Mosquitoes can outsmart popular insect repellent, study finds

The world's deadliest animal could be outsmarting us.

In a new study, researchers discovered that mosquitoes can learn to associate the smell of DEET with food — potentially weakening the world’s most widely used insect repellent.

"If mosquitoes are repeatedly exposed to DEET, it becomes less effective as a repellent," said study co-author Claudio Lazzari of the University of Tours, France, in a statement. This raises concerns that in certain situations, the repellent may even begin to attract some biting insects.

If mosquitoes can learn to associate the scent of DEET with the prospect of dinner, this could make people wearing DEET more attractive to biting insects under the right conditions, according to the study.

DEET as a reward?

"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 study co-author Clément Vinauger of Virginia Tech University, in a news release. "That's a possibility we should take seriously when we think about how repellents are used in the real world."

Lazzari said, "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."

The study, published in the Journal of Experimental Biology, was a collaboration between Vinauger, associate professor at Virginia Tech, and Lazzari of the University of Tours in France.

Mosquitoes can learn

The mosquito species studied was the yellow fever mosquito, a species that spreads dengue fever, Zika, yellow fever, and chikungunya, which infect tens of millions of people each year. Overall, estimates are that mosquitoes kill as many as one million people each year, far more than any other animal.

In the study, researchers trained the mosquitoes using a type of Pavlovian conditioning — the same idea behind Ivan Pavlov’s well-known experiments where dogs learned to link the sound of a bell with food.

According to Virginia Tech University, 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% 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.

Sugar was also studied

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.

“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.”

Don't give up the DEET

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 in a statement.

But this new research says that timing and concentration could matter more than had been thought.

"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.

Doyle Rice is a national correspondent for USA TODAY, focusing on weather and climate.