Mosquitoes can learn to associate the smell of the world’s most widely used repellent with food, and after “training” they can even prefer to bite people who have been sprayed with it. This follows from an experimental study published on Thursday and cited by AFP.
Surprising results obtained by scientists “under very specific laboratory conditions”, according to the study’s lead author, Claudio Lazzari, they “do not question the efficacy” of DEET repellent. The chemical compound DEET, developed in the United States in the 1940s, has helped save many lives from diseases transmitted by stinging insects.
“It is the absolute gold standard among repellents used by the World Health Organization (WHO) to combat the transmission of mosquito-borne diseases,” emphasized Lazzari, acting as professor emeritus of the Research Institute of Insect Biology at the University of Tours in France.
The author of the study claims that it is necessary to develop new, more effective and ecological repellents that will cause fewer allergic reactions. That, he says, requires scientists to better understand why repellents repel mosquitoes. “We don’t know exactly why,” Lazzari admitted. Scientists are trying to find out if substances like DEET are toxic to mosquitoes, if they prevent them from recognizing the smell of humans, or if they simply repel them with an unpleasant smell.
Therefore, an international team of scientists used the principle of conditioning known from the experiment with Pavlov’s dog, which associated the arrival of food with a sound signal. Egyptian mosquitoes (Aedes aegypti), which transmit diseases such as dengue fever, Zika virus, yellow fever or chikungunya virus, were placed by scientists in a net box and offered a bag with warm sheep’s blood.
When the researchers added the smell of DEET repellent, the mosquitoes moved away from the source. Subsequently, however, the scientists fed the insects with warm blood for 20 seconds, while during the last ten seconds they also released the smell of the repellent. They repeated this procedure three times.
When the mosquitoes were then exposed to just the smell of the repellent, more than 60 percent of them tried to bite the cloth, even though there was no blood in it. In the next part of the experiment, one of the scientists set both hands on the mosquitoes – one “clean” and the other sprayed with DEET repellent. According to the results, mosquitoes clearly preferred the hand covered with repellent.
The scientists reached similar results in experiments with sugar instead of blood. In the wild, mosquitoes mainly feed on plant nectar. The co-author of the study, Clément Vinauger from the University of the American state of Virginia, stated, that until now it was generally assumed that repellents work solely on the basis of their chemical composition.
But a new study suggests that mosquitoes don’t just react to the chemical itself, but also to how their brain evaluates it based on previous experience, according to scientists. “We show that the mosquito’s brain can override this response based on experience,” Vinauger explained. “What the insects have learned is as important as the action of the chemical itself. I think this is a fundamental change in perspective.” he added.
However, Lazzari also pointed out that they would be needed in nature “very specific conditions”, to make the mosquitoes behave the same as in the laboratory. The study was published in the Journal of Experimental Biology.
