Waters near croplands impair frogs' immunity.

Pesticide-containing waters leave frogs more susceptible to fungal infections than pristine environments do, new field data suggest.

Tyrone Hayes and his collaborators at the University of California, Berkeley located tadpoles of Rana aurora, a protected frog species, at three sites in California. One site was upstream of any farm and had a comfortable water depth for tadpoles, about 2 feet. Another site, also upstream of agriculture, was so shallow that some frogs were exposed to air, causing some dryness-related distress. The third site was in Salinas Valley, a major area for lettuce and spinach cultivation. Waters there, about 2 feet deep, contain various pesticides that drain from the croplands.

The researchers confined some tadpoles in cages at each site and gave the animals injections of either an inert solution or a dose of bread yeast, a frog pathogen.

Tadpoles exposed to 0.125 or 0.2 gram of yeast per milliliter were assured of survival only if they lived in the deep, pristine site. At the shallower site, those doses killed 20 percent and 80 percent of the animals, respectively. Those numbers demonstrate that dryness-induced stress can compromise frogs' immunity, says Hayes.

In Salinas Valley, all tadpoles exposed to the yeast either died or became comatose, Hayes reported. He concludes that the pesticides compromised the animals' immunity even more than dry conditions did.