Two Frogs, Two Pesticides and their Toxicity

Two pesticides used in highly populated agricultural areas of California appear to be killing frogs that live and breed in the Sierra Nevada Mountains, according to results from a study published in the August 2009 issue of Environmental Toxicology and Chemistry.

The study examined how chlorpyrifos and endosulfan used in the Central Valley of California affect amphibians that breed in the mountains to the east. Toxicity was measured to larval Pacific treefrogs (Pseudacris regilla) and foothill yellow-legged frogs (Rana boylii), which are among the amphibians with declining populations that often live and breed in meadows surrounding the Sierra Nevada. Winds blow insecticide residues into the mountains, and they fall as rain or snow. In these regions, insecticides have longer half-lives because of cooler temperatures and can be spread by melting snow to areas where amphibians live and breed.

As outlined in the article, "Toxicity of Two Insecticides to California, USA, Anurans and Its Relevance to Declining Amphibian Populations" by Donald W. Sparling and Gary M. Fellers, the study used laboratory testing to examine how the insecticides affected the two frogs at environmentally realistic concentrations. During testing, tadpoles were observed at various stages of development to see how the insecticides affected their growth and health.

Sparling performed the study at the Cooperative Wildlife Research Laboratory, Southern Illinois University in Carbondale. Fellers worked through the U.S. Geological Survey, Western Ecology Research Center, at Point Reyes, Calif.

Endosulfan was more toxic than chlorpyrifos to both species, according to the research, and tadpoles of both species developed abnormalities when exposed to high endosulfan concentrations. Endosulfan also affected the growth and development rates in both species. The researchers say this affects the amphibians' behavior and increases their vulnerability to predators and hydrological events such as floods and droughts. The study also shows that chlorpyrifos and endosulfan are highly toxic to both amphibians, with the yellow-legged frogs more sensitive than the Pacific treefrogs to these insecticides.

"The difference in sensitivity is important, because P. regilla populations are still comparatively stable in California, even in montane areas, whereas R. boylii is one of the species that has declined in recent years," according to Sparling and Fellers.

The yellow-legged frogs, which rely more on standing water during reproduction, have seen higher population declines compared with other species.

"Concentrations of insecticides in the Sierra Nevada Mountains of California may have the ability to inflict serious damage on native amphibians," Sparling and Fellers write. "The present study adds to the increasing evidence that pesticides are very harmful to amphibians living in areas that are miles from sources of pesticide application."

Environmental Toxicology and Chemistry is a publication of the Society of Environmental Toxicology and Chemistry.