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The sound of no mosquitoes biting.

The sound of no mosquitoes biting

The search for a perfect mosquito repellent may never end, but it's been years since scientists have come up with anything even a little more effective than the old standby, N,N-diethylm-toluamide, or deet. "Probably the single most important reason for this failure," says Edward E. Davis of the Vector Biology Program at SRI International in Menlo Park, Calif., "is that we still don't know the site or mode of action of any currently effective [mosquito] repellent substances" - most of which have remarkably dissimilar chemical structures. Without insight into what makes a repellent repulsive, it's difficult to design a more effective version.

But in preliminary research reported last week, Davis provided new clues about deet's modus operandi, spurring hope that custom designed compounds someday may put an end to that summertime symphony of little whining wings. Speaking at the International Congress of Entomology in Vancouver, British Columbia, Davis described electrophysiological and behavioral experiments on mosquitoes that suggest deet specifically interferes with antenna-based receptor cells that normally bind and respond to lactic acid.

Lactic acid, which evaporates from the skin of warm-blooded animals, has long been recognized as a mosquito "host attractant" molecule. But scientists have been uncertain what step in a lactic-acid-mediated chain of events deet interrupts. Davis' experiments suggest that deet, when it evaporates from treated clothing or skin, somehow interferes with the initial molecular binding of the attractant acid to a mosquito's sensory cells.

Using tungsten microelectrodes, Davis measured microvolt "action potentials"--changes in electrical activity that initiate neuronal firing -- at lactic acid receptor sites on the tiny hairs, or sensillae, of mosquito antennae. The neurons that normally fire with increased frequency in response to the specific lactic acid cue fired significantly less frequently in the presence of deet; other neurons were much less affected by deet. This suggests that more generalized theories about deet's mode of action--that it is a central nervous system blocker or that it binds to and activates its own, specific "noxious stimuli" receptors -- are probably incorrect, Davis says.

He also performed behavioral experiments, in which he tallied the number and degree of directional changes mosquitoes made in a wind tunnel when exposed to varying combinations of lactic acid and deet. He videotaped their behavior and reviewed the tapes frame by frame. The mosquitoes behaved the same in the presence of lactic acid and deet as they did in the absence of lactic acid, providing new evidence of the acid's attractant role.

In other work, Peter Belton of the Simon Fraser University in Burnaby, British Columbia, glued live mosquitoes' "chins" onto glass microscope slides. He then zapped their antennae with a laser (see diagram) and recorded vibratory resonance and the electrical activity of sensory cells at the antennal bases. His findings confirmed earlier work, done with less-sensitive instrumentation, that male mosquitoes are most sensitive to sounds in the vibratory range made by the beating of female mosquito wings -- about 250 hertz. But he also found that electronic devices that purpot to repel mosquitoes by emitting irritating sound waves are completely outside the hearing range of the tiny insects.
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Title Annotation:research on how repellents work
Author:Weiss, Rick
Publication:Science News
Date:Jul 16, 1988
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