Fruity whiff may inspire new mosquito repellents

Odors from ripening bananas can jam fruit flies’ and mosquitoes’ power to detect carbon dioxide

Fruit flies actually have a love-hate relationship with the smell of fruit. And a new insight into the chemistry of that attraction and repulsion 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 CO2 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 flies. In this case, compounds released by the fruit block the flies’ CO2 receptors, Ray and Riverside colleague Stephanie Turner report online August 26 in Nature.

Mosquitos, in contrast, outright love CO2. They hunt down blood meals by following plumes of the exhaled gas. But as in fruit flies, a fruit compound can jam CO2 receptors in the notorious mosquito Culex quinquefasciatus, the researchers say. This species spreads West Nile fever and the parasites that cause the huge limb swellings of the tropical disease filariasis. Ray proposes that compounds that could keep the mosquito detectors from sensing those plumes might render people hard to find. With a grant from the Bill and Melinda Gates Foundation, he’s starting to test the strategy.

In fruit flies, the aversion to CO2 turned up when researchers shook the insects or zapped them with a little electric shock. The stressed flies released an odor with CO2 as a key component, and unharmed flies fled from that odor.

“There was this paradox,” Ray says. The same fruit flies that avoid stressed flies and even unripe CO2-exhaling fruit, crowd around ripe fruit as well as other strong CO2 sources such as beer.

To study the contradiction, Ray and Turner turned 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 pores on some of the hairs let the researchers check for activity in a neuron bearing the the specialized receptor known to detect CO2. The researchers monitored that activity while releasing both fruit odors and CO2. Two of the fruit odors strongly reduced the neuron’s reaction to CO2.

Those two inhibitor molecules, 1-hexanol and 2,3-butanedione, were quite a surprise, Ray says. “They do not look anything like CO2.”

Yet he and Turner note that earlier studies show that as bananas ripen, concentrations of 1-hexanol increase by 777 percent and 2,3-butanedione by 14,900 percent.

To see whether the inhibitors indeed act on the receptor, “we took the awesome power of fly genetics,” Ray says, and put the receptor protein in a neuron that normally has nothing to do with CO2 detection. Isolated in this alien neuron, the receptor still jammed when the inhibitors wafted by.

Mosquito neurons responded to one of the inhibitors, 1-hexanol. The 2,3-butanedione didn’t produce much of a reaction but a closely related compound, 1-butanal, did, Ray and Turner found.

The new paper looks like a significant contribution toward developing new controls for disease-spreading insects, says neuroethologist Pablo Guerenstein of the Argentine National Research Council in Diamante and Entre Rios National University in Oro Verde, Argentina. Now, Guerenstein says, he wants to know more about the results’ biological significance, such as whether the inhibitors affect the way insects perceive CO2 in the natural blend of odors from a fruit. Also, he points out that the inhibitory 1-hexanol appears in some vertebrate odors, and he would like to know about its role there.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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