Mosquitos can learn to be attracted to a person wearing the repellent DEET, that is if the DEET is lightly applied or fades over time, according to a study published today in the Journal of Experimental Biology. 

The study emphasized the findings do not mean people should stop using DEET, it’s still one of the most effective repellents available, particularly in regions where mosquito-borne disease is common. A user should use enough and make sure that the repellent doesn’t fade as the day or night goes on.

The study was conducted in a laboratory at Virginia Tech University, not in the wild. However, it involved one of the most dangerous mosquitoes, Aedes aegypti, a species that spreads dengue fever, Zika, yellow fever, and chikungunya, which infect tens of millions of people each year.

The was a collaboration between Clément Vinauger, 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 Department of Biochemistry in the College of Agriculture and Life Sciences.

"That's a possibility we should take seriously when we think about how repellents are used in the real world."

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.

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 says mosquitos shift from innate avoidance to the smell of DEET to a learned appetitive response that makes DEET attractive by establishing associations with two rewarding contexts: vertebrate blood feeding and plant sugar feeding.

Researchers in thelab 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.

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 avoid hosts who swat at them, combine smell and vision to track people with surprising precision, and gravitate toward and away from the smell of certain body soaps.

"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."