The insect repellent DEET is nowhere near as good at repelling mosquitoes as believed, says researchers, with mosquitoes learning to ignore it after just a few hours of exposure.
The London School of Hygiene & Tropical Medicine team tested changes in responses to DEET in Aedes aegypti mosquitoes, which are notorious for biting during the day and can transmit dengue fever.
DEET was originally developed by the US army, and was used in the Vietnam war. It's generally regarded as the best mosquito repellent available. But, the team found, just a brief exposure to DEET is enough to make some mosquitoes less sensitive to the repellent, so that they once again head for human skin.
"Our study shows that the effects of this exposure last up to three hours. We will be doing further research to determine how long the effect lasts," says researcher James Logan.
The team found that this insensitivity to the smell could be correlated with a decrease in the sensitivity of odor receptors on the mosquito's antennae following a previous exposure.
"We think that the mosquitoes are habituating to the repellent, similar to a phenomenon seen with the human sense of smell also," says Logan. "However, the human olfactory system is very different from a mosquito's, so the mechanism involved in this case is likely to be very different."
Logan stresses that we shouldn't be throwing our DEET away."On the contrary, DEET is a very good repellent, and is still recommended for use in high risk areas," he says. "However, we are keeping a close eye on how mosquitoes can overcome the repellent and ways in which we can combat this."