Researchers found that yellow fever mosquitoes can learn to associate the smell of DEET with food – potentially weakening the world’s most widely used insect repellent.
Every summer, millions of people spray themselves with DEET to keep mosquitoes away. But new research suggests mosquitoes may be able to learn to associate the repellent with food – and even become attracted to it.
published in the Journal of Experimental Biology, was a collaboration between , associate professor at Virginia Tech, and Claudio Lazzari at the University of Tours in France.
“If someone applies DEET and the concentration fades over time, but a mosquito still manages to feed, the insect may begin associating that smell with a reward,” said Vinauger, part of the in the . “That’s a possibility we should take seriously when we think about how repellents are used in the real world.”
Yes, mosquitoes can learn
The study focused on the yellow fever mosquito, Aedes aegypti, a species that spreads dengue fever, Zika, yellow fever, and chikungunya, which infect tens of millions of people each year.
Researchers trained the mosquitoes using a form of Pavlovian conditioning — the same learning principle behind Ivan Pavlov’s famous experiments in which dogs learned to associate the sound of a bell with food.
Mosquitoes were restrained behind fabric mesh with a bag of warm blood positioned just out of reach. After the mosquitoes began to feed on the blood, researchers introduced the smell of DEET. After repeating the experiment four times, more than 60 percent of the insects tried to feed when presented with only the smell of DEET.
Next, mosquitoes were given a choice between two human hands — one untreated and one coated with DEET at normal concentrations. Untrained mosquitoes avoided the DEET-treated hand. Trained mosquitoes were drawn to it.
The researchers also found mosquitoes could form the same association when sugar, instead of blood, was used as the reward.
“The common assumption has always been that repellents work because of their chemistry — that DEET simply smells bad to mosquitoes and they flee or that its chemistry prevents mosquitoes from smelling us,” said Vinauger, who is also an affiliate of ’s . “But 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. That, I think, is a paradigm shift.”
The findings do not mean people should stop using DEET, Vinauger said. It’s still one of the most effective repellents available, particularly in regions where mosquito-borne disease is common.
“If you’re in tropical regions where disease risk is real, you should use it,” he said.
But the study suggests timing and concentration may matter more than previously understood.
“Instead of applying a lot at once, you may want to reapply regularly so it’s always active and providing continuous protection,” Vinauger said.
He added that treated clothing may also present challenges because DEET concentrations in fabric decline over time.
The study builds on years of mosquito learning and behavior research connected to Vinauger’s work. While pursuing his Ph.D. in Lazzari’s lab in France, and later as a postdoctoral researcher at the University of Washington, Vinauger helped pioneer experiments showing mosquitoes can learn and remember odors associated with blood meals and defensive hosts.
At Virginia Tech, Vinauger’s lab studies how mosquitoes use sensory information to find hosts and adapt to changing environments. His team has shown that mosquitoes remember and , to track people with surprising precision, and gravitate toward and away from the smell of .
“Mosquitoes are remarkable at processing information about their environment,” Vinauger said. “What we are trying to understand is not only how they detect us, but how their brains interpret those cues and turn them into behavior.”
As Aedes aegypti expands into new regions and insecticide resistance grows worldwide, Vinauger said understanding mosquito behavior is becoming increasingly important for public health.
“We need to understand how mosquitoes keep outsmarting our control strategies,” Vinauger said. “And that takes understanding how they work — at the molecular level, the neural level, the behavioral level.”
Marya Barlow for Virginia Tech
