Scientists report a worrying trend in which mosquitoes are becoming resistant to the insect repellent Deet. Deet is an abbreviated form of the chemical N,N-diethyl-meta-toluamide and is the most common active ingredient used in insect repellents. You can apply Deet to the skin or to clothing, and as well as deterring mosquitoes, it helps to provide protection against tick bites, chiggers, and other insects that can transmit diseases.
In recent tests however, scientists are finding that although Deet works in the first instance, after a few hours have passed it seems to lose its effectiveness. Research carried out at the London School of Hygiene and Tropical Medicine say mosquitoes are first deterred by the substance, but then later ignore it. And they warn that although more research is needed, tests should be carried out to find alternatives to Deet.
Deet was first developed by the US military following experiences of soldiers carrying out jungle warfare during World War II. Although Deet has always been effective against mosquitoes and other insects, why they are repelled by the chemical has never been properly established. Some scientists think that it is the smell that mosquitoes do not like, other studies suggest that the yellow oil actually blocks the insects olfactory receptors and disables the insects ability to detect carbon dioxide exhaled by humans.
The research was carried out on a specific mosquito – Aedes aegypti, a particularly deadly species that spreads dengue and yellow fever. Researchers from the London School of Hygiene and Tropical Medicine took these mosquitoes and in laboratory condition, presented them with a human arm covered in Deet. The mosquitoes were initially repelled by the chemical but after a few hours, when the same mosquitoes were offered the human arm again, the researchers found that the Deet was less effective.
To investigate why this might be happening, the researchers attached electrodes to the insects’ antenna.
Dr Logan explained: “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. 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.”
The team discovered in earlier research that genetic changes to the same species of mosquito could make them immune to Deet, although this was only applicable on the laboratory mosquitoes.
Dr Logan said: “Mosquitoes are very good at evolving very quickly,” and added that it was vital to understand both these permanent genetic and temporary olfactory changes that were taking place.
The findings of the gradual build-up of resistance to Deet should not put people off using the chemical in high risks areas where mosquitoes are very common. The scientists are now trying to establish exactly how long the initial effects lasts for before it begins to lose potency. They also want to find out how native species are effected by the time-lapse.
Dr James Logan said: “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.”
Source: BBC News/Science