Pesticide Profile - Phorate

Quick Facts:

• Chemical name: O,O-diethyl S-ethylthiomethyl phosphorodithioate

• Trade Names: Thimet, Rampart, Granutox, Agrimet

• Pesticide Type: insecticide, acaricide, nematicide

• Pesticide Class: organophosphate, cholinesterase inhibitor

• Use: Approximately 3 million pounds are used in the U.S. annually, 80% of which is applied to corn, potatoes, and cotton.

• Phorate is a restricted use pesticide due to high dermal, oral, and inhalation toxicity.

• Phorate is very highly toxic to birds; only 3-4 granules are enough to kill a sonbird or mallard duck.

• Phorate ranks high in the number of occupational incidents resulting in adverse health events.

• Phorate has been determined to have been responsible for numerous bird kills, sometimes involving large numbers of birds.

Chemical Structure

Background

Phorate is an organophosphate pesticide effective against a wide array of insects, mites, and some nematodes. It is used on a variety of field crops but 80% of its use is on corn, potatoes, and cotton. Other crops include: beans, peanuts, sugar beets, sorghum, soybeans, lilies, daffodils, and wheat. It is formulated as a granular end-use product which is incorporated into the soil by in-furrow or drilling application, and as an emulsifiable concentrate which may be applied aerially to foliage. Nearly 3 million pounds of phorate are used annuallyon 2.5 million acres, in the U.S.

Phorate is toxic through oral, dermal, and inhalation routes of exposure. The occupational hazards to humans associated with the application of phorate can be extreme. Toxicity appears to vary with age, with the young being more susceptible. Acute toxicity is similar to other cholinesterase inhibitors, with very high doses resulting in unconsciousness, convulsions, and death. Chronic effects result from low-level exposure manifest in neurological and neuromuscular symptoms which are sometimes prolonged. Animal studies have shown that ingested phorate is excreted very slowly, with only 40% of the ingested amount excreted over a period of six days. 

Phorate is very highly toxic to birds and other wildlife. It has been responsible for numerous incidents of mortality in birds and fish. Birds are exposed to phorate when they ingest granular phorate, when they contact pooled water on fields recently treated with phorate, and by the ingestion of phorate-contaminated insects or small mammals. Phorate ranks among the most acutely toxic organophosphates in use. While the decomposition of phorate occurs within a few days, reported field incidents indicate that phorate and its metabolites can express their toxicity several months after application. Much of the use of phorate in the U.S. is concentrated over environmentally sensitive areas such as the Prairie Potholes region, a critical breeding area for much of the nation's waterfowl. Other high-use regions include the Mississippi River Valley, the central valley of California, the Southeastern Gulf states, and the Great Lakes Region.

Environmental Effects

Fate

• Persistence: Phorate has a soil half-life which varies from 2-173 days, depending on conditions. The degradates of phorate, specifically, phorate sulfone and phorate sulfoxide may be much more persistent in the environment. The mean half-life for phorate sulfoxide is 100 days. Freezing, for example, when phorate is applied to cropland during or before freezing temperatures, preserves the active ingredient, sometimes over the entire winter and increases its persistence in soils. In water, phorate is subject to rapid hydrolysis, however, its degradates are more stable in water. Formaldehyde is a major degradate in water.

• Solubility: phorate is slightly soluble in water; it is very soluble in organic solvents.

• Degradation in plants: phorate is absorbed into plants through the roots when it is applied to the soil. Phorate can be detected in plant residues for up to 28 days. Plants metabolize phorate to its toxic degradates, sulfoxide and sulfone compounds.

• Accumulation: phorate has the ability to accumulate in living tissue at significant levels. In fish, after 14 days depuration, 10% of ingested phorate was retained in the tissues.

Ecotoxicity 

• Phorate is very highly toxic to fish and aquatic invertebrates.

• LC50 cutthroat trout, largemouth bass, bluegill 2-13ug/L

• Phorate has been confirmed as the cause of fish kills when run-off from treated crops entered ponds. 2000-3000 bluegill, bass, crappie, minnows and catfish were killed in one incident in Illinois. 90,000 fish were killed in another incident in Arkansas.

• Phorate is very highly toxic to mammals. In an incident in Kansa, 1996, 3 swift foxes were found dead and phorate was confirmed as the cause of death when stomach contents were analyzed.

• Phorate is very highly toxic to birds. Representative LD50's:

  mallard                             0.62 mg/kg
  red-winged blackbird           1.0   mg/kg
  common grackle                 1.3   mg/kg
  ring-necked pheasant          7.12 mg/kg

Incidents

• Fresno, California, June 1972. Twenty-five ducks and black-necked stilts were found dead in a sugar beet field. Phorate had been applied two days earlier.

• Calipatria, California, November 1978. Granular phorate was applied to an alfalfa field during irrigation, contrary to label instructions. Two days later, the carcasses of 195 birds were removed including ring-billed gulls, cattle egrets, and curlews (species not reported). Phorate was detected in all of the birds examined. Regurgitated crop contents were found to contain nearly 100% crickets with 92.7 ppm phorate.

• Lyman County, South Dakota, 1982. Phorate used on wheat fields was responsible for the deaths of 38 mallards, 4 gadwalls, 9 wigeons, 6 pintails, 7 green-winged teal, 1 red-tailed hawk, and 1 golden eagle.

• Tule Lake, California, November 1986. 50 to 60 mallards and pintails were found dead in a field that had been planted in barley the previous summer. Crop analysis revealed phorate in all seven of the birds analyzed. No evidence of misuse was found.

• Pierre, South Dakota, March 1989. Phorate killed birds in March on a winter wheat field that was treated in September of the previous year. During late winter, a pond had formed in the wheat field from the thaw of snow cover and from rain. 70 Canada geese and other waterfowl were found dead around this pond 2 weeks after it formed. A few days later, 12 Canada geese, ducks and a sharp-tailed grouse were found dead on a second small pond about one-third of a mile away from the first pond. Bald eagles were observed feeding on the dead geese and seven bald and one golden eagle were fatally poisoned by phorate.