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Biting Bugs No Match for Poison Frog's Toxic Skin

By John Barrat
Inside Smithsonian Research

Scientists have long realized that the toxic chemicals produced by the skin glands of poison frogs serve to protect these amphibians from predators that try to eat them. Recent research by Paul Weldon, a biologist at the Smithsonian National Zoological Park’s Conservation and Research Center in Front Royal, Va., and colleagues has revealed these toxins also protect the frogs from mosquitoes, biting flies and other disease-carrying insects.

In the study, Weldon collaborated with John Daly and Thomas Spande of the Laboratory of Bio-organic Chemistry at the National Institutes of Health, both of whom have devoted decades to identifying chemical toxins from poison-dart and other frogs. Experiments using these compounds have been limited because of the small amounts of material available.

Weldon tested the effects of one of these compounds, pumiliotoxin 251D, or PTX 251D, on yellow fever mosquitoes (Aedes aegypti) raised in laboratory colonies.

He first coated an ultra-thin silicone membrane with PTX 251D and placed it over a mosquito food source—sugar water. Most mosquitoes that landed on the membrane to feed turned over onto their backs and slowly flailed their legs, or their legs fell off entirely, an effect known as leg autonomy, and they died.

In a second experiment, Weldon confined the mosquitoes inside a narrow pipette into which he inserted a wire coated with PTX 251D, forcing the mosquitoes to stay in contact with the wire for three minutes. Weldon used different concentrations of PTX 251D in this experiment to try to estimate at what level of concentration the poison-frog alkaloid induces toxicosis in mosquitoes.

These results demonstrated that PTX 251D deters yellow fever mosquitoes from feeding and induces toxicosis with contact, as evidenced by impaired flight and leg autonomy. The researchers also learned that PTX 251D is an effective insecticide at concentrations well below those that occur naturally in poison frogs. Weldon credits the wire contact technique—which he developed at the Department of Entomology at the Walter Reed Army Institute of Research—with his ability to test compounds that are only available in small amounts.

"This is the first microscale toxicity test for insects that I am aware of," Weldon says. "It may be useful in demonstrating the insecticidal properties of other compounds."

Poison frog Dendrobates pumilio from Bocas del Toro, Panama (Photo by Ralph Saporito)

This article originally appeared in Inside Smithsonian Research, Winter 2007.