Mosquitoes Can Learn to Prefer Insect Repellent, Scientists Reveal

(SeaPRwire) –   No one wants to do mosquitoes any favors. There are an estimated 110 trillion of these often deadly pests worldwide, many of which spread diseases like malaria, dengue, West Nile virus, chikungunya, yellow fever, and Zika. But a new study in the Journal of Experimental Biology suggests we might actually be training mosquitoes to bite us—thanks to the alluring scent we emit when we spray on DEET, a supposed mosquito repellent that can sometimes have the opposite effect.

DEET (short for N,N-diethyl-meta-toluamide) was invented in 1946 by a U.S. Department of Agriculture chemist. This anti-mosquito formula was quickly adopted by the military for use in jungles and other tropical environments. The chemical is thought to work in multiple ways: blocking scent receptors so mosquitoes can’t recognize human or animal targets, leaving a bitter taste detected by their feet, or mimicking the smell of natural mosquito-repelling plants. Whatever its mechanism, it keeps bugs away—until it doesn’t.

The problem, identified by a research team including Clément Vinauger (associate professor in Virginia Tech’s biochemistry department), is that while mosquitoes are simple creatures, they’re smart enough to learn. When DEET is freshly applied, it repels mosquitoes for a while. But as the scent fades without disappearing entirely, mosquitoes resume biting and start associating the chemical with a tasty blood meal. This actually draws them to people wearing DEET.

“Mosquitoes are very smart and can outsmart our control tools,” says Vinauger. “DEET is a molecule that can be interpreted by the mosquito brain, and mosquitoes can learn to like it.”

During their studies, Vinauger and colleagues exposed small mosquito groups to feeding trials scheduled for early morning and evening—peak times for the insects to seek blood meals. In the first trial, mosquitoes were in a glass tube; on the other side of a fine mesh was a blood vessel warmed to 98.6°F. After letting them feed through the mesh, researchers exposed them to DEET’s scent, which didn’t stop their feeding. The blood and DEET were then removed, and mosquitoes eventually left the mesh. Next, DEET’s scent was reintroduced without blood—but over 60% of mosquitoes returned to the mesh, expecting a meal.

“By training them, we were able to demonstrate that they can assign value,” says Vinauger. “They can actually turn that molecule that innately is an aversive weapon into something that is attractive to them.”

In the second phase, researchers presented two mosquito groups with two human hands—one sprayed with DEET, one not. Trained mosquitoes (that saw DEET as attractive) attacked both hands; untrained ones only went for the non-DEET hand.

To confirm findings, researchers repeated the experiment with sugar (another mosquito food) instead of blood. Again, insects avoided DEET-treated sugar unless trained to link the chemical to the meal—then DEET was like a dinner bell.

This doesn’t mean DEET isn’t useful—it can still be highly effective. But it suggests better usage: instead of a heavy one-time dose, Vinauger recommends applying the bottle’s recommended amount and reapplying throughout the day to keep skin concentration above the aversive level.

“It’s the frequency of application, how often you need to put it back on the skin” he says.

Repelling mosquitoes isn’t just about DEET. Vinauger suggests well-maintained window screens, bed nets in malaria-prone areas, eliminating standing water (flower pots, wading pools) to reduce egg-laying, octenol (mimics breath/sweat) to lure mosquitoes to zappers, and long sleeves/pants to limit exposed skin. Still, the study shows we should fear mosquito intelligence almost as much as their bites.

“Mosquitoes are very plastic,” says Vinauger, “and the more we look at them, the smarter they appear.”