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Terbufos

Publication Date: 3/94

TRADE OR OTHER NAMES

Trade names include Contraven and Counter.

INTRODUCTION

Terbufos is an organophosphate insecticide and nematicide used on corn, sugar beets and grain sorghum. Primarily formulated as granules, it is applied at planting in a band or directly to the seed furrow. Terbufos controls wireworms, seedcorn maggots, white grubs, corn rootworm larvae and other pests.

Products containing 15% or more terbufos are classified as Restricted Use Pesticides (RUP). Restricted Use Pesticides may be purchased and used only by certified applicators.

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Terbufos products are labeled with a DANGER signal word, and are extremely toxic. Symptoms of acute toxicity often include nausea, abdominal cramps, vomiting salivation, excessive sweating and diarrhea within 45 minutes of ingestion. Absorption into the bloodstream may cause inhibition of cholinesterase, an enzyme essential for normal functioning of the nervous system. This, in turn, can lead to chest tightness, wheezing, blurred vision, fatigue, headache, slurred speech and confusion. Symptoms from skin absorption, such as localized sweating, may be delayed up to 12 hours. At high enough doses, death may result from respiratory arrest, respiratory muscle paralysis and/or constriction of the lungs.

No neurotoxic effects were observed in chickens given a single dose of 40 mg/kg, the highest dose tested (8).

The oral LD50 of terbufos is from 1.3 to 1.57 mg/kg in female rats, and from 1.6 to 1.74 mg/kg in male rats (7). The oral LD50 for technical terbufos in male mice is 3.5 mg/kg, 9.2 mg/kg in female mice, 4.5 mg/kg in male dogs, and 6.3 mg/kg in female dogs (12). Rabbits given a single dose of 0.1 mg to the eyes died within 2 to 24 hours after dosing (8).

The dermal LD50 for rabbits is 1.1 mg/kg for 24 hours (2, 5).

CHRONIC TOXICITY

Slow thinking, memory loss, irritability, delayed reaction times and anxiety have been noted in workers chronically exposed to organophosphates like terbufos.

When rats were fed approximately 0, 0.01, 0.02, 0.046 or 0.09 mg/kg/day for 90 days, the NOEL was 0.02 mg/kg/day. Cholinesterase inhibition occurred at higher doses (12). Similar results were obtained in a 1-year study with rats. The NOEL was 0.025 mg/kg/day, with cholinesterase inhibition occurring at 0.05 mg/kg/day, the highest dose tested (12).

EPA has established a Lifetime Health Advisory (LHA) of 0.001 mg/l of terbufos in drinking water. This means that EPA believes that water containing terbufos at or below this concentration is acceptable for drinking every day over the course of one's lifetime, and does not pose any health concerns. Consumption of terbufos at high levels well above the LHA level over a long period of time has caused damage to the eye and stomach, disturbances in fetal development and cholinesterase inhibition in animals (13).

Reproductive Effects

In a long-term study in rats, no chronic reproductive effects were observed after daily exposure to low doses of terbufos (7).

When rats were fed 0, 0.0125 or 0.05 mg/kg/day for 6 months, there was an increase in the number of litters with dead offspring at the highest dose tested. The reproductive NOEL for this study was 0.0125 mg/kg/day (12).

Rabbits were given doses of 0, 0.1, 0.2 or 0.4 mg/kg/day on days 7 to 19 of pregnancy. Cesarean sections were performed on day 29. No adverse effects on the offspring were seen at any dose. Toxic effects on the mothers occurred at the highest dose tested (12).

Terbufos appears unlikely to cause reproductive effects in humans.

Teratogenic Effects

Terbufos does not cause birth defects in animals, except in extreme situations, and similar effects are not anticipated in humans. There were no birth defects in the offspring of rats given 0.05, 0.1 or 0.2 mg/kg/day on days 6 to 15 of pregnancy (12).

In a similar study on rabbits, no birth defects were observed in the off- spring of rabbits given 0.25 mg/kg/day, the highest dose tested. The mothers exhibited reduced body weight gain at this dose (8).

