Pesticide Profile - Terbufos

Quick Facts

• Chemical name: S-tert-butylthiomethyl O,O-diethyl phosphorodithioate

• Trade names: Counter, Aragran, Contraven, Plydox

• Pesticide type: soil insecticide, nematicide

• Class: organophosphate, cholinesterase inhibitor

• Use: used to control soil pests on corn, sugar beets, and sorghum. 90% of the active ingredient applied is to corn.

• Terbufos is very highly toxic to birds, mammals, and fish. Its degradates are also highly toxic and are more persistent in the environment.

• Terbufos ranks fourth in the U.S. for reported fish kills

• Terbufos has caused avian mortality incidents in the U.S. and Canada involving large numbers of hawks and eagles.

• Terbufos bioaccumulates in the fat tissues of living organisms, as shown in laboratory tests with fish.

Chemical Structure

Background

Terbufos is an organophosphate insecticide and acaricide used mainly on corn in the U.S. Approximately 7.5 million pounds of active ingredient are used annually. Terbufos acts systemically and on contact. It is formulated as a granular product and is applied directly to the soil in furrows. While the parent product breaks down quickly, two of the primary degradates, terbufos sulfoxide and terbufos sulfone, are known to be highly toxic and more persistent in the terrestrial and aquatic environment. Another degradate, formaldehyde, is prevalent when terbufos is introduced into aquatic systems.

Environmental Effects  

Fate

• Persistence: In aerobic soil, hydrolysis and biodegradation are the primary degradation pathways for terbufos. The half-life under conditions which favor microbial growth is 27 days. It increases to 67 days in anaerobic soil. The two important metabolites, terbufos sulfoxide and terbufos sulfone, are more persistent than terbufos, and are equally toxic. The half-lives for the sulfoxide and sulfone degradates are 116 and 96 days, respectively. In aquatic systems, the hydrolysis half-life for terbufos is about 13 days for the range of pH 5, 7, and 9. The primary breakdown product in water is formaldehyde, but the sulfone and sulfoxide degradates are also present. The half-lives for the sulfone and sulfoxide degradates are 32 days and 68 days, respectively.

• Solubility: nearly insoluble in water at 5.0mg/L, terbufos is soluble in acetone, aromatic hydrocarbons, chlorinated hydrocarbons, and alcohols.

• Mobility: The mobility of terbufos in soil varies. In Arkansas loamy sand, it is moderately mobile but in Indiana silt loam, New Jersey sandy loam, and Wisconsin loam soils it is essentially immobile. The primary degradates are considerably more mobile than the parent compound. Terbufos degradates have a high potential for surface and groundwater contamination and have been responsible for many incidents of fish kills after run-off contaminated ponds and rivers.

• Bioaccumulation: studies have shown that terbufos has the ability to bioaccumulate in living tissue. The bioconcentration factors for fish range from 320X to 940X , indicating a moderate potential for bioaccumulation.

Ecotoxicity 

• Terbufos is very highly toxic to freshwater invertebrates by both acute and chronic criteria. Studies have noted developmental abnormalities in aquatic invertebrates at extremely low concentrations of terbufos.

• Terbufos is very highly toxic to both warm and cold water fish species.

• Terbufos is very highly toxic to mammals. The LD 50 for rats has been reported to be from 1.5 to 2.0 mg/kg.

• Terbufos is very highly toxic to passerine bird species. The LD 50 for the red-winged blackbird is 2.1 mg/kg. Chronic effects on terbufos and its degradates have not been adequately studied.

Incidents

•  Illinois, 1991. The Department of Conservation reported a fish kill after runoff of terbufos applied to a cornfield contaminated the immediate watershed. Biologists investigating the site estimate 41,800 bluegill, 38,000 largemouth bass, 5,700 green sunfish, 4,300 black crappie, 407 red-ear sunfish, and 211 hybrid sunfish were killed.

• Terbufos ranks fourth overall in the U.S. for documented fish kill incidents. To summarize the incidents, during the period from 1989 to 1998, seventy-eight incidents have been reported. The numbers of fish killed range from 30 to 90,000. Most of the kills were related to use on corn. Grassy buffer strips did not prevent incidents, in some cases. Incidents generally occurred from 2 days to 3 weeks after application. The incidents were determined by the EPA to have resulted during periods of normal  rainfall and were not attributed to rare or severe storm events. The mortalities oftentimes occurred on soils which are not highly erodible, demonstrating the ability of terbufos and its metabolites to persist and move through the soil.

• Madison, Wisconsin, 1995. An adult female and a juvenile red-tailed hawk were found at the base of a tree in Madison, WI. Meat taken from the crops of the hawks contained 12 and 13 ppm terbufos. The investigator speculated that the prey of the hawks had been rodents from a nearby corn field (USFWS case file 2300).

• Bear Valley, near Klamath Falls, Oregon, 1992. Five bald eagles were found dead in an unidentified site. Their crop contents were analyzed at the Patuxent Wildlife Research Center in Maryland. Four of five crop samples, one from each bird, were found to contain terbufos.

• Dumas, Texas, 1996. In April, 20 migrating Swainson's hawks were killed when they gorged on grubs in a corn field recently treated with a granular formulation of terbufos. Stomach contents of the birds contained soil, grubs, and from 6.5 to 16 ppm terbufos. The registrant of terbufos, American Cyanamid, commissioned a team of scientists to investigate the kill. The consultants concluded that terbufos granules, along with seed corn, had been deposited on the surface of the soil, instead of in the furrows after plowing. The grubs were exposed with plowing and would likely have remained exposed even after terbufos and the seed corn were covered. It is highly likely that proper application methods would have prevented this incident. This kill proves that secondary poisoning is possible after terbufos contaminated invertebrates are consumed by avian predators.

• Felton, Delaware, 1997. Two Canada geese were found dead in a 7 acre field of corn. The geese were feeding in newly planted corn which had been treated with granular terbufos. There were heavy rains prior to the incident. This incident shows that grazing birds, such as geese, are susceptible to the effects of terbufos. The heavy rain may have facilitated the absoption of terbufos into the soil, lessening the contact exposure to geese, or, the rain may have increased exposure to geese if standing water containing terbufos was present on the field. Additionally, the rain may have enhanced the uptake of terbufos by the young corn plants, increasing the systemic effect and exposing the geese to high levels of the chemical and its metabolites as they fed on the corn.