Each summer, people in mosquito country slather themselves with DEET, or diethyltoluamide, the synthetic liquid widely seen as the most effective mosquito repellent around.
But in some situations, they might be turning themselves into mosquito magnets, according to new research published in the Journal of Experimental Biology.
The discovery makes for interesting insights into why DEET is usually so effective. It’s also a cautionary lesson about nature’s adaptability in the face of human ingenuity, and to not take for granted the promise of such seemingly bullet-proof inventions as DEET.
“We need to understand how mosquitoes keep outsmarting our control strategies,” said Clément Vinauger, a Virginia Tech researcher who took part in the research and has spent years plumbing the behavior of mosquitos.
The stakes are much more than a few scratchy bites. Mosquitoes can spread dangerous blood-borne illnesses including malaria, dengue and yellow fever, killing an estimated 1 million people every year.
The use of DEET has been a mainstay of dealing with these biting insects, usually by spreading it on people’s skin or clothes. But despite its widespread use since its invention in the 1940s, it’s not entirely clear why it works. Does it trigger some kind of irresistible physiological reaction in mosquitoes? Or can insects overcome that response and come to tolerate or even like the smell?
To figure that out, Vinauger and his collaborators took a page from the work of Russian physiologist Ivan Pavlov, who famously showed that he could train dogs to salivate at the sound of a bell, because they had learned to associate it with food.
In a sense, the new experiments took it even a step further. What of an animal could become so conditioned that it would seek out a disgusting physical sensation, such as a terrible smell?
To figure that out, the scientists took laboratory-raised Aedes aegypti mosquitoes, a species that spreads yellow fever and dengue. They enclosed individual insects in a plastic cylinder topped with wire mesh. They lowered a warm bag of sheep blood toward the mesh and watched to see how often a female mosquito tried to poke its proboscis into the bag. Some mosquitoes were tested in a DEET-free setting. Others were offered a blood bag while being perfumed with DEET. In a third version, mosquitoes were allowed to feed on the bag unmolested for 10 seconds, then had DEET wafted into the chamber while feeding for another 10 seconds.
For each version, individual mosquitoes went through their routine three times, to drive home the behavioral lesson.
Then the scientists exposed each trained mosquito to the smell of DEET minus the actual blood bag. Most of the ones that had never encountered DEET before or had a constant dose of the chemical while the blood was presented reacted as we might expect. They showed little interest in feeding.
But the ones that had started feeding and then encountered the DEET smell did the equivalent of Pavlov’s salivating dogs. They acted as if they were going to bite, even when there was no blood bag.
To see if this response could be replicated in a more realistic situation, mosquitoes were exposed to the two hands of scientist Ayelén Nally of the University of Buenos Aires in Argentina. Just one of her hands was doused in DEET. Mosquitoes without any special training all headed toward the DEET-free hand. But more than half the trained mosquitoes showed a preference for the hand covered in the insect repellent. (Nally didn’t shed blood for the experiment – there was a mesh barrier blocking the mosquitos.)
The startling results suggest that rather than a hardwired physical response, the repellent might work because it evokes the smell of natural occurring repellents such as chemicals from a plant, the scientists suggested. “What we are showing is that the mosquito’s brain can rewrite that response based on experience. What the insect has learned matters just as much as what the chemical does,” said Vinauger. “That, I think, is a paradigm shift.”
That doesn’t mean people should toss away their DEET. It’s still highly effective in many cases. “If you’re in tropical regions where disease risk is real, you should use it,” he said.
But people might need to use it more thoughtfully. “Instead of applying a lot at once, you may want to reapply regularly so it’s always active and providing continuous protection,” Vinauger said.
That way, mosquitoes won’t get close enough to take a bite and begin associating the smell with a snack. Because if they do, then you might just be putting on mosquito perfume.
Lazzari, et. al. “Associative learning switches DEET valence from aversive to appetitive in Aedes aegypti.” Journal of Experimental Biology. May 28, 2026.
Image: ©Anthropocene Magazine
