DEET is one of the most effective repellents available against blood-sucking insects and is designed to keep mosquitoes, ticks and flies away. But under certain circumstances, its effect could be reversed: In an experiment, mosquitoes learned within a short time to associate the actually off-putting smell with food. As a result, they now preferred to bite people who had “protected” themselves with DEET – the repellent had become an attractant. From the researchers’ point of view, this effect does not speak against the use of DEET, but rather in favor of more careful use that minimizes the risk of conditioning.
When we spend time outdoors on a warm summer evening, they soon start buzzing around us: annoying mosquitoes that are hoping for a tasty blood meal from us. Insect sprays are often used to protect against pests. The most effective of them contain the chemical insect repellent diethyltoluamide, or DEET for short. This repellent is particularly important in regions where mosquitoes transmit dangerous diseases such as malaria, yellow fever or Zika fever. An effective deterrent to the buzzing vectors saves many lives every year.
Adaptable mosquitoes
But now a study suggests that the protective effect of the repellent can be reversed under certain circumstances: As a team led by Claudio Lazzari from the University of Tours in France has shown, the mosquito species Aedes aegypti can learn to associate the actually frightening smell of DEET with food. These mosquitoes are mainly found in the tropics and subtropics and can transmit diseases such as yellow fever, dengue fever, Zika fever and other viral diseases that affect millions of people every year.
For their experiment, the researchers trained some mosquitoes using Pavlovian conditioning, the same principle by which dogs learn to associate the ringing of a bell with food – except in this case the bell was the smell of DEET and the food was a bag of warm blood hidden behind a piece of cloth. Before training began, the mosquitoes liked to bite through the fabric into the blood bag, but stayed away when they smelled DEET. For conditioning, the team first had the mosquitoes begin their blood meal and then sprayed DEET as they fed. And sure enough, after just four repetitions of this training, the smell of DEET alone caused 60 percent of mosquitoes to try to bite through the fabric – even if there wasn’t a bag of blood behind it.
Experiences shape stinging behavior
To find out whether mosquitoes can adapt to DEET in humans, Lazzari’s colleague Ayelén Nally used her own hands and sprayed one of them with DEET. The result was clear: While untrained mosquitoes avoided the hand protected with the repellent, the conditioned animals were particularly attracted to it. They then even preferred to stab the researcher in the hand that had been treated with the repellent. In further experiments, the team also demonstrated that prior conditioning with sugar water instead of blood produces the same effect.
From the researchers’ perspective, the results open up a new perspective on how DEET works. “Until now, the general assumption was that repellents work because of their chemical composition – that DEET simply smells bad to mosquitoes and so they flee, or that its chemical composition prevents mosquitoes from smelling us,” explains co-author Clément Vinauger of Virginia Polytechnic Institute and State University. “However, we show that the mosquito’s brain can reprogram this response based on experience. What the insect has learned is just as important as the effect of the chemical. This is, in my opinion, a paradigm shift.”
What does this mean in practice?
The researchers emphasize that the new findings do not fundamentally question the use of DEET. “If you are in tropical regions where there is a real risk of disease, you should use it,” says Vinauger. But to prevent mosquitoes from getting used to it, there are a few rules to follow: “If someone applies DEET and the concentration wears off over time so that the mosquito still manages to suck blood, the insect might start to associate that smell with a reward,” he explains. “Rather than applying a large amount at once, you may want to reapply regularly to ensure it is always effective and provides continued protection.”
Source: Claudio Lazzari (University of Tours, France) et al., Journal of Experimental Biology, doi: 10.1242/jeb.251935