The link between amphibian declines and pesticide use has been puzzling because pesticide levels in nature are well below those that are lethal in the laboratory. New research helps explain this mystery: the presence of predators can make the pesticide carbaryl more toxic to tadpoles.
“When we include some of the natural ecology, even low concentrations of a pesticide can be highly lethal to amphibians,” says Rick Relyea of the University of Pittsburgh, Pennsylvania, in the December issue of Ecological Applications.
Sold under the commercial name Sevin, carbaryl is a neurotoxin used worldwide to control insects on crops, rangelands, and forests. Carbaryl works by inhibiting the neuroen-zyme acetylcholinesterase, a mechanism common to many other carbamate and organophosphate pesticides.
Extending his previous work linking pesticide toxicity to predator stress in the gray tree frog (Hyla versicolor), Relyea treated five more amphibian species with both carbaryl and predator stress (caged red-spotted newts, Notophthalmus viridescens). The additional amphibian species were American toads (Bufo americanus), bullfrogs (Rana catesbiana), green frogs (R. clamitans), leopard frogs (R. pipiens), and wood frogs (R. sylvatica). Relyea treated newly hatched tadpoles and monitored their survival for 16 days.
Relyea found that the combination of carbaryl and predator stress was more lethal in green frogs and bullfrogs (up to 8 and 46 times, respectively) than carbaryl alone.
For example, few tadpoles of either species died when treated only with 1.6 mg/L of carbaryl. But when caged newts were added, most of the tadpoles of both species died; 80 percent of the green frog tadpoles and 98 percent of the bullfrog tadpoles died after 16 days.
This synergistic effect may also be at play in natural aquatic environments, where carbaryl levels can be much higher (up to 4.8 mg/L) than those that had effects in this study. “This suggests that apparently safe concentrations of carbaryl (and perhaps other pesticides with similar modes of action) can become more deadly to some amphibian species when combined with predator cues,” says Relyea.
Relyea, R. 2003. Predator cues and pesticides: A double dose of danger for amphibians. Ecological Applications 13:1515-1521.
Red-spotted newts (Notophthalmus viridescens) Photo by John White