August 22 (HealthDay News) -- The reason DEET-based insect repellants are effective against mosquitoes is because the little blood suckers don't like the smell of the chemical, says a University of California, Davis study.

The finding disproves the long-held belief that DEET interferes with mosquitoes' sense of smell and ability to target a person.

"We found that mosquitoes can smell DEET, and they stay away from it. 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," chemical ecologist Walter Leal, a professor of entomology, said in a UC Davis news release.

He and his colleagues set up odorless sugar-feeding stations, including some that contained DEET, and found that DEET actively repelled Southern house mosquitoes, which can carry West Nile virus, St. Louis encephalitis, and lymphatic filariasis, a disease caused by threadlike parasitic worms.

The study was published in the Aug. 18-22 issue of the journal Proceedings of the National Academy of Sciences.

DEET, the most common active ingredient in insect repellents, was developed in the United States in the 1940s. Worldwide, about 200 million people use DEET-based repellants to protect themselves against disease-carrying insects.

"Despite the fact that DEET is the industry standard mosquito repellant, relatively little is known about how it actually works," UC Davis research entomologist William Reisen said in the news release. "Previous studies have suggested a 'masking' or 'binding' with host emanations. Understanding the mode of action is especially important, because DEET is used as the standard against which all other tentative replacement repellants are compared."

SOURCE: University of California, Davis, news release, Aug. 18, 2008