ShareThisNext to Pymatuning Reservoir in northwestern Pennsylvania are 700 water-filled tanks that may provide the answer to just how risky pesticides are to the environment and human health.
The tanks are filled with tadpoles, and University of Pittsburgh ecologist Rick Relyea has used them over the past several years to show how lethal many of the most commonly used pesticides are, even in concentrations below allowable levels.
That's significant not just because frogs are an important part of nature's food web but because these delicate creatures may function as an early warning system for environmental threats to human beings.
Over the last four years, Relyea and his colleagues have shown how ubiquitous chemicals like the weedkiller Roundup, malathion and endosulfan kill off large proportions of some frog species and may be contributing to the worldwide decline in the amphibian population.
In 2005, he showed that Roundup, the most commonly used herbicide in the world, killed more than 70 percent of the tadpoles in his tanks when it was present in just a third of the maximum concentration expected in nature. The chemical also killed more than 80 percent of land frogs after just one day of exposure to the recommended dosage of Roundup Weed & Grass Killer.
Last year, his team demonstrated that malathion, which is used for mosquito and insect control around the world, was deadly for one species, the leopard frog.
Malathion didn't kill the leopard frog tadpoles directly. Instead, it obliterated tiny creatures known as zooplankton, which normally eat algae.
Without the zooplankton nibbling away, the algae in the tanks grew so thick that it blocked sunlight from reaching another type of algae that lived on the bottom of the tanks and is the primary food source for the leopard frog tadpoles.
The chain reaction starved many of the tadpoles and kept them from metamorphosing into frogs.
In another study last year, the Relyea team found that a "cocktail" of 10 common pesticides killed nearly all the leopard frog tadpoles in the tanks, even when each one was under the limits considered safe.
Closer analysis showed the main culprit in the cocktail was endosulfan, a nerve agent used to combat such insects as aphids and cabbage worms, but which is risky enough that it has been banned in the European Union and some Asian and West African nations.
Unlike other nerve agents, Relyea said, endosulfan works by overstimulating an animal's nerves. Tadpoles affected by the insecticide, he said, "would turn cartwheels. They would just spin and spin and spin and then die."
The same cocktail, however, had almost no effect on another common species, gray tree frog tadpoles. In fact, those tadpoles tended to thrive once their leopard frog competitors were wiped out.
Even though Relyea has focused much of his recent research on the dangers of pesticides, he doesn't consider himself "anti-pesticide."
"As a kid I grew up on a farm spraying all kinds of pesticides, so I recognize the benefits. But we really haven't put a lot of attention on the side effects (they have) on organisms that we're not trying to kill. We're not surprised when pharmaceuticals have unintended side effects, so we shouldn't be surprised when pesticides do."
And while his research hasn't proved the pesticides have human health effects, he said we should pay close attention to that possibility.
"I think what's clear is that pesticides have an immense potential for unintended impacts, and organisms -- humans or otherwise -- are immensely complicated, and those unintended impacts are really hard to predict," especially when you consider that today's commercial pesticides as a whole contain more than 800 active ingredients.
Relyea's ongoing work already has had some real-world results. After the Roundup study was published, Congress demanded that the U.S. stop spraying the herbicide over wetlands in Colombia, where the government was trying to stop illegal coca cultivation.
In the future, he would like to see the federal Environmental Protection Agency start to use at least one amphibian species in standard testing of new pesticides.
Right now, the EPA tests pesticides on lab rats or mice, birds, fish and a small plankton called Daphnia. The fish are supposed to stand in for all 6,000 species of amphibians, Relyea said, but "sometimes that's not a reasonable stretch."
E-mail Mark Roth of the Pittsburgh Post-Gazette at mroth(at)post-gazette.com.
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
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