Mosquitoes may learn to like DEET, a common insect repellent

Pesky mosquitoes on the hunt for blood may find the smell of a common repellent attractive, not repulsive. That’s the finding of new lab tests.

The chemical is known as DEET. It’s been one of the most effective insect repellents for decades. But in new lab tests, yellow-fever mosquitoes (Aedes aegypti) exposed to DEET learned to associate the chemical with food. This suggests these insects can link unpleasant odors with rewards. That turns a bad experience into an inviting one.

For now, it’s not clear if this might happen outside the lab, too.

How DEET works, exactly, is a bit unclear, says Clément Vinauger at Virginia Tech in Blacksburg. He studies animals’ brains and behaviors. Some studies have suggested mosquitoes don’t like DEET’s smell or taste. Others hinted that the repellent scrambles mosquito senses. That would keep the insects from detecting the otherwise enticing body odors that normally lure them in for a blood meal.

The new results suggest that mosquitoes do detect DEET, Vinauger says. They also can learn, changing their behaviors based on prior experiences.

DEET-licious

Vinauger’s team ran tests to see how mosquitoes acted when they encountered DEET several times.

They housed mosquitoes in a central container connected to two flasks. One flask held clean air. The other contained DEET. At first, the mosquitoes fed on blood from an artificial feeder while exposed to only clean air. Then, the researchers cranked up the DEET dial.

They aimed to train the mosquitoes to associate the repellent with blood. 

To test how well the training worked, the researchers put trained and untrained mosquitoes in narrow tubes. A team member sprayed DEET on one hand but not the other. They placed a hand near each end of a tube with a mosquito inside. Then the team watched what happened.

Mosquitoes that earlier had experienced DEET while allowed to feed on blood attempted to bite the repellent-treated hand. But untrained mosquitoes steered clear. The researchers shared their findings May 28 in the Journal of Experimental Biology.

Punishment or reward?

The findings suggest mosquitoes are smelling DEET, concludes Anandasankar Ray. He’s a neuroscientist at the University of California, Riverside. Ray did not take part in the work. “And [mosquitoes] can be trained to be attracted to it by offering a reward with it.”

The chemical is not masking our scent, Ray says. But he notes that mosquitoes pick up odors with their legs as well as using their antennae. In the new tests, they couldn’t land on the repellent-treated hand. Because the insects must land to slurp blood, DEET should rebuff the mosquitoes before they can start to feed. DEET’s scent would be “paired with a bitter-touch contact,” Ray says. “It would be a punishment for them rather than a reward.”

Vinauger suspects mosquitoes could learn to link DEET with a meal when its repellent effects have largely worn off. That could happen several hours after people first put it on their skin. There might be traces of DEET left, but not enough to deter the insects, he says. “Mosquitoes might still land, drink some blood and not be repelled.”

If they manage to snag blood, the insects might learn to link the smell with the meal. Afterward, they might begin hunting for a combination of human odor and DEET. 

The results don’t suggest that people should stop using DEET, Vinauger says. “It’s still the gold standard in terms of protection.” It’s an important option for deterring mosquitoes where they commonly spread disease. But the chemical is sold by different manufacturers at varying concentrations. And each product may come with unique instructions. Following those instructions is key to using enough DEET to keep the irksome insects from landing.