Insecticides from the Central Valley of California are drifting into montane regions where amphibian population declines have been severe. Pesticides have been measured at ng/L (ppt) concentrations in snow and surface waters and at μg/kg (ppb) levels in amphibian tissues. Pesticides are playing a significant role in the declines of native, pond-dwelling amphibians in the Sierra Nevada. Higher concentrations of pesticides have been measured in watersheds where amphibians have experienced the greatest declines, pesticide-induced cholinesterase depression is greatest in the southern Sierra Nevada where the declines have been most striking, and field experiments involving Pseudacris regilla
(Pacific chorus frog) have shown that survival is lower, genotoxicity is greater and the rate of deformities is higher in areas most impacted by pesticides.
Chlorpyrifos is the most widely used organophosphorus pesticide in California. Chlorpyrifos blocks acetylcholinesterase (AChE) at neural synapses and hence lead to repeated firing of neurons. This can cause death through respiratory failure. In addition, chlorpyrifos can be degraded into chlorpyrifos oxon that is at least 100 times more toxic. Endosulfan is the second most commonly used pesticide in California. Endosulfan also impairs neurological function. Our experiments have shown that endosulfan is the most toxic of the commonly used pesticides in California. Whereas all three forms of endosulfan are toxic, a mixture of alpha and beta endosulfan resulted in an LC50 of 0.3 μg/kg body weight in Rana boylii (foothill yellow-legged frog) and ca. 3 μg/kg body weight in P. regilla and Bufo boreas (western toad). Approximately 86% of adult P. regilla collected from an area that had experienced declines had trace amounts of one or more of the endosulfans. Although endosulfan use is less than that of chlorpyrifos in California, its longer half life and high toxicity may make it more dangerous.
Laboratory assays have provided additional evidence that anuran tadpoles are sensitive to pesticides. R. boylii was more sensitive to both pesticides than was P. regilla. The estimated median lethal concentration or LC50 for chlorpyrifos was 265.8 μg/L in P. regilla and 90.6 μg/L for R. boylii. Time to metamorphosis increased with concentration of chlorpyrifos in both species and cholinesterase activity declined with exposure concentration in metamorphs of both species. For endosulfan, the LC50 was 7.87 μg/L for P. regilla and 0.33 μg/L for R. boylii. All R. boylii exposed to concentrations > 0.8 μg/L died before metamorphosis. Endosulfan reduced the rates of growth and development in both species, and caused severe twists in the bodies of early stage larvae exposed to endosulfan at 50 and 200 μg/L. P. regilla remains relatively abundant and is broadly distributed throughout California. In contrast, R. boylii is among the species experiencing severe population declines. The calculated LC50 and no adverse effects level (NOAEL) of 0.003 μg/L for endosulfan for R. boylii adds to the increasing evidence that pesticides are very harmful to amphibians living miles from the source of pesticide application.
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