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4 hours ago
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It is a spray used worldwide to protect humans from mosquito bites, but now research suggests Deet can become attractive to the insects if they associate it with feeding.
Deet – which has the chemical name N,N-diethyl-meta-toluamide – is widely used in insect repellants, with the UK Health Security Agency recommending products with 50% Deet as the first choice to protect against mosquito bites.
Such protection is important given that in some countries mosquito bites can spread fatal diseases such as dengue, Japanese encephalitis, malaria and Zika.
However, a new study suggests that, as Pavlov’s dogs learned to associate the ringing of a bell with food, so mosquitoes can learn that the presence of Deet indicates the possibility of a blood meal.
“For a long time, it was believed that repellants worked solely because of their chemical properties, either by being toxic or unpleasant to mosquitoes and driving them away, or by blocking their ability to detect humans. However, our findings suggest that the reaction can be modified by experience,” said Prof Claudio Lazzari, from the University of Tours, France. “We believe this represents a significant change in our understanding of repellants.”
While studies have previously suggested mosquitoes were less bothered by Deet after first exposure, the team behind the new research said it had been unclear why this was.
Writing in the Journal of Experimental Biology, the researchers said they initially observed how trapped mosquitoes attempted to bite a bag of warm blood that they could not quite reach.
Further investigation revealed that 60% of mosquitoes that fed when presented with warm blood alongside exposure to Deet subsequently showed biting attempts when exposed to Deet alone.
This compared with 17% of insects given no prior training, 13% of mosquitoes previously presented with Deet alone, 17% that previously fed on warm blood with no exposure to Deet, and 23% that previously fed on warm blood and were exposed to Deet but not simultaneously.
In another test, the team found almost 60% of mosquitoes that previously fed on blood while exposed to Deet subsequently attempted to reach and bite a researcher’s Deet-treated hand. By contrast, untrained mosquitoes universally tried to bite the researcher’s other, untreated hand.
Dr Nina Stanczyk of ETH Zürich university, who has previously studied the effectiveness of Deet towards mosquitoes, welcomed the research.
“Mosquitoes have been shown to have impressive learning abilities, but the fact they can associate such a strong repellent smell with their food and are then attracted to it afterwards is remarkable, and important for us to be aware of for the future,” she said.
However, experts emphasised that the findings did not mean that travellers should ditch their Deet.
“People should understand that Deet does not lose its effectiveness through normal use, but only under specific laboratory conditions designed to reveal how it works on mosquitoes,” said Lazzari.
Prof Francesca Romana Dani, an entomologist at the University of Florence, who was not involved in the study, said it was unlikely that, under normal conditions, mosquitoes would change their response to Deet based on previous experience, not least as the same insect may encounter different repellants during subsequent blood meals.
“Furthermore, although a single mosquito can take multiple blood meals, they do so every few days, so it’s important to evaluate how long the memory of a blood meal taken in the presence of Deet will last,” she said.
Stanczyk said travellers should continue wearing repellants with confidence. “The study authors state it was challenging to make mosquitoes feed a first time in the presence of Deet, and that the highest risk an association would form is when [the] repellant starts to wear off,” she said. “Therefore, the most important point for travellers is to regularly reapply repellant as instructed by the product label.”
