Every summer millions of people spray themselves with insect repellant to keep mosquitos away, but research suggests the blood sucking insects can learn to associate the repellent with food. Deet — which has the chemical name N,N-diethyl-meta-toluamide — is widely used in insect repellants and recommended by the UK Health Security Agency. Mosquito-related illnesses kill about 700,000 people annually according to the World Health Organization (WHO), making repellant vital in countries where mosquito bites can spread malaria, Zika virus, dengue and Japanese encephalitis.But a new study suggests the insects start to associate the smell of the repellant with a reward over time — in some cases they become attracted to it."If someone applies DEET and the concentration fades over time, but a mosquito still manages to feed, the insect may begin associating that smell with a reward," said Clément Vinauger, associate professor at Virginia Tech. "That's a possibility we should take seriously when we think about how repellents are used in the real world."A new study suggests the insects start to associate the smell of the repellant with a reward (Getty/iStock)For the study, published in the Journal of Experimental Biology, researchers focussed on the yellow fever mosquito, Aedes aegypti, a species that spreads dengue fever, Zika, yellow fever, and chikungunya, which infect tens of millions of people each year.Researchers trained the mosquitoes using a form of Pavlovian conditioning — the same learning principle behind Ivan Pavlov’s famous experiments in which dogs learned to associate the sound of a bell with food.Mosquitoes were restrained behind fabric mesh with a bag of warm blood positioned just out of reach. After the mosquitoes began to feed on the blood, researchers introduced the smell of Deet. After repeating the experiment four times, more than 60 per cent of the insects tried to feed when they could smell Deet.Next, mosquitoes were given a choice between two human hands, one untreated and one coated with Deet at normal concentrations. Untrained mosquitoes avoided the Deet-treated hand, but rained mosquitoes were drawn to it.Clément Vinauger, associate professor in the College of Agriculutre and Life Sciences at Virginia Tech. (Virginia Tech)“The common assumption has always been that repellents work because of their chemistry — that Deet simply smells bad to mosquitoes and they flee or that its chemistry prevents mosquitoes from smelling us,” said Vinauger.“But what we are showing is that the mosquito’s brain can rewrite that response based on experience. What the insect has learned matters just as much as what the chemical does. That, I think, is a paradigm shift,” he added. The findings do not mean people should stop using Deet, Vinauger said. It’s still one of the most effective repellents available."If you're in tropical regions where disease risk is real, you should use it," he said.But the study suggests timing and concentration may matter more than previously understood."Instead of applying a lot at once, you may want to reapply regularly so it's always active and providing continuous protection," Vinauger said.