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GUY RAZ, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Guy Raz.

Memorial Day marks the unofficial start of summer. It's time for warm days, ice cream, swimming pools, and this unwelcome sound.

(Soundbite of mosquito buzzing)

RAZ: Mosquitoes. Now, they'll stay away if you spray on some DEET. It's been the gold standard for repellants for over five decades, but it's also sort of smelly. It's feels oily, and it can even melt plastic. So, scientists are looking for alternatives.

And as NPR's Nell Greenfieldboyce reports, they've found some promising new chemicals.

NELL GREENFIELDBOYCE: When I first called Ulrich Bernier, he couldn't talk because he was too busy being bitten by mosquitoes. Later on, I asked him how many mosquito bites he had.

Mr. ULRICH BERNIER (Chemist, U.S. Department of Agriculture): I probably have on the order of 300 to 400 on my left arm.

GREENFIELDBOYCE: He's used to it though. He's a chemist with the U.S. Department of Agriculture in Florida, who tests insect repellants.

Mr. BERNIER: The test that we do involves a cage of mosquitoes, and in that cage, we have 500 female mosquitoes.

GREENFIELDBOYCE: Bernier routinely sticks his arm in there, but not his bare arm. He's tightly covered up, except for one small window of inviting flesh. That little window is protected by a thin cloth that the mosquitoes can bite right through. In tests the cloth gets coated with candidate repellants.

Mr. BERNIER: If the mosquitoes fly away, then we know it's repellant. If they land on the cloth and walk around, we know we were at the threshold level of repellency.

GREENFIELDBOYCE: And when they start biting, the repellant is failing. Bernier recently used this test to look at some interesting new chemicals. They came from researchers at the University of Florida. The scientists had systematically searched through a long list of possible repellants tested over the last 50 years. It turned out that some of the best belonged to one class of chemicals. By carefully analyzing this class of chemicals, they came up with some clues that would let them synthesize some new repellants that they thought would work.

Mr. BERNIER: I was surprised to see just how good these things were.

GREENFIELDBOYCE: Bernier found, that in his tests, some of the compounds repelled insect attack way longer than DEET. If you put DEET on a cloth, it works for only around 17 days. Some of the new compounds lasted over three times as long - up to 73 days. After trying so many different repellants that people have submitted over the years, it was nice to see some that actually were better than DEET.

GREENFIELDBOYCE: And he liked something else, too. In the past, most repellants were discovered just by testing random chemicals, not by designing them.

Mr. BERNIER: The biggest implication from this particular study is that we were able to actually predict repellency based on models, synthesize compounds, test them in the laboratory, and get good agreements.

GREENFIELDBOYCE: The results are reported in the proceedings of the National Academy of Sciences.

Ms. LESLIE VOSSHALL (Scientist, Rockefeller University, New York): Any efforts to use any approach to improve on DEET should be commended.

GREENFIELDBOYCE: Leslie Vosshall is a scientist at the Rockefeller University in New York, who's also on a mission to beat DEET. She just recently discovered how DEET works its magic on bugs, something that's long been a mystery. She found that DEET blocks the action of odor receptors.

Ms. VOSSHALL: Which means that when you're spraying DEET on your skin or your clothes, you create a little microenvironment of this chemical that when an insect approaches, makes them blind to odors.

GREENFIELDBOYCE: And as mosquitoes are blind to your odor, then to them, it's like you're invisible.

Vosshall is now trying to find other chemicals that block these odor receptors. Because her lab's working with cells and proteins rather than mosquitoes and people, she can quickly and cheaply screen massive numbers of chemicals - hundreds of thousands.

Ms. VOSSHALL: And we're finding lots of interesting things that look nothing like DEET that are much more potent, at least in the lab.

GREENFIELDBOYCE: But the big question is how they'll work if they're smeared on people's skin. There's a lot more testing to do before any of these repellants could show up in your bug spray.

Nell Greenfieldboyce, NPR News.

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