What Mosquitoes Want: Secrets of Host Attraction
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T HE M OSQUITO G ENOME: A NOPHELES GAMBIAE 1 NEWS 2 Sexual predator 3 What Mosquitoes Want: Deinocerites cancer 4 (Crabhole mosquito) 5 6 The struggle for Secrets of Host Attraction sex gets extreme. 7 Adult males 8 Why do mosquitoes feast on some people and leave others alone? emerge first and 9 Researchers are trying to find out, hoping it will help them design the per- grab female pupae credit: University of Florida/FMEL 10 fect mosquito trap with their forelegs while keeping other males at 11 bay. Copulation, which lasts up to 30 minutes, 12 Life isn’t fair. Whereas some people never But the ultimate goal is a far cry from takes place as the female emerges from her 13 seem to get bitten by mosquitoes—and often such pricey gadgets, says Willem Takken skin—a phenomenon dubbed “pupal rape.” 14 don’t even seem to notice the critters— of Wageningen Agricultural University in Distribution: Coastal areas along the Gulf of 15 others spend their evenings frantically swat- the Netherlands, a pioneer in the field. Mexico, Caribbean, and Central America. 16 ting them, usually to no avail. If you’re in the He’d like a simple, $1 or $2 trap that peo- Breeding habitat: Has found a unique niche in 17 latter category, you’ve probably wondered: ple in developing countries could affix to the burrows of land crabs. 18 Feeding: Night feeder; host preferences Why me? Is it thin skin? My gorgeous body their doorposts to keep out the mosquitoes unknown. 19 odor? Sumptuous blood vessels begging to that spread deadly diseases. Key targets are Diseases: None known. 20 be punctured? Or is it all between the ears, as Anopheles gambiae, the species that trans- 21 some people say, and you simply fuss and mits malaria, and Aedes aegypti, 22 fret more about mosquito bites? which spreads dengue and yel- 23 Rest assured, it’s not your imagination: low fever. Deinocerites cancer 24 Several studies have shown that to 25 mosquitoes, all people really aren’t creat- Blood, sweat, and cheese 26 ed equal. Besides factors such as heat and For almost a century, re- 27 carbon dioxide, mosquitoes use odors to searchers have been trying to di- 28 find their victims, and humans ap- vert mosquitoes from their tance and second thoughts about insecticide 29 pear to exude different amounts pursuit of human blood. use have sparked a renewed interest in alter- on October 24, 2011 30 of the volatile compounds the The field blossomed in native control methods. 31 insects love. the 1950s, when dozens Scouting for potentially attractive com- 32 By studying mosquito be- of entomologists in pounds, researchers are taking a closer look 33 havior, entomologists are try- several countries set at the more than 300 chemicals present on 34 ing to tease out these favorite out to discover what human skin. Martin Geier of the University 35 smells. It’s a complex story, attracts females—the of Regensburg, Germany, for instance, takes 36 they say. Millions of years of only mosquitoes that skin rubbings and then chemically removes a 37 evolution have resulted in bite—to their hosts. certain group of compounds—say, the ke- 38 sophisticated odor-based Anthony Brown tones or the fatty acids. If one group attracts www.sciencemag.org 39 navigation systems that dif- of the University mosquitoes, it can be further separated into 40 fer greatly from one of Western On- its individual components, he says. 41 mosquito species to the tario in London, To test how compelling single compounds 42 next, depending on where Canada, for in- or mixtures are, researchers use a specialized 43 it lives and which host it stance, built hu- instrument called an olfactometer, whose 44 prefers. Even so, chemical man-shaped steel central part is a Y-shaped wind tunnel. Two 45 and behavioral studies— tanks, which he different odors are blown into the short legs 46 often using human volunteers as called robots, dressed them of the Y; when mosquitoes are set loose at the Downloaded from 47 bait—have helped identify some of the up, and then counted the other end, they fly upwind and, like quiz 48 smells that tempt several mosquito number of mosquitoes that show contestants choosing between two 49 species. And recently, molecular re- landed on them in a forest. doors, decide whether to go left or right. Re- 50 searchers have begun identifying the He found, among other searchers can also fixate mosquitoes, apply 51 receptors that pick up these odors and things, that the robots became miniature electrodes to their nerves, and test 52 translate them into neural signals. more attractive if their skin whether exposing them to a whiff of some 53 Researchers hope to use odor cues was 37°C (the temperature of compound elicits an electrical signal. 