DEET, the widely used mosquito repellent, does not block the insects' sense of smell but simply stinks to them, U.S. researchers reported on Monday.
This in turn could help in the development of better mosquito repellents, according to the team at the University of California, Davis.
They managed to record the signals from individual neurons in the antennae, which mosquitoes use to smell, and determined that they were not blocked but stimulated in a way that suggested the odor was simply unpleasant.
"We found that mosquitoes can smell DEET and they stay away from it," said Walter Leal, a professor of entomology who led the work. "DEET doesn't mask the smell of the host or jam the insect's senses. Mosquitoes don't like it because it smells bad to them."
DEET, the common name for N,N-diethyl-3-methylbenzamide, is the most common active ingredient in insect repellents. The U.S. Centers for Disease Control and Prevention and other health groups recommend DEET as the best way to avoid the bites of mosquitoes, ticks and other disease-carrying insects.
It was developed by scientists at the U.S. Department of Agriculture and patented by the U.S. Army in 1946.
Most experts believed DEET worked by blocking the insect's ability to detect 1-octen-3-ol, a volatile substance that is contained in human sweat and breath. That would mean that the insects, which find their human victims by smell, would fly right on by a DEET-disguised arm or ankle.
Writing in the Proceedings of the National Academy of Sciences, Leal and his group described how they tested this theory.
Collaborator Zain Syed discovered the precise neurons on the antennae of mosquitoes that detect DEET, and found they are right next to the neurons that sense 1-octen-3-ol.
The DEET neurons reacted strongly to DEET, and more strongly when more DEET was used, the researchers reported. They would not have been stimulated if DEET was merely blocking them.
"I was so delighted when I first encountered the neuron that detects DEET, a synthetic compound," Syed said in a statement. "I couldn't believe my eyes because it goes against conventional wisdom."
It also reacted, even more strongly, to compounds known as terpenoids, which help make up the distinctive aromas of eucalyptus, cloves, menthol and camphor.
"In the future, this new knowledge can be incorporated into developing new repellents and maybe in control strategies," Major Dhillon, president of the American Mosquito Control Association, said in a statement.