The length of time that it takes for the toxic effects of a common crop pesticide to emerge raises questions about U.S. EPA's standard approach to assessing pesticides' safety, according to a new study.
University of Pittsburgh researchers exposed nine species of tadpoles to endosulfan -- a common pesticide used on cotton, tomatoes, melons, squash and tobacco crops -- for up to eight days at levels EPA says are typically found in nature.
After four days, three species seemed unaffected. But four more days after being moved to clean water, 97 percent of leopard frog tadpoles died, as did up to 50 percent of spring peeper and American toad tadpoles.
Their findings, published in the September issue of Environmental Toxicology and Chemistry, are noteworthy because most scientists and EPA typically use four-day tests to determine whether a pesticide is safe, said Rick Relyea, an associate professor of biological sciences at Pittsburgh.
"For many pesticides, probably four days is a good assumption," Relyea said. "The problem is that it's clearly not good for every pesticide, so now we have to ask, for which pesticide is it a good assumption?"
The study followed earlier work by Relyea that found endosulfan to be 1,000 times more toxic to amphibians than other pesticides.
"We knew it was really deadly to fish, but in the first four days, the leopard frogs were not dying," Relyea said. "Something weird was going on here. We found [the lag effect] was clearly happening in three different members of frogs, so this was clearly something that must be not that rare."
Part of the problem with EPA's risk-assessment model, Relyea said, is that it does not test pesticides on amphibians, even though tadpoles and other amphibians are sensitive to pollutants and many scientists consider them indicator species. Instead, the agency relies on testing four groups -- fish, mammals, birds and crustaceans -- then extrapolates the data.
"This is the standard set of model organisms, and it's there to represent what's out there in an efficient way," Relyea said. "There's no doubt it's efficient. The question is, is it correct?"
EPA said risks to fish are generally assumed to be representative of risks to other aquatic vertebrates, including amphibians. Additionally, the agency said specific tests for amphibians are not sufficiently vetted yet to be used. EPA also said amphibians tend to be less sensitive than fish to pesticides.
Endosulfan is an organochlorine, as is DDT, which EPA banned in 1972. Endosulfan is already banned in other countries, and last month, pesticide manufacturer, Bayer CropScience, announced it would stop selling products containing the pesticide.
EPA said its scientists would evaluate the Pittsburgh research along with other comments the agency receives as it reviews endosulfan's risk assessment. As a part of that review, EPA said it would evaluate the need for longer toxicity studies. It expects to make a final decision on endosulfan by the end of this year.
"If other studies consistently show that post-exposure toxicity estimates differ substantially from those recommended by [the American Society for Testing and Materials] and other global partners, then the agency will consider revisiting the design of aquatic toxicity studies in general," EPA said.
Click here to read the report.