September 5, 1998

Common pesticide clobbers amphibians

By J. Raloff

Over the past decade, biologists have been chronicling a dramatic worldwide disappearance of frogs and toads. A new Canadian study lends strong support to the suspicion that agricultural chemicals are contributing to this mysterious amphibian decline.

Michael Berrill of Trent University in Peterborough, Ontario, and his colleagues collected newly laid eggs of the wood frog, green frog, and American toad from the wild. Some of the eggs were incubated in water containing endosulfan, an organochlorine pesticide, while others were allowed to hatch and the tad-poles received a 4-day exposure to the compound.

The study simulated conditions that the animals might face as they breed in drainage ditches, ponds, and other wet spots around orchards and farms, says coauthor Bruce Pauli, a biologist with Environment Canada in Hull, Quebec. Because of endosulfan’s toxicity to fish, regulations discourage farmers from using endosulfan near open water. However, aerial drifting of the pesticide can leave concentrations of 0.4 milligrams per liter or more at 3 meters beyond the perimeter of sprayed agricultural fields.

In the amphibian tests, concentrations of endosulfan were low -- 0.03 to 0.4 milligrams per liter of water.

Though eggs exposed to endosulfan hatched normally, Berrill’s team reports in the September Environmental Toxicology and Chemistry, the highest pesticide concentrations caused the resulting tadpoles to temporarily exhibit a depressed "avoidance behavior." This would have rendered them vulnerable to predation, the researchers say.

Tadpoles exposed to the pesticide after hatching, however, fared worse. All three species tested experienced high death rates -- especially at the higher exposures. Timing of the exposure made a surprising difference. Up to 30 percent of those dosed immediately after hatching died, while groups exposed as 2-week-old tadpoles sometimes suffered 100 percent mortality.

All three species of tadpoles also incurred significant sublethal toxicity --   even in the lowest exposure groups, except for toads exposed immediately after hatching. Typically, affected tadpoles exhibited hyperactivity, characterized by "whiplike convulsions," followed by temporary paralysis. The survivors of the highest exposures also grew unusually slowly.

In the future, Pauli says, his group hopes to follow amphibians through metamorphosis to assess whether early, low-dose exposures to such pesticides may be responsible for some of the deformities plaguing North American frogs (SN: 10/11/97, p. 230).

Currently, endosulfan is commonly used to fight insects, aphids, and mites on a broad range of fruits, vegetables, ornamental plants and oilseed and cotton. Berrill’s team concludes that the hazard it presents to frogs and toads "is sufficiently great to warrant its replacement by less toxic alternatives wherever possible."

Don Sparling, a contaminant ecologist with the U.S. Geological Survey’s Patuxent Wildlife Research Center in Laurel, Md., agrees that "this study gives us cause for concern." Indeed, he notes, "we know so little about the effects of pesticides on amphibians that almost anything we find in these studies is going to be new."

In May, Interior Secretary Bruce Babbitt convened a cabinet-level briefing on amphibian declines. It led to the creation of the Taskforce on Amphibian Declines and Deformities to coordinate federal monitoring and research. Its first meeting is slated for Sept. 21.   

From Science News, Vol. 154, No. 10, September 5, 1998, p. 150.
Copyright Ó 1998 by Science Service.

Michael Berrill
Trent University
Watershed Ecosystems Program
Peterborough, Ontario K9J 7B8
Canada

Stephanie Hanna
U.S. Department of the Interior
Communications and Media Affairs
1849 C Street, N.W.
Mailstop 6013 MIB
Washington, DC 20240

Bruce Pauli
Environment Canada
Canadian Wildlife Service
100 Boulevard, Gamelin
Hull, Quebec K1A 0H3
Canada

Don Sparling
USGS Patuxent Wildlife Research Center
12100 Beech Forest Drive
Laurel, MD 20708-4038