Abstract

Methoprene is an insect growth regulation hormone analogue, widely used for aquatic mosquito control. Other uses include crop and home pest control and oral and topical flea control in dogs and cats. Methoprene interferes with an insect's normal maturation process, preventing larvae to mature to reproductive adult stages. It is available in suspension, emulsifiable and soluble concentrate formulations, as well as in briquette, aerosol, and bait forms.

In most animal species, methoprene has very low toxicity when ingested or inhaled, and slightly toxic by dermal absorption. The oral LD₅₀ for methoprene in rats is greater than 34,600 mg/kg, and greater than5000 mg/kg in dogs. The inhalation LD₅₀ in rats is greater than 210 mg/L. Reported dermal LD₅₀ values in rabbits are 2000-3000 mg/kg. The target organ primarily affected by methoprene after long-term exposure is the liver. In mammals, methoprene is rapidly and completely broken down and excreted, mostly in urine and feces. Unchanged methoprene is excreted in cattle feces in sufficient amounts to kill some dung-breeding larvae. Methoprene does not appear to be teratogenic or mutagenic or pose any reproductive hazards.

Methoprene is slightly toxic to birds. For one commercial methoprene product, the reported 5-8 day LC₅₀ values are greater than 10,000 ppm for mallard ducks and bobwhite quail, and its acute oral LD₅₀ in chickens is greater than 4640 ppm. An acute oral LD₅₀ of greater than 2000 mg/kg was determined in mallards. Non-lethal effects in birds have been seen at acute oral doses of 500 mg/kg. Signs appeared as soon as 2 hours post-ingestion and persisted up to 2 days; signs included slowness, reluctance to move, sitting, withdrawal, and incoordination. Chronic feeding trials in birds have shown no reproductive effects.

Methoprene is slightly to moderately toxic to fish. The reported 96-hour LC₅₀ values for a commercial methoprene product were 4.6 mg/L in bluegill sunfish, 4.4 mg/L in trout, and greater than 100 mg/L in channel catfish and largemouth bass. Methoprene may have some potential for accumulation in bluegill sunfish and crayfish.¹

Methoprene is highly toxic to some species of freshwater, estuarine, and marine invertebrates, though the acute LC₅₀ values are greater than 100 mg/L in freshwater shrimp, and greater than 0.1 mg/L in estuarine mud crabs. Commercial methoprene products have shown to have little effect on waterfleas, damselflies, snails, tadpoles, and mosquito fish. Tests with earthworms showed little if any toxic effects on contact, and methoprene is nontoxic to bees.

Methoprene has a low persistence in soil and degrades rapidly in water. When applied at the extremely high application rate of 1 pound per acre, a commercial preparation of methoprene had a half-life of less than 10 days. In soil, microbial degradation appears to be the major route of methoprene's disappearance. Methoprene is also readily degraded by sunlight. In pond water, half-lives of about 30 and 40 hours have been documented, at initial concentrations of 0.001 mg/L and 0.01 mg/L, respectively. At normal temperatures and levels of sunlight, technical methoprene is rapidly degraded by aquatic microorganisms and sunlight. Commercial methoprene products are biodegradable and nonpersistent in plants, and plants grown in treated soil are not expected to contain methoprene residues.¹

Preliminary investigations by Cliburn on the effects of methoprene on various life stages of different amphibian species (B. woodhousei, R. catesbeiana, and R. pipiens) have been completed. Acute toxicity studies on R. catesbeiana, and R. pipiens larvae indicate LC₅₀ values of >10,000 ppb. B. woodhousei adult LC₅₀ values were >1000 ppb (highest dose tested). Chronic studies indicate a 22 day LC₅₀ of >1000 ppb for B. woodhousei and a LC₅₀ of >1000 ppb for R. catesbeiana, and R. pipiens. No other adverse effects were reported.² Other research has shown that the degradation products of methoprene can experimentally cause developmental deformities in Xenopus similar to those seen naturally in North American amphibians.³

Based upon current toxicity studies, methoprene appears to have low toxicity in most animal species.

References

1.  EXTOXNET. 1996. Extension Toxicology Network Pesticide Information Profiles. Methoprene. http://ace.orst.edu/info/extoxnet/

2.  Environmental Protection Agency. 2001. June 2001 Update of the March 1991 Methoprene R.E.D. Fact Sheet. http://www.epa.gov/pesticides/biopesticides/ingredients/

3.  La Clair JJ, JA Bantle, J Dumont. 1998. Photoproducts and metabolites of a common insect growth regulator produce developmental abnormalities in Xenopus. Environmental Science & Technology 32:1453-1461.