary exposure analysis 
using tolerance level residues and percent of crop treated where 
possible.  The ARC for phorate for the U.S. population average is 
0.000491 mg/kg/day.  For the U.S. population average, the ARC occupies 
98% of the ADI.  For children 1 to 6 years of age, the ARC occupies 235% 
of the ADI, and for non-nursing infants, it occupies 331% of the ADI. 
The ARC is based on current tolerance levels, and, where possible, on 
percent of crop treated.  Due to the significant contribution made by 
milk to the diet of children and non-nursing infants, data regarding the 
reduction of residues through cooking and processing are required.


         4. SUMMARY OF REGULATORY POSITIONS AND RATIONALES

     Based on a high acute toxicity of phorate to avian species and the 
current registered uses of phorate, there exists a high potential for 
adverse effects to avian species from exposure to phorate granules at or 
near the soil surface.  This potential for exposure to phorate is 
demonstrated from results of Level I studies and is confirmed by bird 
kill incidents.  The Agency is currently evaluating these data in the 
context of a comparative risk assessment of granular pesticides which 
may pose a risk to birds.  Based on this assessment, regulatory action 
may be taken.

     The Agency is not placing phorate into Special Review at this time 
for hazards to aquatic organisms. Available field reports and laboratory 
data indicate that the concentrations of phorate in the aquatic 
environment resulting from the registered uses of phorate might expose 
aquatic species to residue levels exceeding risk criteria for Special 
Review. Upon receipt and evaluation of the aquatic field study (due in 
1991), a determination will be made regarding further regulatory action.

     Unique warning statements required include revised and updated fish 
and wildlife toxicity statements, reentry statements, and protective 
clothing statements.

     The Agency is requiring special acute and subchronic eye studies to 
evaluate phorate's effect on the eye.

     The Agency will not approve significant new uses for this chemical 
since many of the tolerances are still not adequately supported.

     The Agency will continue to restrict the use of products containing 
phorate. Phorate meets the risk criteria of 40 CFR 152.170 due to acute 
oral and dermal toxicity and bird toxicity.

     The Agency is still unable to fully assess phorate's potential for 
contaminating groundwater. Upon receipt of a terrestrial field 
dissipation study, and other requested environmental fate data, the need 
for groundwater monitoring will be determined.

              5. SUMMARY OF OUTSTANDING DATA REQUIREMENTS

Toxicology                                    Time Frame

Special Testing for Eye Effects               9-18 Months
Metabolism                                      24   "
Mouse Short-term Study                          12   "

Environmental Fate/Exposure

Hydrolysis                                       9 Months
Photodegradation in Water                        9   "
Photodegradation on Soil                         9   "
Aerobic Soil Metabolism                         27   "
Anaerobic Soil Metabolism                       27   "
Lab Volatility                                  12   "
Soil Dissipation                                27   "
Confined Rotational Crop                        39   "
Accumulation in Fish                            12   "
Foliar Dissipation                              18   "
Soil Dissipation                                18   "

Fish and Wildlife

Avian Reproduction                              24 Months
Estuarine/Marine Organism Testing (TEP)         12   "
Freshwater and Estuarine Fish Early Life
  Stage Life-Cycle Study                        15   "
Freshwater and Estuarine Invertebrate
  Life-Cycle Study                              15   "
Aquatic Organism Field Testing              January, 1991

Residue Chemistry

Residue data - Raw Agricultural Commodities     18 Months
Processing Studies                              24   "
Poultry Metabolism                              18   "
Cooking Studies                                 24   "
Storage Stability                               15   "
Residue Analytical Methods                      15   "

Product Chemistry

Majority of Data                              9-15 Months


                    6.  CONTACT PERSON AT EPA

William H. Miller
Product Manager (16)
Insecticide-Rodenticide Branch
Registration Division (TS-767)
Environmental Protection Agency
Washington, DC  20460
Tel. No. (703) 557-2600

DISCLAIMER:  THE INFORMATION 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.