54 to lure mosquitoes into the perfect trap the human body) than at low- Recent studies have confirmed what 55 or otherwise outwit them—say, by de- er temperatures, if they ex- Brown and others discovered half a century 56 signing repellents that foul their sense of haled CO2, or if they wore a ago: that for most mosquito species, CO2, 57 smell. Garden parties and golf getaways wet jerkin—or, better still, one heat, and moisture are key attractants. But 58 might be the first beneficiaries; in- soaked in human sweat. these lead a mosquito to any warm-blooded 59 deed, one U.S. company is already By the mid-1960s, animal—bird, cow, or human. That might 60 marketing the $500 to $1200 most research on host be fine for species that aren’t too picky, 61 Mosquito Magnet, which pur- attraction had stopped, such as Culex pipiens, a West Nile vector in 62 portedly attracts mosquitoes Footwork. Anopheles gambiae in part because DDT the United States. But those that dine al- 63 by emitting a compound called (left) bites mainly on the feet made mosquito extermi- most exclusively on humans, such as An. 64 1-octen-3-ol, as well as heat, CO2, and ankles; An. atroparvus nation so easy. Lately, gambiae and Ae. aegypti, need much more 65 and water vapor. prefers the face. however, emerging resis- specific attractants. SOURCE:AND RING CARDÉ MIRIAM COOPERBAND 90 4 OCTOBER 2002 VOL 298 SCIENCE www.sciencemag.org T HE M OSQUITO G ENOME: A NOPHELES GAMBIAE Hunting for cues, Bart Knols, a re- researchers, who joined the field just 2 years tors is produced only in mature females—an searcher in Takken’s group, noticed in 1995 ago, will help make sense of it all. They are important clue that it might be involved in that An. gambiae had a predilection for bit- making some headway: In a study published host seeking. So far, the group hasn’t been ing its victims on the feet and ankles—even on page 176, a team led by Laurence able to link any of the odors known to attract when their entire bodies were exposed. (This Zwiebel at Vanderbilt University in An. gambiae to any of the receptors. Still, the clearly set it apart from related species, such Nashville, Tennessee, has scoured the newly study is a “nice breakthrough,” says Takken, as An. atroparvus, a mosquito from Holland that goes mainly for the face.) A native of the Dutch province of Limburg, Knols also real- Death trap. The Mosquito Magnet’s popularity is ized that foot odor bears a remarkable re- growing in the United States (bottom). Something semblance to the pungent cheese from that much simpler and cheaper is needed to divert region. And sure enough, An. gambiae mosquitoes in developing countries. turned out to be heavily attracted to the smell of Limburger cheese. The finding, after making snickering headlines around the globe, led researchers to tempt different mosquitoes. “It became sort of a madhouse,” Knols recalls. “People started taking Limburger cheese all over the world.” But the stinky dairy product turned out to be an acquired taste, he says; just those few mosquitoes that feed primarily on hu- mans were strongly attracted. Knols says the common denominator between feet and cheese is obvious: a bacterium used in cheese production, called Brevibacterium linens, which is a close rel- ative of Brevibacterium epidermis, a bug known to reside in the warm, humid on October 24, 2011 clefts between human toes. Both turn that might speed the discovery of other, more glycerides into a specific set of break- powerful attractants. down products, such as fatty acids. Takken’s group is now trying to find Building the perfect trap out exactly which products provide Whether chemical lures can be fashioned the draw. into an irresistible mosquito trap, much less Over the years, researchers have one that would be cheap and effective in de- found that individual species have veloping countries, isn’t clear. But there is a their own idiosyncratic tastes for var- precedent. In many East African countries, www.sciencemag.org ious attractants. Ae. aegypti find lac- simple traps have helped virtually eradicate tic acid—which humans produce on tsetse flies, the carriers of sleeping sickness their skin but other mammals don’t— and a livestock disease called nagana. (One sublime; to An. gambiae, it’s only so- trap consists of a simple black-and-blue so. With ammonia, it’s the other way cloth, baited with acetone and octenol—or, around. And even in Aedes, Geier ex- alternatively, buffalo urine—and sprayed plains, lactic acid alone isn’t all that with insecticide.) attractive; rather, it boosts the appeal Mosquitoes, however, could pose a more Downloaded from of several other compounds.