Pesticide Profile - Ethyl parathion

Quick Facts:

• Chemical name: O,O-diethyl O-p-nitrophenyl phosphorothioate

• Pesticide type: insecticide, miticide

• Class: organophosphate

• Mechanism of action: cholinesterase inhibitor

• U.S. regulatory status: restricted use only. Approved for nine crops: alfalfa, barley, corn, cotton, canola, sorghum, soybean, sunflower, and wheat. In 1991, most uses on row crops and fruit were voluntarily cancelled under an agreement with the registrant and the EPA. This action was taken to mitigate the risk to workers exposed during application and post-application.

• Ethyl parathion is one of the most toxic pesticides in use. According to the U.S. Fish and Wildlife Service, "substantial evidence verifies that mortality of migratory birds and other non-target organisms occur even when (ethyl) parathion is applied with complete conformance with the label."

Background

Ethyl parathion was formulated in 1946 as a broad-spectrum, nonsystemic insecticide and was the first commercially successful organophosphate insecticide marketed. It is moderately persistent in the environment, and one of its major degradates, paraoxon, is many times more potent as an inhibitor of cholinesterase. Ethyl parathion has been shown to bioconcentrate in tadpoles and fish in laboratory experiments; this is significant in that other studies have shown the persistence of ethyl parathion to be 690 days at 20 degrees Celsius. Ethyl parathion has been shown to be hazardous at sub lethal levels. Sub lethal exposure has resulted in lowered cold tolerance in Northern bobwhites (Rattner et al. 1982), and impaired reproductive behavior in laughing gulls (White et al. 1983), mallard and bobwhite (Bennet and Bennet 1990, Bennet et al. 1991). Buerger et al. (1991) documented that Northern bobwhite are more susceptible to predation after the birds were dosed with levels of ethyl parathion expected under actual field conditions and released into the wild.

Even as ethyl parathion's use is restricted to only nine crops in the U.S., the regional concentration of some of these crops leads to high levels of the insecticide in the surrounding environment. Sunflowers and wheat grown in the Prairie Pothole region of North and South Dakota and adjacent Minnesota are subject to some of the highest usage rates in terms of pounds per acre. This region accounts for 50% or more of the total waterfowl reproduction in the U.S. Ducks and geese of the prairie states are at immediate risk from acute primary and secondary, and chronic poisoning from ethyl parathion. Its environmental persistence greatly increases the risk to birds and other wildlife.

Environmental Effects

Fate

• Persistence: A review article (Mulla, 1981) reports that the half-life of ethyl parathion in soils ranges from six weeks to sixteen years. Microbial mediated degradation is an important route of dissipation, ethyl parathion is relatively resistant to photodegradation on soil surfaces. The EPA has cited tests that show the persistence in aerobic soil to be 50-140 days, and 6-88 days in anaerobic aquatic soils and sediments. The hydrolytic half-life is long, at 180 days. Pimentel (1971) has reported that ethyl parathion persists in 20 degree Celsius aerobic water for up to 690 days.

• Slightly soluble in water at 24.0 ppm.

• Bioaccumulation: Data from fish studies show that ethyl parathion may accumulate in tissues. A bioconcentration factor of 335 was noted for one species of fish (Yu 1975). When American kestrels were fed tadpoles exposed to parathion, brain cholinesterase values were depressed to 45% normal resulting in death. Amphibians are relatively resistant to cholinesterase inhibitors and may bioaccumulate enough ethyl parathion to be toxic to avian predators in a field setting.

Ecotoxicity

• Ethyl parathion is very highly toxic to aquatic invertebrates with nearly all EC50's less than 1.0 mg / l.

• Ethyl parathion is very highly toxic to freshwater fish on an acute basis.

• Acutely toxic to mammals through oral and dermal routes of exposure. Reproductively toxic, and chronically toxic when administered to test mammals at levels far below acutely toxic doses. Ethyl parathion toxicity increases with increases in the exposure period; animals exposed to multiple applications of ethyl parathion are more likely to be adversely affected.

• Highly toxic to honeybees. Ethyl parathion is a hazard to pollinators and plants dependent on these pollinators for reproduction.

• Ethyl parathion is very highly toxic to avian species on an acute oral basis.

Fulvous whistling duck            LD50            0.125 mg / kg
Mallard                                LD50            1.9     mg / kg
Red-winged blackbird             LD50             2.4    mg / kg

Very highly toxic through dermal exposure, 1.8 mg / kg on the feet of house sparrows for 24 hours resulted in 50% mortality.

Incidents

• Oklahoma, 1994. Fourteen Mississippi kites were found dead and two were found moribund. A nearby cotton field had been sprayed with ethyl parathion. Laboratory analysis showed that the birds had been secondarily exposed to ethyl parathion after eating contaminated insects. (Franson, 1994).

• More than 216 immature and adult laughing gulls were found dead near cotton fields sprayed with 1.0 lb. / acre ethyl parathion. Laboratory tests confirmed ethyl parathion as the cause of death. It was concluded that adults died from the ingestion of poisoned insects and the chicks died either from ingestion of contaminated insects regurgitated to them by their parents, or by starvation due to the death of their parents (White et al. 1979).

• Delaware, 1990. Twenty purple martins were found in a residential yard 200 yards away from a field that had been sprayed with ethyl parathion.

• Etter, Texas, 1981. 1500 geese and 100 ducks were found dead at a lake. The birds had consumed winter wheat growing in the area. Laboratory analysis of crop contents confirmed the presence of ethyl parathion.

• Northern New York, 1982. Nearly 3200 birds were found dead in a field containing scattered rye seeds treated with ethyl parathion. Species found include: Red-winged blackbird, common grackle, mourning dove, Cooper's hawk, red-tailed hawk, blue jay, Eastern meadowlark, and song sparrow.

• Swisher County, Texas, 1988. 200 Canada geese were found dead in a lake in Northern Texas. Fifteen meters from the lake, a field of winter wheat had been treated with 0.57 lbs. / acre of ethyl parathion to control aphids. Laboratory analysis on wheat residues and crop contents clearly indicated ethyl parathion.