Smell of dinner: Can mosquitoes learn to love DEET?

DEET has long been considered the gold standard when it comes to preventing mosquito bites. Sprayed before a hike or picnic, or during a trip to mosquito-dense areas around the world, the world’s most widely used insect repellent is expected to drive mosquitoes away in the opposite direction with its scent.

but the study yesterday, Journal of Experimental Biology This suggests that mosquitoes may learn to associate the smell of DEET with dinner and become attracted to DEET rather than away from it. The findings challenge long-held assumptions about how DEET works and what mosquitoes can learn.

Training changed mosquito response to DEET

For the study, researchers from France’s University of Tours and Virginia Tech looked at whether women were female. Aedes aegypti Mosquitoes that transmit dengue, Zika, yellow fever, and chikungunya may learn to associate DEET with food rewards.

The researchers used a type of Pavlovian conditioning in which the mosquito sucks warm blood through an artificial membrane. Twenty seconds after the meal, the researchers released DEET into the feeder. After repeating this process three more times, the mosquitoes were exposed to DEET but not fed.

When the trained mosquitoes sniffed out the DEET odor alone, more than 60% tried to suck the bait again, exhibiting what researchers called the “bite attempt response” (BAR). This compares to approximately 20% of untrained mosquitoes that performed BAR when exposed to DEET alone.

Mosquitoes may learn that DEET isn’t so bad after all.

In another experiment, mosquitoes were given a choice between two human hands. One hand was treated with DEET and the other hand was untreated. All untrained mosquitoes avoided the DEET-treated hands. However, trained mosquitoes were significantly more likely to turn to the treated hand.

Reward learning not limited to blood

In the final experiment, the researchers exposed mosquitoes to DEET and the insects fed on sugar. When trained mosquitoes were then exposed to DEET alone (no sugar), most mosquitoes developed BAR. The results of this study suggest that the mosquito’s learned behavior is not limited to blood.

“These results suggest that the neural circuits that process the detection and interpretation of ‘reward’, or positive reinforcement, are not specific to any particular type of reward,” lead author Dr. Clement Vinauger, associate professor of biochemistry at Virginia Tech, told CIDRAP News.

“In practice, this means that mosquitoes can learn the association between odors and sweet rewards beyond the window of human blood feeding,” he says. “In other words, the mosquito’s ability to learn could impact many more aspects of daily life than we currently think.”

DEET is dose dependent

Commercial concentrations of DEET are highly effective at repelling mosquitoes. “For this reason, in our experiments, we needed to give the mosquitoes a chance to start feeding before they smelled the DEET,” Vinaugar says. “Conversely, the mosquitoes will simply refuse the food.”

“However, its repellent effect is dose-dependent,” he added. “And there is a risk that even lower doses could be detected by the mosquito’s olfactory system without being effectively repelled. In this case, you can imagine that mosquitoes that experience this might learn that DEET is not so bad after all.”

What the insects learn is just as important as what the chemicals do. I think that’s a paradigm shift.

A common assumption about DEET is that it works because of its chemical properties. DEET either blocks mosquitoes from smelling us, or it simply smells terrible to mosquitoes and drives them away. “But what we show is that the mosquito’s brain can rewrite its responses based on experience,” Binaugar said in the Virginia Tech paper. news release. “What the insects learn is just as important as what the chemicals do. I think that’s a paradigm shift.”

Proper use is the key to maintaining effectiveness

DEET was first developed by the U.S. Department of Agriculture in the 1940s and was originally used by the U.S. military during jungle combat. Currently, it is the cheapest and most effective insect repellent in the world.

Vinaugar says DEET should not be abandoned, especially in areas where mosquito-borne diseases are endemic. But the findings suggest that the repellent may become less effective as DEET concentrations on skin and clothing decline over time, suggesting that concentration may be more important than expected.

“If a mosquito bites someone who has applied DEET to their skin several hours before, and the concentration of the repellent is too low to repel the mosquito, but strong enough for the insect to smell, the mosquito may be more likely to bite the person who smells the DEET,” lead author Dr. Claudio Lazzari, a professor at the University of Tours, wrote in the Company of Biologists. News release.

Our study highlights the importance of following the manufacturer’s recommendations for dosage and reapplication frequency.

The authors say further research is needed to determine whether mosquitoes can learn that DEET means dinner in the real world. “The laboratory experiments we conducted represent a very specific scenario,” Vinaugar says. “They don’t directly reflect the real-world conditions we would expect.”

At the same time, this data provides guidance on how to most effectively use insect repellents. “Our study highlights the importance of following manufacturers’ recommendations for the dosage and reapplication frequency of the particular repellent used to reduce the risk of mosquitoes learning to outwit our control tools,” he added.