The effects of herbicide and fertilizer runoff on amphibians in rural areas has been reported before. We’re talking about frogs that develop extra limbs and other deformities. And of course, we’re talking about frogs dying. But a University of Pittsburgh study, funded by the National Science Foundation, shows that the active ingredient in popular pesticides like some Scotts Ortho products — it’s called malathion — is preventing tadpoles from maturing because it wreaks havoc with the food chain they need to grow. Here’s excerpt from Science Centric Web site which pulls its information from the Oct. 1 issue of Ecological Applications:

Gradual amounts of malathion that were too small to directly kill developing leopard frog tadpoles instead sparked a biological chain of events that deprived them of their primary food source. As a result, nearly half the tadpoles in the experiment did not reach maturity and would have died in nature.

“The chain of events caused by malathion deprived a large fraction of the leopard frog tadpoles of the nutrients they needed to metamorphose into adult frogs,’ said study author Rick Relyea, an associate professor of biological sciences in Pitt’s School of Arts and Sciences. “Repeated applications sustained that disruption of the tadpoles’ food supply. So, even concentrations that cannot directly kill tadpoles can indirectly kill them in large numbers.”

Here’s a thoughtful article on the issue from 2005: read here.