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Farm chemicals can hammer frog populations: study supports link between weed killer and parasites.

Atrazine, the second-most widely used agricultural pesticide in America, can pose a toxic double whammy to tadpoles. The weed killer not only increases the likelihood that massive concentrations of flatworms will thrive in the amphibians' ponds, a new study reports, but also diminishes the ability of larval frogs to fight infection with these parasites.

Moreover, the new data show, runoff of phosphate fertilizer into pond water can amplify atrazine's toxicity. The fertilizer boosts the production of algae on which snails feed. The snails serve as a primary, if temporary, host for the parasitic flatworms, which can sicken frogs.

Amphibian populations around the world have been declining recently, with many species on the brink of extinction. Infection with trematodes, tiny flatworms, can trigger debilitating limb deformities, and severe infections can kill the amphibians.

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Researchers wanted to know why high rates of those deformities began showing up in the mid-1990s. The study suggests that one answer lies in atrazine's quick rise to dominance in U.S. agriculture.

Val R. Beasley of the University of Illinois at Urbana-Champaign and his colleagues quantified more than 240 separate factors in 18 Minnesota wetlands that might affect amphibian trematode infection rates. In the Oct. 30 Nature, the team reports that atrazine concentrations stood out as the prime correlate with trematode infection rates in Minnesota's declining northern leopard frog.

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The weed killer and its breakdown products accounted for 51 percent of the likelihood these frogs would be sickened by trematode infections. The presence of phosphate fertilizer by itself showed no effect. But when atrazine was present, the pair accounted for 74 percent of the probability that frogs would host infections.

To test whether these chemicals could cause the infections, the researchers raised young tadpoles in tanks designed to imitate woodland ponds. The team added atrazine, phosphate or both--at concentrations that would be present in waters not far from farmland.

Where atrazine was present, four times as many snails developed as did in water free of the weed killer, reports Jason Rohr of the University of South Florida in Tampa, the Nature study's lead author. These experiments indicate, he says, that as snail populations climbed so did the number of incubating trematodes.

Because these tank studies were conducted in Pennsylvania, where leopard frogs are in serious decline, the researchers substituted related species--green and pickerel frogs. Among surviving green frogs, trematode infections were significantly higher when atrazine was present. The same was not true for pickerel frogs, but this species did experience a high rate of mortality when trace quantities of atrazine laced the water. Pickerels that died could not be tested for trematodes.

Finally, in the presence of atrazine, the young frogs made one-half to one-seventh as many parasite-clearing immune cells as those in pesticide-free water.

This study "links a couple pieces of the puzzle together," notes Joseph Kiesecker of the Nature Conservancyin Fort Collins, Colo. Other studies had shown that atrazine could impair frog immunity and that trematode infections could cause limb deformity and lethal illness in frogs. The new study, he says, now shows atrazine can play a major role in both problems.

"What really impressed me about the new work," adds Tyrone Hayes of the University of California, Berkeley, "is that it looked at a huge number of factors describing a complex environment and asked which of these 240 things contributes [to the infections]. And the most important one turned out to be atrazine."

Rohr says his team will also look at preserved tadpoles from the tank experiment to see if they exhibit reproductive abnormalities.

Syngenta, which registered atrazine in the United States and remains a leading manufacturer, declined to comment.
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Title Annotation:Environment
Author:Raloff, Janet
Publication:Science News
Geographic Code:1USA
Date:Nov 22, 2008
Words:610
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