Researchers at the University of California, Davis, recently identified a powerful scent produced by both humans and birds that apparently lures Culex mosquitoes to their next blood meal.

The source, a substance called nonanal, emits a fruity or floral odor and also is made commercially, used in manufacturing perfumes and flavors. When given off in the air, nonanal works synergistically with carbon dioxide, another known mosquito attractant, to send a strong message to the biting insects. Mosquitoes detect smells with the olfactory receptor neurons of their antennae.

“Nonanal is how they find us,” said Walter Leal, a professor of entomology and leader of the research team.

“Mosquitoes are very sensitive to this compound,” he added. “It’s a very common chemical produced by humans and animals, and also industry. It’s everywhere.”

The discovery ultimately could lead to new ways of controlling mosquitoes, which carry disease-causing viruses, including West Nile virus. A West Nile infection can result in serious neurological illness. Mosquitoes from the Culex family--the focus of this study--carry West Nile.

Birds, the main host of mosquitoes, are a reservoir for West Nile. The insects transmit viruses to humans--as well as to birds, horses, dogs, cats, bats, chipmunks, squirrels and domestic rabbits--when they bite and take a blood meal.

“If you can decrease the biting of mosquitoes, you can decrease the incidence of disease,” Leal said.

Leal and postdoctoral researcher Zain Syed screened hundreds of naturally occurring compounds emitted by multiracial and multiethnic humans, and multiple species of birds, including chickens and pigeons. They collected chemical odors from 16 human subjects, including two African Americans, two Chinese, eight Caucasians, three Latinos and one East Indian, who ranged in age from 20 to 55.

They found varying amounts of the substance among the individuals, but everyone produced it. “We didn’t find anybody in our samples who was completely depleted of the compound,” Leal said.

They also baited mosquito traps with a combination of nonanal and carbon dioxide. “We were drawing in as many as 2,000 a night in Yolo County, near Davis,” Syed said. “Nonanal, in combination with carbon dioxide, increased trap captures by more than 50 percent, compared to traps baited with carbon dioxide alone.”

The researchers believe that nonanal results from the breakdown of dietary fatty acids. Eliminating the intake of fatty acids, however, is not the answer to preventing mosquito bites, Leal said. “You can’t get rid of fatty acids,” he said. “We need them.”

Moreover, finding ways to block nonanal may not be the route to mosquito control. First, humans produce varying levels of the substance and scientists do not yet know the effects of different amounts on mosquito behavior. In fact, in some cases insects are drawn by small amounts of attractants, while repelled by high amounts.

“We know there are variations in how much nonanal humans produce, but we don’t know whether they are related to more or less attraction,” Leal said. “We want to go back to the field and try different doses of nonanal to see if mosquitoes are attracted by more levels - or whether there is a point where the attraction starts to decline.”

Furthermore, researchers also need to learn more about the body’s natural repellants, since these, too, likely play a role in controlling mosquito bites. “It’s very common to hear stories about people who go into the field and one gets lots of mosquito bites, while another person doesn’t get any,” Leal said.

“All of us make natural repellants and attractants, so we could be dealing with more than one compound,” he added. “Some of us may be very attractive to mosquitoes--not necessarily because we generate nonanal, but because we don’t generate the natural repellent. We don’t know all the chemicals yet. People are saying: ‘now you can mask nonanal and everything will be solved.’ It’s very appealing to say that, but it’s not that simple.”

In 2008 Leal and his team discovered how the insecticide DEET repels mosquitoes. Scientists had long thought that DEET, an insect repellent developed more than 50 years ago by the U.S. Department of Agriculture, works by masking a human’s smell or by jamming the insect’s senses, interfering with its ability to locate a host. But Leal showed that mosquitoes can smell DEET--and hate it. They avoid it simply because they dislike its odor.

The nonanal study, which was published in the Proceedings of the National Academy of Sciences, was funded by the National Science Foundation, the National Institutes of Health, and through a cooperative agreement with Bedoukian Research, a supplier of specialty aroma and flavor ingredients headquartered in Connecticut.

—By Marlene Cimons, NSF.

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