Quick Facts

• Chemical name: O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate

• Trade names: Brodan, Detmol UA, Dowco 179, Dursban, Empire, Eradex, Lorsban, Paqeant, Piridane, Stipend

• Pesticide type: insecticide

• Class: organophosphate

• Mechanism of action: cholinesterase inhibitor

• Usage patterns: Approximately 24 million pounds were used annually in 1999 in the U.S., making chlorpyrifos on of the most commonly used pesticides. The EPA's decision to cancel most of the household uses of chlorpyrifos has reduced usage by half. Agricultural uses on orchards and row crops persist.

• Regulatory status: US EPA announced a drastic cancellation of most household-use products in June 2000, based on human health risks. The principal manufacturer of chlorpyrifos, Dow Chemical, will cut production by approximately 12 million pounds.

Chemical Structure

Background

Chlorpyrifos, a broad-spectrum organophosphate insecticide first registered for use in 1965, is one of the top five residential pesticides in use in the U.S. Registered uses include a wide variety of food crops, turf and ornamental, and greenhouse. An estimated 70% of urban usage is as a termiticide. 

The US EPA determined in June of 2000 that chlorpyrifos, common trade name Dursban, was sufficiently hazardous to human health as to warrant cancellation of nearly all the household-use products. Because of its extensive use, chlorpyrifos metabolites have been found in the urine 82% of adults sampled from the National Health and Nutrition Examination Survey, while the Minnesota Children's exposure Study found that 92% of 89 children evaluated had measurable urinary concentrations of 3,5,6-TCP, the primary urinary metabolite of chlorpyrifos.

Environmental Effects

Fate

• Persistence: Chlorpyrifos is moderately persistent in soils. The half-life is usually between 11 and 180 days but can range from 2 weeks to over a year, depending on soil type, climate and other conditions. In water at pH 7.0 and 25 degrees Celsius the half-life was 35-78 days in one study. Residues persist on plants from 10 to 14 days, and there is data that indicates that chlorpyrifos may accumulate in some crops.

• Solubility: slightly soluble in water at 2.0 mg / L.

• Bioaccumulation: Chlorpyrifos has the potential to bioaccumulate. It has been found to accumulate in the fat tissue of rats in one study, in the wing fat tissue of 800 hunter- killed northern bobwhite in a 1989 study, and in the tissues of fulvous whistling ducks collected in Florida. Bioconcentration factors in fish studies have been documented at values of 58 to 5100.

Ecotoxicity 

• Chlorpyrifos is very highly toxic to freshwater fish, aquatic invertebrates, and estuarine and marine organisms. Chlorpyrifos accumulates in the tissues of aquatic organisms. Chlorpyrifos binds tightly to soil particles and is relatively insoluble in water. It persists in the sediments of aquatic habitats and is hazardous to benthic invertebrates and other bottom feeders. Numerous fish kills have been reported. After residential use of Dursban, approximately 500 bream were found dead in a Georgia pond where a swimming pool had backwashed in to the pond. Commercial use of chlorpyrifos to treat pine seedlings resulted in fish kills in a Saline River watershed in Arkansas. The kills occurred years after application and included crappie, bass, bullhead, catfish and spotted suckers. Most of the kills occurred after heavy rains. After use for termite eradication, about 2000 bluegill were killed after Dursban had been unknowingly injected into an underground water supply, which discharged into a lake.

• Chlorpyrifos is highly toxic to honeybees and is a hazard to pollinators.

• Chlorpyrifos can be very highly toxic to birds.

  • Common Grackle LD50 - 5.62 mg / kg
  • Rock Dove LD 50 - 10.0 mg / kg
  • Red-winged Blackbird LD 50 - 13.1 mg / kg
  • Sandhill Crane LD 50 - 25-50 mg / kg

Studies that examined the effects of temperature combined with exposure to chlorpyrifos showed increased mortality and increased cholinesterase inhibition with colder temperatures. This study was conducted using northern bobwhite. The study indicates that temperature stresses increase the sensitivity of birds to chlorpyrifos by about two fold.

 

The EPA required chronic avian reproduction studies because chlorpyrifos is persistent, bioaccumulates, and has multiple applications in certain scenarios.  Reproductive studies show that chlorpyrifos administered at sub acute levels to mallards reduces clutch size and the number of surviving ducklings.

Incidents

• Between the years of 1992-1994, twenty-six red-tailed hawks, one Cooper's hawk, and one barn owl were killed in Central California as a result of agricultural spraying with chlorpyrifos. These kills were reported to the Ecological Incident Information System (EIIS), which is compiled by the EPA. Many of the incidents involved more than one bird, and approximately half of the dead birds were tested in a laboratory and found to be positive for chlorpyrifos exposure. 

• Over eighty incidents of fish kills have been reported to the EIIS from 1992 through 1999, involving millions of fish and other aquatic creatures. Fish species killed include darters, gar, bream, trout, bass, sunfish and catfish.

• Use on a Florida ranch in 1988 resulted in a kill involving many species of animals over a broad taxonomic range: 427 Florida soft-shelled turtles, 27 banded water snakes, American alligators, wood storks, white ibis, 75 bowfin, 1200 Seminole killifish, and 142 unidentified birds, as well as many other species.

• One hundred American robins were killed in Florida after chlorpyrifos use on a golf course in 1996; seventy-eight Canada geese were killed after chlorpyrifos use on a golf course in Suffolk, New York in 1974.

• Many incidents of American robin kills after chlorpyrifos use on residential lawns have been reported from many areas of the country, included one kill in Citrus County, Florida which involved sixty robins on one residential lawn. A chlorpyrifos-impregnated flea collar contaminated a birdbath and killed eighteen cedar waxwings in Grant, Wisconsin in 1993.