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Amphibian Population Decline Linked to Malathion Use

(Beyond Pesticides, January 9, 2009) Environmental Toxicology and Chemistry has published a study (Vol. 27(12):2496–2500) entitled “Effects of Malathion on Embryonic Development and Latent Susceptibility to Trematode Parasites in Ranid Tadpoles.” It shows that malathion used as an agricultural insecticide is responsible for interfering with the normal development of pickerel frog embryos, thus leaving them more susceptible to parasite invasion.

Malathion is present in natural water sources that have been exposed to urban and agricultural runoff. This organophosphate pesticide can be applied by planes in mosquito control program, and as esult enters water from the air.

Although direct lethal and sublethal effects of chemical contaminants have been documented, latent and long-term effects have been less well documented. Therefore, researchers sought to fill this knowledge gap and found, as suspected, that tadpole survival rates decreased and malformations and susceptibility to parasite encystment rates increased as a result of exposure to malathion concentrations mimicking those found in actual water sources.

Tadpoles are being exposed to increasing numbers of parasites in waters that are warming as a result of global climate change, and the researchers who performed this study speculate that, as a consequence, those exposed to malathion will have weakened immune systems that render them less able to defend themselves from invasion. Indeed, trematode infection was observed in tadpoles seven weeks after embryonic exposure to low concentrations of malathion.

This study shows that declines in amphibian populations are related to the agricultural application of malathion, which causes various kinds of damage to frog embryos and tadpoles that are, as a consequence, increasingly susceptible to parasite invasion. A similar study, published in October, 2008, found that some tadpole populations are being further threatened by malathion, as the chemical kills zooplankton, their primary food source. A 2007 U.S. Geological Survey study found malathion’s breakdown product 10-100 times more toxic to amphibians than the parent product.

Malathion is one of a variety of agricultural and household chemicals that threaten amphibians’ health and our drinking water. For more information on water contamination, visit Beyond Pesticides’ brochure, Threatened Waters: Turning the Tide on Pesticide Contamination.


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