Wafting pesticides taint far-flung frogs.

Federal researchers have added new evidence to the growing case that agricultural pesticides blowing into California's wilderness areas have played a role in mysterious declines in frog populations.

Traces of the common pesticides Diazinon and chlorpyrifos showed up in more than half the Pacific tree frogs sampled in Yosemite National Park, but in only 9 percent of the frogs tested at sites upwind of agricultural areas, report U.S. Geological Survey scientists Gary Fellers and Donald Sparling.

Fellers, based at the Point Reyes National Seashore in California, and Sparling, at the Patuxent Wildlife Research Center in Maryland, spoke at a USGS symposium on amphibian declines held last week in Reston, Va.

Parts of California that may look like frog heaven have been anything but that during the past 15 years. The California red-legged frog now ranks as a threatened species on the U.S. list; the mountain yellow-legged frog and Yosemite toads have been proposed for listing.

The idea that drifting pesticides might somehow be harming frogs isn't new, and studies have already confirmed parts of the scenario (SN: 9/5/98, p. 150). For instance, Fellers and his coworkers last year reported that air currents can transport pesticide residues into remote areas. Just what those low exposures might be doing to frogs has remained a troubling question, Fellers says.

Now, he and Sparling have checked Pacific tree frogs at six locations scattered around California. The researchers chose tree frogs as a proxy for rare species. Along the coast upwind of inland farms, "the frogs seem to be doing rather well," Fellers says. In contrast, frogs in wilderness areas downwind of heavy agriculture were contaminated with low concentrations of pesticide.

The researchers report the first evidence in California that sublethal pesticide doses affect frogs: Tissue samples showed hampered activity for the enzyme cholinesterase, which keeps nerve cells firing normally. Fellers speculates that frogs with this condition might not be bopping, fleeing, or mating in top form. Details of the work will appear in a future ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY.

"It's an important study," comments Carlos Davidson of the University of California, Sacramento, who studies the geographic distribution of amphibian declines in California. "Just because we have evidence for one cause doesn't mean the other ones are wrong," he cautions. For example, he says he's convinced that the practice of stocking waterways with trout is devastating native amphibians in some spots.

At the USGS symposium, veterinary pathologist Carol Meteyer of the Wildlife Health Research Center in Madison, Wis., described what she calls the first systematic comparison of deformed frogs from different sites. Other researchers have blamed the deformities on widespread environmental contaminants, parasites, or even predators that mutilate frogs. However, Meteyer says, her high-detail X-ray study reveals that frogs lacking a leg or two are also missing pelvis parts. That wouldn't come from a survivable predator attack, she says.

She also notes that deformed frogs at four Vermont sites are usually missing limbs, but those in Maine have extras. Says Meteyer: "To me, this says there are different agents out there," including, possibly, mixes of pesticides.