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La Clair, JJ, JA Bantle and J Dumont. 1998. Photoproducts and metabolites of a common insect growth regulator produce developmental deformities in Xenopus. Environmental Science and Technology 32:1453-1461. In laboratory experiments with the African clawed toad, Xenopus laevis, embryos, La Clair et al. show that the insect growth regulator S-methroprene itself poses little risk to toads at levels commonly encountered in the environment. But, when toad embryos encounter the breakdown products of methoprene caused by this compounds reaction with sunlight, water and microorganisms, the exposures "dramatically interfere with normal amphibian development." Methoprene and the relevant breakdown products interfere with the retinoid hormone system. S-methoprene is widely used in the US to control mosquito populations. The levels of S-methoprene and its breakdown products that should result from "proper use" of the compound (i.e., at the recommended spray level) are low compared to those necessary in these experiments to cause the observed deformities. La Clair et al. note, however, that the breakdown products may be concentrating in the environment in ways that are not easily mimicked in laboratory experiments. They also note that methoprene is widely used in many insecticidal applications, and even if each application is carried out correctly, the combined impact may push exposures to a level that is biologically significant. They note that S-methoprene is added to sprays against fleas in preparations available in grocery, pet and pest-control stores. At the concentration level in these consumer preparations, a single application to a 10-pound pet would be enough to contaminate approximately 110,000 liters (roughly 29,000 gallons) These results are important for two reasons. First they show that frog deformities can be produced by contamination. Second, they confirm, once again, that studies of contamination must be carried out in ways that reflect the real world. In this case, it would not have been sufficient merely to observe the impact of the active ingredient (S-methoprene) itself. Important effects were evident only once the experiment focused on the breakdown products of the active ingredient. Other researchers, however, contend that retinoids are not important to frog deformities. More... |