Elevated pesticide threatens amphibians.

Breezes crossing California's agricultural heartland, the Central Valley, can ferry farm chemicals to elevations high in the Sierra Nevadas. Mountain-water concentrations of endosulfan--a much-used Central Valley insecticide--are strong enough to threaten certain frogs and toads, a new laboratory study shows.

Donald W. Sparling, a wildlife toxicologist at Southern Illinois University in Carbondale, airlifted amphibian eggs collected at relatively pristine Sierra Nevada sites to his lab, where he then incubated them to adulthood in water that was clean or treated with endosulfan at between 0.007 and 15 parts per billion (ppb). Other researchers have recorded concentrations of the pesticide in mountain ponds of around 0.3 ppb, he says.

The highest concentration killed every exposed animal among all three species tested. At around 3 ppb, half of the Western toads (Bufo boreas) and Pacific tree frogs (Pseudacris regilla) died. The animals that survived tended to be about two-thirds as big as usual, Sparling reported.

It took endosulfan concentrations of only 0.3 ppb to wipe out half of the foothills yellow-legged frogs (Rana boylii) in the study. Some of those frogs and a few Western toads succumbed at even 0.15 ppb--well within the range of concentrations seen in snowmelt and pond water at some Sierra Nevada sites, says Sparling. This finding suggests why populations of both species are on the decline in those mountains, he says.--J.R.