Deet works the first time, but loses its power after only a few hours The popular insect repellent Deet appears to be losing its effectiveness against mosquitoes, according to research published today.Scientists from the London School of Hygiene and Tropical Medicine say mosquitoes are first deterred by the substance, but then later ignore it. The research, published in the journal Plos One was carried out on Aedes aegypti, a species of mosquito that spreads dengue and yellow fever.
Dr James Logan from the London School of Hygiene and Tropical Medicine, 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."
Deet - or N,N-diethyl-meta-toluamide - is one of the most widely used active ingredients in insect repellents. It was developed by the US military, following its experience of jungle warfare during World War II.
However, there are concerns that some mosquitoes are growing resistant to it.The findings come from an experiment conducted by scientist in which they took some mosquitoes in the laboratory, and tempted them with a human arm covered in Deet. Initially the repellent deterred the insects off their potential meal.
However, a few hours later researchers found that the Deet was less effective.
Dr Logan explained: "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.  We concluded that mosquitoes are very good at evolving very, very quickly."