May 28th 2026|2 min read
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DEET is the gold standard for preventing mosquito bites. Although there is some debate over precisely how it works to ward off the insects, most researchers agree that a key part of its effectiveness is unpleasantness—the bugs just hate to be around it. Effective as it is, however, some mosquitoes seem capable of getting over their revulsion. This has led to the speculation that the insects might be getting used to the chemical.
Claudio Lazzari at the University of Tours set out to investigate, by running a version of the experiments made famous by Ivan Pavlov, a Russian physiologist, in the late 1800s in which dogs were conditioned to associate sounds with food. In those experiments, Pavlov routinely made a sound with a metronome or buzzer and then promptly fed the dogs. Eventually, when he made the sounds but did not provide food, he found that the animals still salivated, as they would before a meal.
In his experiments, Dr Lazzari presented hungry mosquitoes with warm sheep’s blood while they were being exposed to a gentle breeze of ordinary air or of air blown past a piece of paper soaked in DEET. With colleagues, he then repeated the experiments with sugar water, instead of blood, on a different set of mosquitoes.
The researchers then bravely presented their own hands to the captive insects. One hand was sprayed with DEET while the other was left untreated. To their dismay they watched as 60% of the mosquitoes that had been presented with blood or sugar water while being exposed to DEET flew (and bit) the hand coated in the repellent. In contrast, all mosquitoes trained with ordinary air avoided the repellent entirely.
Dr Lazzari’s conditioning in the laboratory, reported this week in Journal of Experimental Biology, was important for the mosquitoes in developing their fondness for DEET. But the chances are good that a similar scenario is taking place in the real world, where the effects of DEET typically weaken over time, especially when people are sweating or not applying enough of the stuff in the first place.
In such circumstances, hungry mosquitoes are most likely to be able to put up with the faint odour of DEET, bite a lightly protected person, begin associating the repellent with food and become all the more dangerous to those whom they encounter later in their lives. ■
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This article appeared in the Science & technology section of the print edition under the headline “When DEET means dinner”
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