In two articles published in the August 1 issue of the journal Ecological Applications, Relyea and his doctoral students Nancy Schoeppner and Jason Hoverman found that even when applied at concentrations that are one-third of the maximum concentrations expected in nature, Roundup® still killed up to 71 percent of tadpoles raised in outdoor tanks.

Relyea also examined whether adding soil to the tanks would absorb the Roundup® and make it less deadly to tadpoles. The soil made no difference: After exposure to the maximum concentration expected in nature, nearly all of the tadpoles from three species died.

Although Roundup® is not approved for use in water, scientists have found that the herbicide can wind up in small wetlands where tadpoles live due to inadvertent spraying during the application of Roundup®.

Studying how Roundup® affected frogs after metamorphosis, Relyea found that the recommended application of Roundup® Weed and Grass Killer, a formulation marketed to homeowners and gardeners, killed up to 86 percent of terrestrial frogs after only one day.

“The most striking result from the experiments was that a chemical designed to kill plants killed 98 percent of all tadpoles within three weeks and 79 percent of all frogs within one day,” Relyea wrote.

Previous studies have determined that it is Roundup®’s surfactant (polyethoxylated tallowamine, or POEA, an “inert” ingredient added to make the herbicide penetrate plant leaves) and not the active herbicide (glyphosate) that is lethal to amphibians.

This research was funded by the National Science Foundation, Pitt’s McKinley Fund, and the Pennsylvania Academy of Science.