New Evidence that Common Pesticide Kills Frogs

A pesticide used to kill everything from head lice and fleas at home to agricultural pests in farm fields and mosquitoes in neighborhoods can decimate populations of frogs whose habitat is exposed to the poison, according to new research.

The pesticide, malathion, doesn't directly affect frogs, but repeated low doses leads to the elimination of key links in the tad pole food chain, effectively destroying the ecosystem that supports leopard frogs, according to the University of Pittsburgh study, published in Ecological Applications and funded by the National Science Foundation. The zooplankton -- tiny floating animals -- were killed, leaving their algae food to grow unfettered, which choked off so much light that it starved bottom-dwelling algae that tadpoles eat. That, in turn, starved the tadpoles.

In the experiment, 43% of the leopard frog tadpoles in the exposed ecosystem died, while wood frog tadpoles were not affected.

The experiment mirrored real-world conditions, in which the pesticide is applied in relatively small doses repeatedly over time. Government regulations require only tests of toxicity to specific species, not ecosystems, and they do not require testing on amphibians.

"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," Relyea said. "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."

The results should be applicable to other pesticides, including carbaryl, diazinon, endosulfan, esfenvalerate, and pyridaben, according to Relyea.

The study is part of a nine-year project by Pitt researcher Rick Relyea to determine whether pesticides are one cause of the global amphibian decline. A previous study implicated the common weed killer "Roundup" in frog deaths. The study is well-timed, since 2008 is the Year of the Frog, an international effort to bring attention to the crisis.

That malathion is toxic is not news. Farm workers who hand-pick crops must by law wait as long as six days after an application to enter fields, according to the Agency for Toxic Substances and Disease Registry. Like many pesticides, it is a man-made chemical that came into wide use in the 1950s and has remained in heavy use since.

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