Science gets the deets on DEET

Insect repellent appears to repel mosquitoes by confusing them

The insect repellent DEET doesn’t actually repel insects — it confuses them. New research suggests that DEET gums up sniffing machinery, sabotaging an insect’s sense of smell.

Nerve cells that send odor-related signals to the brain respond differently to DEET if it’s sniffed alone or with other scents, experiments with fruit flies reveal. DEET’s effects also vary depending on what variety of smell-receiving machinery catches the scent, and whether it’s a whopping dose or just a whiff, a team of researchers reports online September 21 in Nature.

“The effects of DEET are not straightforward,” says neuroscientist Maurizio Pellegrino of the University of California, Berkeley, a member of the research team. “We think it confuses the odor coding — the insect doesn’t know exactly what it is smelling.”

While the highly effective compound has been protecting people for decades from loathsome mosquitoes, ticks and chiggers, it hasn’t been clear how DEET does its stuff. As any Little League parent or backyard barbecue chef knows, a mosquito that lands on a DEET-slathered arm takes off in a hurry. DEET also protects people before a bug makes contact.

Some evidence had suggested that the compound, technically N,N-diethyl-meta-toluamide, hits a specific cellular baseball mitt, an odor receiver that catches repellent scents, triggering a “get-the-heck-out-of-here” signal.

“The idea was, when an insect detected such an odorant, it would fly away; it didn’t want any part of it,” says Joseph Dickens of the U.S. Department of Agriculture’s Invasive Insect Biocontrol and Behavior Laboratory in Beltsville, Md.

But the new work suggests that DEET isn’t repulsive or frightening to bugs. It’s baffling, blinding them to the delicious scent of human.

“It’s as if you are hungry and you love hamburgers,” says Pellegrino, who did the work while at Rockefeller University in New York City. “If DEET is present, it doesn’t smell like hamburger anymore, even if a hamburger is right in front of you.”

Fruits flies have more than 60 cellular mitts for catching scents. When the mixtures of molecules that make up odors drift by, the mitts snatch particular molecules, nerves fire, and an odor message is sent to the fly’s brain. When the researchers exposed flies to DEET alone and combined with other scents, the nerves didn’t fire in a consistent pattern. Changing up the concentration of DEET altered the nerve firing as well, Pellegrino and colleagues report.

The results suggest that DEET corrupts the odor signal, perhaps by contorting parts of the sniffing machinery in such a way that wafting molecules other than DEET are misinterpreted.

Surveying the sniffing machinery of fruit flies from around the world revealed further evidence that DEET might render the flies’ odor receivers incapable of catching other scents. One fruit fly strain, Boa EsperanÒa from Brazil, was insensitive to DEET, responding to scents as if DEET weren’t there. Comparison of the proteins that make up the Brazilian strain’s sniffing machinery revealed structural differences that may prevent DEET from being caught.

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