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Fruity whiffs can mask the scent of carbon dioxide for flies, mosquitoes: study suggests new approach to making insect repellents.

Fruit flies actually have a love-hate thing with the smell of fruit. And a new insight into the chemistry of that relationship could lead to novel repellents for other insects, researchers say.

Carbon dioxide is a known turn-off to fruit flies when it emanates from stressed peers. "Drosophila sniff C[O.sub.2] and avoid it like crazy," says neurobiologist Anand Ray of the University of California, Riverside. But ripe fruit puffs out the gas and still attracts plenty of the flies. In this case, compounds in the fruit block the flies' C[O.sub.2] receptors, Ray and Riverside colleague Stephanie Turner report online August 26 in Nature.

Mosquitoes, in contrast, love C[O.sub.2]. They hunt down blood by following plumes of the exhaled gas. But as in fruit flies, a fruit compound can jam C[O.sub.2], receptors in the mosquito Culex quinquefasciatus, the researchers say.

This species spreads West Nile fever virus and the parasites that cause the limb swellings of filariasis. Ray proposes that compounds that could keep the mosquito detectors from sensing those plumes might render people hard to find.

The paper looks like a significant contribution toward developing new controls for disease-spreading insects, says Pablo Guerenstein of CONICET, Argentina's research council in Diamante and Entre Rios National University in Oro Verde.

An aversion to C[O.sub.2] turned up in fruit flies when researchers stressed some of the insects. Stressed flies released an odor with C[O.sub.2] as a key component, and unharmed flies fled from that odor. "There was this paradox," Ray says, because fruit flies avoid stressed flies but crowd around ripe fruit and other C[O.sub.2] sources.

To study the seeming contradiction, Ray and Turner looked to the sensory detectors on the insects' antennae. "Each antenna is shaped like a strawberry, with hundreds of tiny hairs on it," Ray says. Inserting minute electrodes into some of the hairs let the researchers check for activity in a neuron bearing the specialized receptor known to detect C[O.sub.2]. The researchers monitored that neuron's activity while releasing both fruit odors and C[O.sub.2]. Two of the fruit odors strongly reduced the neuron's reaction to C[O.sub.2].

Those two molecules, 1-hexanol and 2,3-butanedione, were quite a surprise, Ray says. Yet, he and Turner note, earlier studies showed that as bananas ripen, concentrations of 1-hexanol increase by 777 percent and 2,3-butanedione by 14,900 percent.

To test the receptor independent of the neuron, Ray says, the team used "the awesome power of fly genetics" to put the receptor in a neuron that normally does not respond to the gas. The new neuron responded to CO., and also showed reduced activity when the fruit molecules wafted by.

Mosquito neurons also failed to respond to C[O.sub.2] in the presence of 1-hexanol and 1-butanal, a close relative of 2,3-butanedione.

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Title Annotation:Molecules
Author:Milius, Susan
Publication:Science News
Geographic Code:1USA
Date:Sep 26, 2009
Words:490
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