Science News - January 29, 2004

Natural stresses magnify pesticide's side effects

Low concentrations of the popular pesticide Sevin are up to 46 times more lethal to certain frogs when tested in combination with cues from predators—a condition that more closely mimics nature, according to new research from the University of Pittsburgh. The study, published on December 15 (Ecol. Appl. 2003, 13, 1515–1521), adds to a growing body of research on multiple stressors on frogs that suggests that the U.S. EPA’s standards for testing pesticide toxicity are inadequate.

Traditional toxicity tests for pesticides measure lethal effects on test animals (usually fish) in a controlled laboratory setting, but Rick Relyea, the study’s author, was curious about whether the approach predicted what actually happens in nature. Relyea exposed tadpoles of six native Pennsylvania frog species to carbaryl, the generic name for Sevin, in the presence of a common natural stressor, the predatory red-spotted newt. Carbaryl concentrations ranged from 0.03 milligrams per liter (mg/L) to 6.5 mg/L, which encompass the levels of up to 4.8 mg/L that are found in ponds and wetlands after nearby fields are sprayed, he says.

The mere presence of caged newts in the tanks had a negligible effect on the survival of tadpoles, who see and smell the predators, Relyea says. The same was true of low concentrations of carbaryl alone (0.03–1.6 mg/L). But when carbaryl was added to tanks with caged newts, the combination was up to 8 times more deadly to green frogs and 46 times more deadly to bullfrogs. “This suggests that apparently safe concentrations of carbaryl, and perhaps other similar pesticides, can become more deadly to some amphibian species when combined with predator cues,” Relyea says.

“Relyea is pioneering a new area of research that examines interactions between natural and human-caused stressors,” says Andy Blaustein, director of the environmental sciences program at Oregon State University. Several studies by Blaustein and others have shown that when human-caused stressors, including pesticides, fertilizers, and elevated levels of UV-B light, combine with natural stressors such as parasites, predator cues, and pond drying, the effect on frogs is much greater than that of each agent acting alone. Although there is as yet no evidence that these synergisms are causing the worldwide decline in amphibians, the results imply that they have the potential to cause high rates of death under natural conditions, Relyea says.

“Because EPA’s toxicity test is as far from the real world as possible, it does not reflect the risk that wildlife may actually face under natural conditoins,” says Don Sparling, a wildlife toxicologist at Southern Illinois University. Scientists’ increasingly sophisticated understanding of pesticide interactions with natural stressors makes it clear that EPA should reinstitute a requirement for field testing of pesticides, which was dropped in 1992, he says.

“EPA does some field testing. For instance, the interim re-registration of the herbicide atrazine on October 31, 2003, requires field monitoring to sample atrazine concentrations in surface water,” says Dave Deegan, an EPA spokesperson. When EPA evaluates a chemical for registration, it considers published research on effects on wildlife, he adds.
“The recommendations from the Scientific Advisory Panel [on atrazine and amphibians] included a strong vote for field studies with native species as integral to the determination of atrazine impacts,” says Dave Skelly, an ecologist at Yale University who is a member of the panel. However, these recommendations are absent from the interim re-registration for atrazine.

Risk assessors must figure out how to embrace the variation inherent in field experiments, but Relyea’s findings add another justification for why it will be worth integrating ecology into toxicology, Skelly says. —JANET PELLEY