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Frogicide. A component of the herbicide Roundup kills tadpoles.

Popular Herbicide Kills Tadpoles

The popular herbicide Roundup is widely used to kill weeds, but a new study suggests it may also kill tadpoles in the wild. The findings could, in part, explain global amphibian declines and are unexpected because Roundup was thought to have little or no effect on animals in the environment.

In addition to controlling garden weeds, Roundup is used to reduce weeds in crop fields and forests. The active ingredient is glyphosate, a chemical that inhibits amino acid synthesis in plants but is thought to be relatively nontoxic to animals and biodegradable. Researchers have shown that herbicides can cause developmental abnormalities in amphibians, but no one had shown that herbicides can kill off tadpoles at concentrations that occur in the environment.

To test this, Rick Relyea, a biologist at the University of Pittsburg in Pennsylvania, simulated a pond ecosystem by filling 1000-liter tanks with well water, plankton, various tadpole species, and other organisms at the same densities found in nature. He then added Roundup to the water to achieve a concentration of 3.8 mg/L, the highest concentration likely to be found in nature.Within a day, Roundup completely eliminated leopard frog and grey tree frog tadpoles and nearly exterminated the wood frog tadpoles. Spring peeper tadpoles were not as dramatically affected, he reports in the April issue of Ecological Applications. Relyea suspects the lethal ingredient in Roundup is not glyphosate, but polyethoxylated tallow amine (POEA), a surfactant that allows the herbicide to penetrate the waxy surfaces of plants and may interfere with the function of the tadpole gills. This surfactant is not regulated under environmental laws because it is not considered an active ingredient in the herbicide.“This is a well-done study that furnishes us with new understanding about the environmental effects of agricultural surfactants”, says biologist Reinier Mann at the Universidade de Aveiro in Portugal. Eric Sachs, the scientific director of Monsanto, the company that manufactures Roundup, acknowledges that POEA can harm aquatic life. "That is why Roundup clearly states on the label that it is not to be applied directly to water," says Sachs.But Relyea counters that, because frogs live and breed in many shallow water environments near forests and fields, the herbicide is likely to enter the frog's water at toxic concentrations.Related sites:
Relyea's website
The Relyea study