"We found that mosquitoes can smell DEET and they stay away from it," said noted chemical ecologist Walter Leal, professor of entomology at UC Davis.
"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," Leal added.
DEET, the common name for N,N-diethyl-3-methylbenzamide, is the most common active ingredient in insect repellents.
Developed by scientists at the U.S. Department of Agriculture and patented by the U.S. Army in 1946, DEET is considered the "gold standard" of insect repellents.
However, DEET's mode of action or how it works had puzzled scientists for more than 50 years.
Researchers found that mosquitoes detect DEET and other smells with their antennae. Leal and researcher Zain Syed discovered the exact neurons on the antennae that detect DEET (N,N-diethyl-3-methylbenzamide).
These neurons are located beside other neurons that sense a chemical, 1-octen-3-ol, known to attract mosquitoes.
"I was so delighted when I first encountered the neuron that detects DEET, a synthetic compound. I couldn't believe my eyes because it goes against conventional wisdom. So I repeated the experiment over and over until we discussed the findings in the lab," Syed said.
The reserahcers set up odourless sugar-feeding stations, including some that contained DEET, and found that DEET actively repelled Culex quinquefasciatus mosquitoes, also known as Southern house mosquitoes.
The mosquito transmits West Nile virus, St. Louis encephalitis, and lymphatic filariasis, a disease caused by threadlike parasitic worms.
"Despite the fact that DEET is the industry standard mosquito repellent, relatively little is known about how it actually works," said UC Davis research entomologist William Reisen.
"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 repellents are compared," Reisen
The study is published in the Proceedings of the National Academy of Sciences (PNAS).