Pesticide Profile - Fenthion

Fenthion Withdrawn!

The risk posed to Piping Plovers and other birds in Florida has now been stopped. ABC and its partners have achieved a major victory in the ongoing battle against the controversial pesticide fenthion. In March 2003, drug and chemical manufacturer Bayer announced its intention to voluntarily withdraw fenthion (used under the trade name Baytex) from the market. At the urging of ABC and its partners, EPA approved this withdrawal and agreed for all sales to end on June 30, 2004, and all uses to end by November 30, 2004.

For more than two years ABC led a campaign to remove fenthion from the market due to its extreme toxicity to birds. In collaboration with Defenders of Wildlife and the Florida Wildlife Federation, ABC filed a law suit against EPA in 2002, citing violations of the Endangered Species Act and the Migratory Bird Treaty Act in the continued use of this highly toxic organophosphate pesticide, which is used to kill adult mosquitoes in four Florida counties. Fenthion has been implicated in the deaths of hundreds of birds in Florida including an endangered Piping Plover. All other counties in the United States have switched to less toxic alternatives.

In the past, fenthion was sold for many different uses, including flea and tick shampoos for dogs, as well as its formulation to kill birds, marketed as the "Rid-a-Bird Perch." However, these products were removed from the market due to concerns regarding human and animal health.

Quick Facts:

• Chemical name: O,O-dimethyl )-4-methylthio-m-tolyl phosphorothioate

• Trade names: Bay 29493, Baycid, Baytex, Dalf, DMTP, Entex, Lebaycid, Mercaptophos, Prentox Fenthion 4E, Queletox, Spotton, Talodex, Tiguvon

• Pesticide Type: insecticide

• Class: organophosphate

• Mechanism of Action: cholinesterase inhibitor

• Laboratory studies have shown fenthion to be mutagenic, carcinogenic, and embryotoxic.

• Fenthion has reproductive effects on fish and birds in the laboratory.

• Routes of exposure: contact and stomach insecticide; fenthion is highly toxic to bird through dermal contact and inhalation.

• Historical Use: Rid-A-Bird, a fenthion impregnated perch, was used to control pest birds in urban, industrial, and agricultural settings. Fenthion is readily absorbed through the skin and highly effective as an avicide. High levels of secondary poisoning result when predatory birds (hawks, owls, falcons) prey on fenthion-exposed birds. These uses were canceled by the manufacturer in 1998 due to massive mortality of predatory birds associated with fenthion use.

Chemical Structure

Background

Fenthion was developed in 1960 and commercialized by Bayer Agriculture. Fenthion has been used in pet sprays and shampoos to control external parasites and it has been widely used as an ectoparasite insecticide for livestock (lice and fly control). Concerns over the environmental risks posed by livestock applications prompted the manufacturers to voluntarily cancel all of these uses. Phase-out of all livestock applications began in March 2000 and will continue over the next two years to allow for depletion of existing stocks. Undoubtedly, the use which posed the most extreme risk to birds was its use as an avicide. Fenthion was applied as a paste to perches to kill pest birds. Countless numbers of raptors and other non-target birds were killed over the period from 1964 to 1997, before this use was voluntarily canceled by the manufacturer.

Human Health Effects

Fenthion is one of the more potent cholinesterase inhibitors, having an acute No Observed Adverse Effect Level (NOAEL) of 0.07 mg/kg/day. Fenthion use has been restricted such that the human exposure scenarios are limited to occupational handlers and residential exposure as a result of mosquito spraying in Florida. The EPA has ordered further testing to investigate past studies indicating that fenthion is mutagenic: causing genetic aberrations in laboratory studies. Fenthion has been identified as a carcinogen in mice, after a two-year NIH study found that tumors occurred at a rate significantly higher in the fenthion treated group than in the control group.

Fenthion is readily absorbed through the skin and, although application methods are engineered to reduce deposition of pesticide droplets near areas where humans might contact the chemical, studies have shown that unacceptable levels of residue are found in areas where humans are likely to be exposed. Cumulative exposure may be of concern, since fenthion is stored in body fat. One study has indicated that a single dose of fenthion may have prolonged action, perhaps as a result of it being sequestered in fat and released later for metabolism (U.S. Public Health Service, Hazardous Substance Data Bank, 1995). The EPA is currently evaluating exposure in children, who are most likely to contact fenthion on lawns and in the household.

