Mosquitoes Become Less Repelled By DEET After First Exposure
February 21, 2013

Mosquitoes Become Less Repelled By DEET After First Exposure

Brett Smith for - Your Universe Online

The insect repellant DEET has been widely used to keep mosquitoes away and has been invaluable in countries affected by outbreaks of disease such as malaria and dengue fever, both transmitted by the annoying pest.

However, a new report in the open-access journal PLoS ONE finds that mosquitoes are less repelled by DEET after an initial exposure.

According to the team of U.K. researchers who conducted the study, mosquitoes will ignore the smell of DEET only a few hours after being exposed to it.

"Our study shows that the effects of this exposure last up to three hours,” study co-author James Logan, from the Department of Disease Control, London School of Hygiene and Tropical Medicine (LSHTM), said in a statement. "We think that the mosquitoes are habituating to the repellent, similar to a phenomenon seen with the human sense of smell also. 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.”

Developed by the U.S. military during World War II, DEET, more properly known as N,N-diethyl-meta-toluamide, is one of the most popular active ingredients in insect repellents. However, some recent studies have shown that it has no effect on some insects.

In the study, researchers introduced A. aegypti mosquitoes, known for transmitting dengue fever, to a human arm covered in DEET. Initially, the insects were repelled by the chemical. However, a few hours later they were reintroduced to the DEET-treated arm and this time the Deet was found to be less effective.

To try and uncover the mechanism behind the repellent´s reduced effectiveness, the researchers repeated the DEET exposure tests with electrodes attached to the insects' antenna.

"We were able to record the response of the receptors on the antenna to DEET, and what we found was the mosquitoes were no longer as sensitive to the chemical, so they weren't picking it up as well,” Logan told the BBC News. "There is something about being exposed to the chemical that first time that changes their olfactory system - changes their sense of smell - and their ability to smell Deet, which makes it less effective."

Previous studies by the same team found that certain genetic alterations to the same species of mosquito will make them ignore DEET altogether. They noted that it was unclear if any of these laboratory mutations had occurred in the wild.

Despite his study´s results, Logan stressed that people should continue to use DEET, especially if they are living in an area where the insects are spreading deadly illnesses.

In the meantime, Logan said his team plans on continuing to study how mosquitoes react to different repellents in the event that they eventually become resistant, or even immune, to DEET.

"The more we can understand about how repellents work and how mosquitoes detect them, the better we can work out ways to get around the problem when they do become resistant to repellents,” he said.

Logan added that his team will also begin looking at how DEET exposure affects other species of mosquito, such as the one know to transmit malaria.