Tadpole Tests Show Lethality of One Pesticide

Northern Leopard Frog

Photo: BuBZ / Wikimedia Commons

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Here's the gist of a new study: Take a little bit of a poison, and it may not be dangerous. Mix a little bit of a lot of "safe" poisons, and it can be deadly.

The poisons in this case are the world's 10 most widely used pesticides. The dead in this case are amphibians. The University of Pittsburgh research on "contaminant cocktails" is published in Oecologia.

Lead author Rick Relyea tried three mixtures to see how they affected the tadpoles of gray tree frogs -- whose songs bring spring forests to life -- and leopard frogs, whose populations are in decline across North America. One set of tadpoles were given the five most popular insecticides (insect killers), one set given the five most popular herbicides (weed killers) and one set the double-whammy: all 10 of the most popular pesticides. Each pesticide was administered in such a low dose that it would have met all federal guidelines; the dose was lower than the levels detected in water supplies.

(For reference, the five insecticides he tested are carbaryl, chlorpyrifos, diazinon, endosulfan, and malathion. The five herbicides are acetochlor, atrazine, glyphosate, metolachlor, and 2,4-D.)

The result?

The five-herbicide cocktail left the tadpoles of both species apparently unharmed. Score one for the chemical poison-makers of the world.

The five-insecticide cocktail, however, killed 99% of all leopard frog tadpoles... as did the double-whammy 10-pesticide cocktail. Gray treefrogs were unaffected, and even thrived in the absence of their pesky neighbors.

But as important as the implications of the chemical cocktail is the deadliness of one particular pesticide. Relyea found that endosulfan, the insecticide, was particularly deadly to leopard frog tadpoles, killing 84% in a low dose. The chemical is banned in the European Union and several other nations, but still widely used in the good-old U.S.A. The insecticide cocktail would not have been nearly so deadly if endosulfan were not in the mix.

“Endosulfan appears to be about 1,000-times more lethal to amphibians than other pesticides that we have examined,” Relyea said. “Unfortunately, pesticide regulations do not require amphibian testing, so very little is known about endosulfan's impact on amphibians, despite being sprayed in the environment for more than five decades.”

2008 is the Year of the Frog, when zoos and wildlife advocates the world over are focusing attention on the "amphibian crisis." Since 1980, 120 species of frogs, toads, salamanders and other amphibians have gone extinct, and as many as half of the remaining 6,000 species are endangered. The causes are many, and not all well-known: The causes almost certainly include a fungus and habitat loss, but may also include global warming and chemical contamination.

Amphibians, with their permeable skin, are considered important early warning signals for problems in the environment that may affect human health.

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