Mutagenic Effects

Several tests have shown that terbufos is not mutagenic. These include a dominant lethal study in rats, an Ames test, a DNA repair chromosomal aberration test, and a test for DNA repair in rat liver cells (8, 12).

Carcinogenic Effects

No tumors were found in mice given 1.8 mg/kg/day, the highest dose tested, for 18 months. The same results occurred in a 2-year study with rats given up to 0.40 mg/kg/day, the highest dose tested (8, 12). EPA has stated that terbufos does not increase the risk of cancer in humans (13).

Organ Toxicity

Because terbufos inhibits cholinesterase, this pesticide can affect the eyes, lungs, skin, and central nervous system, depending on the route of exposure, and the concentration.

Fate in Humans and Animals

In rats given a single oral dose of terbufos, 10% remained in the liver six hours after dosing. Breakdown products were found in the kidney 12 hours after dosing. Of the original dose administered, 83% was excreted in the urine within seven days after dosing, and 3.5% was found in the feces. Terbufos and its metabolites did not accumulate in tissues (7, 12).

No detectable residues have been found in the eggs, milk or body tissues of animals (hens and cows) fed very high dietary doses of terbufos and its cholinesterase inhibiting metabolites (7).

ECOLOGICAL EFFECTS

Effects on Birds

Terbufos is extremely toxic to birds. Its acute LD50 in bobwhite quail is 28.6 mg/kg. Its dietary LC50 in bobwhites is 143 to 157 ppm (7).

There were no effects on bird reproduction from chronic exposure to terbufos (7, 11).

Effects on Aquatic Organisms

Terbufos is extremely toxic to fish, and aquatic invertebrates (7). The acute LC50 for terbufos in freshwater fish species is 0.77 to 20 ppb (7). The LC50 for terbufos in Daphnia magna, a small freshwater invertebrate, is 0.31 ppb (7).

Effects on Other Animals (Nontarget species)

Terbufos is extremely toxic to mammals, and reptiles (7). It is not toxic to bees when used properly.

ENVIRONMENTAL FATE

Breakdown of Chemical in Soil and Groundwater

Terbufos is moderately persistent in the soil. It is rapidly converted to its metabolites which tend to persist in the soil and may be detected at harvest time (1). Terbufos and its metabolites quickly degrade during the first 15-30 days after application, then gradually stabilize. Only 3% of the original application stayed in field-study soils after one month, with 1.5% of the chemical present after 60 days (4).

In a study on silty clay loam soil in South Dakota, the half-life of terbufos was about two weeks. The half-life for the metabolite, terboxon sulfone, was two to three times longer (1).

When applied to a silt loam soil, the half-life for terbufos was calculated at 15 days, while the total residue half-life was 22 days. After 106 days, the total residues were less than 1.0 ppm (6).

Terbufos dissipation is generally faster in soils with very low organic carbon, while binding increases with increasing organic carbon content. Sandy soils lose more of this chemical than do soils with clay over the same time period (4).

Terbufos is generally immobile and is therefore unlikely to leach or contaminate groundwater (4, 12). Much of the chemical can be recovered near the site of application. In one study, over 90% of the applied terbufos was recovered in the top four inches of a soil profile despite heavy rainfall and thorough incorporation down to two-and-one-half inches (4).

Soil moisture does not appear to affect the degradation of terbufos. This chemical will breakdown at about the same rate in soils regardless of the level of wetness (4). As temperature increases, terbufos degrades more quickly.

Being of low water solubility, terbufos is not often found in groundwater. Terbufos has been found in a few groundwater samples collected from locations across the United States. The concentration found was approximately 11 ug/l (10).

Breakdown of Chemical in Surface Water

Terbufos hydrolyzes rapidly. At a concentration of 4.6 ppm, its hydrolysis half-lives were 4.5, 5.5 and 8.5 days at pH 5, 7, and 9 respectively (7, 12). In another study, terbufos hydrolyzed with a half-life of 2.2 weeks at pH 5, 7 and 9. Formaldehyde was the major degradate detected (7).

