Pity the poor mosquito magnet. At any picnic or barbecue, the bugs are all over him like a cheap suit, while everyone else dines pest-free. It's not your imagination; it's chemistry. Some people really are more appealing to mosquitoes. And scientists are just beginning to understand why.
"People differ in their ability to attract mosquitoes, and each person's attractiveness can even change from day to day," says Ulrich Bernier, a chemist at the U.S.D.A.'s Mosquito and Fly Research Unit in Gainesville, Florida. Bernier and his colleagues have analyzed hundreds of natural compounds from human skin and have already identified several chemicals that act as attractants and some substances that seem to cloak the human bouquet. These elixirs combine to make some individuals three times more tempting than others. "We suspect the overall response of mosquitoes to humans is based not only on attractants but also on compounds that help hide us," says Bernier.
The Gainesville group is part of government efforts to control the insect that annoys billions and kills millions worldwide by spreading infectious diseases such as malaria, yellow fever, and encephalitis. Mosquitoes use a number of cues to find their victims: Color contrasts and movement help guide them by daylight, for example, and body heat may direct them to areas where skin is thin and blood vessels dense. But the most potent cues seem to be chemical. Scientists have known since the 1920s that carbon dioxide in exhaled breath attracts mosquitoes; in the 1960s, researchers also fingered lactic acid, a by-product of human metabolism found in sweat.
Yet neither substance, alone or in combination, can rival a bare human arm for mosquitoes' attention, so other bodily vapors must be involved. It's taken three more decades to find them. The problem is that skin-borne chemicals have to evaporate readily in order to reach airborne mosquitoes, and such volatile compounds are hard to nab for study. But techniques for analyzing small amounts of volatiles have grown more sophisticated. A few years ago, Bernier and chemists at the University of Florida used a novel approach to transfer compounds evaporating from the skin to tiny glass beads. With this method, Bernier found hundreds of substances, known and unknown, in each sample. The scientists then exposed laboratory mosquitoes in airtight compartments to the native compounds. After painstaking trials with countless variations, they homed in on one mixture that seemed especially alluring. It contained just three ingredients: lactic acid; acetone, an organic solvent produced when the body burns fat; and dimethyl disulfide, a compound produced when bacteria break down proteins. Alone, each substance makes only mildly effective bug bait; lactic acid itself, for example, attracts less than 20 percent of mosquitoes. Combined with acetone, it draws about 80 percent. But that duo can't compete with human flesh. Given a choice, mosquitoes still prefer a red-blooded host.
Dimethyl disulfide proved to be the pièce de résistance. With that addition, the blend becomes more tempting than some people— a first in tests of synthetic attractors. "We're somewhere in the range of human [appeal]," says Bernier. "But our goal is to be better than humans at attracting mosquitoes." That way, scientists may be able to devise lures that draw mosquitoes away from people and into traps.
A mosquito-filled chamber (top) allows researchers to test what lures and repels the pests. Researchers use glass beads that humans have rubbed between their palms to collect mosquito-attracting compounds. Photographs courtesy of Peggy Greb/ARS/USDA
Once he'd figured out how to attract mosquitoes, Bernier could zero in on natural repellents. He tainted the attractant blend with one chemical after another until the mosquitoes got turned off. A single compound— Bernier won't reveal what it is until it's patented— cut the insects' interest to 6 percent. And unlike deet and other commercial repellents, which work only when mosquitoes get close to or contact the skin, this one seems to keep insects from sensing their target. When Bernier exposed laboratory mosquitoes to the natural repellent and then offered his own skin for consumption, "most of them didn't even know there was an arm there too. It's like their receptors are jammed."
Such masking compounds could be used in slow-release dispensers that would render nearby mosquitoes oblivious to the presence of humans. They might also be safe enough to smear on the skin like ordinary repellents. But the prospect that has mosquito-control experts most excited is a systemic repellent: a potion that would step up the body's production of its own cloaking chemicals. The discovery of natural repellents sets the stage for this development, but it may take decades more to flesh out.
"We still have to figure out how the body produces these substances and how those processes can be manipulated to make a person invisible to mosquitoes," says Donald Barnard, head of the Mosquito and Fly Research Unit. Diet may have something to do with it, he says, but the connection hasn't been well researched. B vitamins have been shown to have no effect on mosquito appeal, and garlic's legendary protection against bloodsuckers doesn't seem to extend to mosquitoes. Fat-burning exercise might release acetone on the skin, and sweating could increase lactic-acid emanations. But genetic determinants, such as the density of skin pores, might also come into play. "We just don't know much about the factors that affect these sorts of things," says George Preti, an organic chemist who specializes in body and breath odors at the Monell Chemical Senses Center in Philadelphia.
To make matters more complicated, Preti says, human metabolism probably isn't the only pathway that produces the body's attractants and repellents. Bacteria make the fatty acids that cause some kinds of body odor, and they may generate other volatiles, including dimethyl disulfide. Researchers in the Netherlands have reported that the sulfur compounds produced by a strain of toe-dwelling bacteria may explain why some mosquito species have a fondness for feet.
Whether made by hu-mans or microbes, the same spectrum of compounds turns up on most people's skin, says Barnard. But the amount of each chemical varies from one individual to another, and those variations may account for the insects' favoritism. Exactly how the native vapors interact to lure mosquitoes or keep them at bay is still unclear: Some chemicals, for example, produce different results at different concentrations. "In very low concentrations, deet acts as an attractant," Barnard says.
And high concentrations may not be desirable either. "Dimethyl disulfide stinks pretty bad, and acetone isn't something I'd want to spend a lot of time in a closed room with," Barnard says. So you can't just slap a random mixture of lactic acid, acetone, and dimethyl disulfide on an unsuspecting friend and expect to create your own mosquito magnet. "If it were that simple, we'd have figured it out by now."
See "How Attractive Are You? To Mosquitoes, That Is" by Tara Weaver-Missick, www.ars.usda.gov/ is/AR/archive/ feb00/mosq0200.htm. Also look up Discover's August 1997 Light Elements, "Why Mosquitoes Suck" by Patricia Gadsby; available at www.discover.com.
