DEET is the gold standard for insect repellents, deterring ticks, flies and mosquitoes. Yet there may be a chink in the quintessential repellent's armour. 'If mosquitoes are repeatedly exposed to DEET, it becomes less effective as a repellent', says Claudio Lazzari, from University of Tours, France, raising concerns that in certain situations the repellent may even begin to attract some biting insects. In research published in Journal of Experimental Biology , Lazzari, Clément Vinauger from Virginia Tech, USA, and colleagues have discovered that mosquitoes can learn to associate the smell of DEET with food, meaning that they could be more likely to bite people that smell of DEET, placing the indispensable repellent at risk.
Lazzari and colleagues made their discovery using a bizarre twist of Pavlov's famous experiment, where dogs learned to associate the arrival of food with a ringing bell, but first the team had to find a way of confirming when the voracious insects were attracted to something tasty.
Knowing that yellow fever mosquitoes (Aedes aegypti) like nothing more than a meal of warm blood, David De Luca (University of Tours) restrained the insects behind a fabric mesh before offering them a bag of warm blood placed just out of reach, to see how enthusiastically the insects stabbed at it with their proboscises in the hope of grabbing a bite.
Sure enough, the insects were keen, especially when De Luca rewarded them by lowering the bag within reach, so they could feast. However, when the team offered the insects a blood meal when surrounded by the scent of DEET, the mosquitoes steered clear.
But could the insects learn to associate the scent of DEET with the offer of dinner? The team fed the insects with warm blood for 20 s, squirting the scent of DEET into the enclosure during the final 10 s of dining, repeating the procedure three more times before checking how the insect responded to the scent of DEET alone.
This time the mosquitoes went almost wild, with more than 60% trying to take a bite when they smelled a whiff of DEET.
And when the team offered the trained insects the choice between Ayelén Nally's two hands (University of Buenos Aires, Argentina), one of which was bathed in DEET while the other was clean, the insects tried to bite Nally's DEET-scented hand.
Finally, Charly Dufour (University of Tours) tried an alternative strategy, attempting to train the mosquitoes to associate a sugary treat with the scent of the repellent, and the mosquitoes got the idea, biting enthusiastically whenever they picked up the scent of DEET.
Mosquitoes can learn to associate the scent of DEET with the prospect of dinner, which could make people wearing DEET more attractive to biting insects under the right conditions.
'If a mosquito bites someone who applied DEET to their skin several hours earlier and the concentration of the repellent is too low to repel the mosquito, but still strong enough for the insect to smell it, the mosquito may be more likely to bite people who smell of DEET', says Lazzari.
The mosquitoes' ability to learn to associate the repellent with food could also tell the researchers something about how the repellent works. 'It is the information that DEET conveys to insects that may cause them to decide not to bite', says Lazzari, explaining that DEET probably mimics some natural plant insect repellents, which keep insects at bay.
But, he adds that DEET is still the most effective insect repellent available, protecting people from mosquito-borne diseases. 'It saves lives!', he exclaims.