Breakdown of Chemical in Vegetation

Terbufos moves from the soil into plants, where it is broken down rapidly. Little of the parent compound is found in plants. Fifty-seven days after seeding and application, the total residues in broccoli were very low, while the marketable heads of broccoli harvested 90 days after seeding held only traces (less than 0.01 ppm, fresh weight) of residues. Under the same conditions, marketable cabbage and cauliflower had trace to nondetectable levels of total residues (6). Field corn banded with 1.12 kg/ha had no detectable residues 60 days after treatment. Sweet corn and popcorn grain harvested at maturity also showed no residue even though the surrounding soil contained 10-14 ppm (6).

PHYSICAL PROPERTIES AND GUIDELINES

Terbufos is a clear, slightly brownish-yellow liquid which is usually formulated into granules for agricultural applications. Its molecular weight is 288.41. It decomposes at temperatures .

Exposure Guidelines:

NOEL (rat): 0.00125 mg/kg/day
ADI: 0.000125 mg/kg/day (8)
LHA: 0.001 mg/l (13)
RfD: 0.000125 mg/kg/day (12)

Physical Properties:

CAS #: 13071-79-9
Chemical name: S-[[(1,1-dimethylethyl) thio] methyl]O,O-diethyl phosphorodithioate
Chemical class/use: organophosphate insecticide
Density: 1.105 at 24 degrees C (12)
Solubility in water: 10-15 mg/L (9); 5.5 mg/L (liquid at 19 degrees C) (3)
Solubility in solvents: acetone, aromatic hydrocarbons, chlorinated hydrocarbons, and alcohols.
Melting point: minus 29.2 degrees C (9)
Boiling point: 69 degrees C (9); 55 degrees C at 0.02 mm Hg (7)
Decomposition point: greater than 120 degrees C (7)
Vapor pressure: 34.6 mPa at 25 degrees C (12)
log P: 595 (12)

BASIC MANUFACTURER

American Cyanamid
One Cyanamid Plaza
Wayne, NJ 07470

Review by Basic Manufacturer:

Comments solicited: November, 1992
Comments received: December, 1992

REFERENCES

  1. Ahmad, N., D.D. Walgenbach and G.R. Sutter. 1979. Bull. Environm. Contamination Toxicol. 23, 423-429.
  2. Meister, R.T. (ed.). 1986. Farm Chemicals Handbook. Meister Publishing Co., Willoughby, OH.
  3. Bowman, B.T. and W.K. Sans. 1979. J. Environ. Sci. Health. B14(6): 625-34.
  4. Felsot, A., L. Wei and J. Wilson. 1982. Environmental chemodynamic studies with terbufos ("Counter") insecticide in soil under laboratory and field conditions. J. Environ. Sci. Health. B17(6): 649-673.
  5. Occupational Health Services Inc. Material Safety Data Sheet for Terbufos. 4/15/87. OHS: NY.
  6. Szeto, S.Y., M.J. Brown, J.R. Mackenzie and R.S. Vernon. 1986. Degradation of terbufos in soil and its translocation into cole crops. J. Agric. Food Chem. 34: 876-879.
  7. U.S. Environmental Protection Agency. Sept. 9, 1988. Pesticide Fact Sheet: Terbufos, Fact sheet No. 5.2. Office of Pesticide Programs, US EPA, Washington, DC.
  8. U.S. Environmental Protection Agency. Aug. 30, 1989. Pesticide Tolerance for Terbufos. Federal Register 54 (167): 35896-7.
  9. Windholz, M. et al. (eds.). 1983. The Merck Index. 10th ed. Merck & Co., Inc.
  10. Williams, W.M, et.al. 1988. Pesticides in Groundwater Data Base: 1988 Interim Report. U.S. Environmental Protection Agency, Office of Pesticide Programs, Washington, DC.
  11. Walker, M.M. and L.H. Keith. 1992. EPA's Pesticide Fact Sheet Database. Lewis Publishers. Chelsea, MI.
  12. U.S. Environmental Protection Agency. Aug. 1988. Health Advisory: Terbufos. Office of Drinking Water, US EPA, Washington, DC.
  13. U.S. Environmental Protection Agency. Jan. 1989. Health Advisory Summary: Terbufos. US EPA, Washington, DC.