For the first time, scientists have found evidence linking agricultural runoff to the rise in grotesque hind-limb deformities in frogs. In the past, the deformities were associated with a common parasite, the burrowing trematode worm, which seemed to affect the development of tadpoles. Now, writing in this week’s issue of the Proceedings of the National Academy of Sciences, researchers have found that exposure to even minute quantities of the pesticides atrazine and malathion appears to make frogs more vulnerable to the parasite by weakening their immune systems. Both pesticides are controversial but common in the U.S., where they are used to kill weeds, mosquitoes, and other insects. Wild frogs with missing or extra hind limbs have been seen in at least 43 states and five Canadian provinces, giving rise to concerns about the fate of amphibian populations and the possible implications for human health.