The breakdown products of the three most commonly used organophosphorus pesticides in the Central Valley are 10 to 100 times more toxic to amphibians than their parent compounds, which are already highly toxic to amphibians, says a report Wednesday from the U.S. Geological Survey.
"Since some of the parent pesticide compounds are already at concentrations sufficient to cause significant amphibian mortality in the Sierra Nevada, the higher toxicity of the breakdown products poses a serious problem," says Gary Fellers, coauthor of the study published in the journal Environmental Pollution.
Donald Sparling, a research biologist and contaminants specialist at Southern Illinois University, and Mr. Fellers, a research biologist and amphibian specialist at the USGS Western Ecological Research Center in Sacramento, conducted laboratory tests to determine the acute toxicity -- the lethal dosage causing death in 96 hours or less -- of chlorpyrifos, malathion and diazinon, and their breakdown, or “oxon” derivatives on tadpoles of the foothill yellow-legged frog.
Organophosphorus pesticides have been implicated in the declines of several amphibian species in the Central Valley and in downwind areas, including the Cascades frog, California red-legged frog, mountain yellow-legged frog and the foothill yellow-legged frog, which inhabit foothill regions east of the Central Valley, the scientists say.
More than 6.5 million pounds of organophosphorus pesticides were used in California during 2004, the most recent year for which data are available. Researchers estimate that this accounts for about 25 percent of such pesticide use nationwide.
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