More than half a century after DEET's invention, scientists still don't know how the popular mosquito repellent works.
Now, using a combination of artificially accelerated evolution and painstaking anatomical observation, researchers have answered a fundamental question about DEET's mechanisms -- and in the process showed that mosquitoes may become resistant to it.
"It's a fundamental piece of research. It will give us a lot more knowledge, rather than just going out and spraying something," said study co-author Linda Field, a molecular biologist at England's Rothamsted Research institute.
How DEET -- short for N,N-diethyl-meta-toluamide -- actually works has been a subject of scientific controversy since its discovery after World War II by U.S. Army researchers who tested thousands of man-made compounds, and were simply happy to find one that repelled bugs.
Researchers later hypothesized that DEET prevented insects from detecting lactic acid, one of the odors mosquitoes follow to unlucky animals. But DEET still worked in the absence of lactic acid, leading some scientists to speculate that DEET interferes with some other molecule's detection. However, other scientists think DEET acts directly on an as-yet-unidentified olfactory receptor, irritating them instead of just hindering them.
The argument would be of little interest to anyone except entomologists, but for the fact that some mosquitoes aren't deterred by DEET. For people in the developed world, that's annoying. For people in tropical areas where insect repellents and insecticides control mosquito-borne disease, it's a pressing concern.
Field and Nina Stanczyk, a University of Nottingham biochemist, started their study by resting a DEET-sprayed arm on a mesh cage, just out of reach of female Aedes aegypti mosquitoes. (Only female mosquitoes bite; like males, they typically feed on flower nectar, but require nutrients from blood in order to lay eggs). Those that tried to feed were removed and bred separately. Within a few generations, more than half were DEET-resistant.
Field cautioned that laboratory results shouldn't be automatically extrapolated to the natural world, but a similar dynamic could well exist, especially in heavily populated areas where humans are the predominant source of blood. "If a small percentage are insensitive, they have a much better chance of getting a blood meal, and are much more likely to pass on their genes. You'd likely see a buildup of the trait," said Field.
The researchers then attached electrodes to their mosquitoes' antennae, allowing them to monitor responses to DEET and other compounds. DEET indeed proved to have an effect on its own, in the absence of any other odor. Stanczyk proceeded to painstakingly attach electrodes to the mosquitoes' sensillum -- tiny, single molecule-detecting structures found by the thousands on each antenna. She identified the type of sensillum that responded to DEET. In DEET-resistant mosquitoes, those sensillum are shaped unusually.
For now, that's the extent of the researchers' knowledge.
"Somewhere inside that sensillum, the odor molecule gets in, is picked up by a binding protein, goes to a receptor protein, and that triggers the behavioral response," said Field. "Our question now is, what has changed? The next step is finding out what's going on inside."
Image: 1) James Jordan/Flickr. 2) Mosquito sensillum./Linda Field.
See Also:
- Whining and Dining: Love, Mosquito Style
- New Mosquito Repellents Beat DEET
- Green Mosquitoes Could Control Killer Disease
- Universal 'Death Stench' Repels Bugs of All Types
Citation: “Behavioral insensitivity to DEET in Aedes aegypti: A genetically determined trait residing in changes in sensillum function.” By Nina Stanczyk, John Brookfield, Rickard Ignell, James Logan, and Linda Field." Proceedings of the National Academy of Sciences, Vol. 107 No. 18, May 4, 2010.
Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.
