A half-century after its discovery, DEET is still the unquestioned king of mosquito repellants. But researchers have discovered that under the right circumstances the mosquitoes can fight back…

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More than half a century after DEET’s invention, scientists still don’t know how the popular mosquito repellent works. Now, using a combination of artificially accelerated evolution and painstaking anatomical observation, researchers have answered a fundamental question about DEET’s mechanisms – and in the process showed that mosquitoes may become resistant to it. “It’s a fundamental piece of research. It will give us a lot more knowledge, rather than just going out and spraying something,” said study co-author Linda Field, a molecular biologist at England’s Rothamsted Research institute.

Field and Nina Stanczyk, a University of Nottingham biochemist, started their study by resting a DEET-sprayed arm on a mesh cage, just out of reach of female Aedes aegypti mosquitoes. (Only female mosquitoes bite; like males, they typically feed on flower nectar, but require nutrients from blood in order to lay eggs). Those that tried to feed were removed and bred separately. Within a few generations, more than half were DEET-resistant. Field cautioned that laboratory results shouldn’t be automatically extrapolated to the natural world, but a similar dynamic could well exist, especially in heavily populated areas where humans are the predominant source of blood. “If a small percentage are insensitive, they have a much better chance of getting a blood meal, and are much more likely to pass on their genes. You’d likely see a buildup of the trait,” said Field.