Environmental Effects

Fate:  

Persistence: The half-life of fenthion in water under field conditions has been reported to range from 2.9 to 21.1 days for various ocean, river, swamp, or lake waters. The persistence may be markedly increased in salt marsh environments where light and oxygen are limited. In one study, 50% of applied fenthion remained in river water two weeks later, while 10% remained four weeks later (Khan, 1977). Data describing the degradation of fenthion on soil is variable. Some reports indicate that fenthion is short-lived on soil, with half-lives of around one day.The U.S. Department of Agriculture lists a soil half-life of 34 days under most conditions. In some soils, fenthion residues may persist for approximately four to six weeks (Harding, 1979). Fenthion binds tightly to soil particles and is relatively immobile in most soil types.

Solubility: Fenthion is nearly insoluble in water with estimates ranging from 2mg/kg water to 7.5 mg/l at 25 degrees C. Soluble in many organic solvents.

Bioaccumulation: Fenthion is a lipophilic compound that is sequestered in the fat tissues of animals. Biologically unchanged fenthion has been recovered from fat samples of cattle, fish, and amphibians after fenthion exposure.

Ecotoxicity

• Fenthion is extremely toxic to honeybees, with an LD50 of 0.319 ug/bee indicating a high risk to pollinators and plants dependent on pollination for reproduction.

• Fenthion is extremely toxic to aquatic invertebrates, particularly crustaceans and freshwater mussels.

• Amphibians are extremely sensitive to fenthion with LC50's of 4.9 and 0.84 in the Bullfrog and frog, respectively. The most conservative LD50 converts to a NOEL (no observable effects level) of just 0.0084 ug/L for amphibians.

• Mammals appear not to be as sensitive to fenthion as other animals; however, chronic effects due to the bioaccumulation of fenthion in fat tissue have not been adequately studied. Additionally, if mammals are relatively resistant to fenthion, then high levels of the toxin may accumulate with repeated exposure. Predatory birds are then at risk if they feed on exposed rodents or other small mammals.

• Fenthion is extremely toxic to birds. The use of fenthion as an avicide to control pest birds resulted in massive mortality of predatory raptors. Fenthion presents a hazard to birds via direct oral and dermal routes of exposure. One study showed that 70-90% of European starlings tested died after contact with Rid-A-Bird perches for only 3-5 seconds on three consecutive days (Garrison et al. 1988). Proposed risk assessment models have shown that pesticides with high dermal toxicity are usually of higher overall risk to birds (Mineau, work in progress.)

Some representative acute, oral LD 50's:

• Red-winged blackbird LD50 1.8 mg/kg

• Mourning dove             LD50 2.68 mg/kg

• Mallard                        LD50 1.0 mg/kg

• American Kestrel          LD50 1.3 mg/kg

• House Sparrow            LD50 2.4 mg/kg

• House Finch                 LD50 10.0 mg/kg

• Bobwhite                      LD50 4.0 mg/kg

Incidents 

The incidents listed below refer only to avian mortality associated with ULV spraying of fenthion. Hundreds of birds, mainly raptors have been killed by secondary exposure of fenthion after consuming poisoned pest birds, such as starlings.

• Grand Forks, North Dakota, 1969. After the aerial application of fenthion at 0.07 lbs./A 10 1500 acres for mosquito control, an estimated 5000-25,000 birds were found killed (Seabloom, 1973). The method of application of fenthion was Ultra Low Volume (ULV), which is the preferred method of application in most cases, today, for mosquito control. Investigators salvaged more than 400 birds representing 37 species including several species of warbler, American Robin, and Swainson's Thrush. The maximum allowable application rate for aerial spraying is 0.1 lb/A, which is higher than the application rate reported for this kill.

• In 1978, after ULV aerial spraying over wet meadows in Wyoming, numerous dead or dying birds were observed by researchers including Savannah Sparrows, Wilson's Phalarope, and several species of blackbird (DeWeese et al., 1983).

• Kenya, 1989. A study which examined the effects of fenthion on non-target birds during a planned kill of Quelea found 17 species of bird in addition to the quelea, dead near the sprayed fields. Quetox, a 60% fenthion solution, was sprayed over the fields.

• Other incidents involving bird mortality from the use of fenthion for mosquito control have been reported. In California, American goldfinch, gulls, ducks, shorebirds,green-backed heron, egrets, and many other species of passerine birds have been found after fenthion sprays for mosquito and/or midge control. In an accident in Louisiana in 1970, more than 1000 birds were reported dead after fenthion application. In Massachusetts and Idaho, robins, sparrows, catbirds, and sandpipers have been killed.