MONDAY, May 3, 2010 (HealthDay News) -- Certain mosquitoes are not turned off by the widely used insect repellent DEET, and now scientists say they know why.
These "insensitive" mosquitoes have defects in their ability to smell the compound, and this genetically determined trait is easily passed from generation to generation, the researchers reported in the May 3-7 online edition of the Proceedings of the National Academy of Sciences.
The findings, from researchers in the United Kingdom and Sweden, could present new molecular targets, paving the way for better, safer repellents to stave off the flying vermin of summer.
"DEET is one of the most effective insect repellents of four that are approved," said Michel Slotman, assistant professor in entomology at Texas A&M University in College Station, who was not involved in the study. "But we haven't understood how it works."
"These [newly discovered] molecular targets will pave the way for the discovery of novel insect repellents if resistance to DEET becomes a problem a few decades down the road," added Walter S. Leal, professor of entomology at the University of California-Davis, and the scientist who discovered that mosquitoes are repelled by the smell of DEET. "Now that it has been corroborated that mosquitoes smell and avoid insect repellents, let the race for identification of DEET-sensitive odorant receptors begin," he said.
Before Leal determined that mosquitoes dislike DEET's smell, researchers had thought the compound interfered with the insects' ability to smell.
N,N-Diethyl-m-toluamide, or DEET -- the most common active ingredient in bug repellents -- is used by about 200 million people worldwide to protect against disease-carrying insects, according to the U.S. Department of Agriculture. Mosquitoes can cause malaria, encephalitis, West Nile and yellow fever.
Although DEET is deemed safe and effective, many people don't like to use it because "it smells and leaves an oily residue," Slotman said. "It's not the most pleasant thing to apply."
And even it's highly effective, some mosquitoes are not deterred by the smell, and scientists have wanted to know why.
The authors of the new study identified female yellow fever mosquitoes (Aedes aegypti) that were not repelled by DEET.
They then bred these females with a random group of male mosquitoes and found that the "insensitivity" gene was dominant and quickly passed on.
In just one generation, the proportion of DEET-resistant mosquitoes jumped from 13 percent to higher than 50 percent.
As it turned out, "smell" receptors on the antenna predicted how a mosquito responded to DEET. Differences in these neurons may explain why some mosquitoes stay away from DEET-lathered humans and some do not.
"[The researchers] found olfactory receptor neurons on the antennae of female mosquitoes that respond to DEET in a dose-dependent manner," Leal explained. "These authors showed that repellency is directly correlated with the sensitivity of these DEET-detecting olfactory receptor neurons."
In other words, the sensitivity of the neurons on the antennae determine the mosquito's ability to detect DEET. Some mosquitoes have a "defect in DEET detection," Leal said.
But DEET is still likely to be the mainstay of summer defenses against mosquitoes and other bothersome insects.
"It is not entirely surprising that mosquitoes develop resistance to DEET as all insects develop resistance to human-made compounds," Leal said. "The good news is that DEET resistance in wild populations develops so slowly that it has been used for almost five decades and it is still a 'gold standard' insect repellent."
The U.S. Environmental Protection Agency has more on DEET.