In the dead of night, 200 bloodthirsty creatures make their way through a tented mesh arena the size of an ice rink. Scientists study them from afar, pumping in air scented with the aroma of human prey. They’re hoping to reveal just what triggers these deadly predators.
This isn’t the setup for an upcoming zombie movie. “It’s the world’s largest multichoice smell test for mosquitoes”—a giant experiment to figure out why the deadly insects prefer some people over others, explains study leader Conor McMeniman, a molecular biologist at Johns Hopkins University. The results, he says, could help scientists devise new traps and repellants to help protect people from deadly mosquito-borne diseases.
Taking this work out of the lab “really strengthens our understanding of [mosquito] behavior,” says Manuela Carnaghi, an insect behavioral ecologist at the University of Greenwich who was not involved with the research. “We’re moving toward more realistic studies.”
Mosquitoes are the world’s deadliest animal. They kill 700,000 people a year, typically by spreading diseases such as dengue and yellow fever. In sub-Saharan Africa, Anopheles gambiae is a particularly deadly pest; the mosquitoes are the main cause of malaria in the region, killing more than 600,000 people a year.
Body odors, carbon dioxide (via exhaled breath), and body heat all alert mosquitoes to a person’s presence. The bugs pick up scents through olfactory neurons on their antennae—they can smell body odor up to 60 meters away—and use body heat to zero in on their target.
As anyone who’s attended a backyard barbecue will tell you, though, some people get bitten a lot more than others. Most researchers have studied the insects’ preferences in relatively small, confined laboratory settings, limiting their ability to see how different scents attract mosquitoes over large distances, or how mosquitoes decide between multiple competing scents.
To examine their hunting strategies in a more natural setting, McMeniman and colleagues asked volunteers to sleep outdoors in eight tents that flanked the borders of a screened-in arena. Inside, hungry mosquitoes buzzed in search of a blood meal. The scientists funneled air containing body odors and carbon dioxide from the tents through ducts into landing areas inside the arena that were heated to 35°C, close to humans’ average body temperature.
Using infrared sensors, the team watched which landing platforms the peckish mosquitoes chose. Even in the large arena—with lots of different smells and sounds and amid real-world weather conditions—the insects gravitated more toward some humans than others, the team reports today in Current Biology.
“We have these tiny animals and these huge, huge areas … and they can still find these tiny platforms just 10 centimeters in diameter, just from odors,” says study co-author Diego Giraldo, a neurobiologist at Johns Hopkins. “It’s very striking.”
Afterward, the researchers analyzed the chemical makeup of the participants’ skin to suss out what gave them their signature smell. As has been found in previous studies, more mosquitoes seemed to prefer people whose scent contained a blend of carboxylic acids, the oily secretions that hydrate and protect our skin. Two of those carboxylic acids are also found in Limburger cheese, McMeniman notes, a known lure for mosquitoes.
Mosquitoes were less likely to buzz over to scents with lots of the chemical eucalyptol, found in plants like sage and eucalyptus trees. The team also compared body odor bait with carbon dioxide bait, and found that mosquitoes had a significant preference for body odor.
These findings “confirm a lot of the things that have already beeh established in other studies,” Carnaghi notes. Going forward, Carnaghi says she’d like to see future iterations of this work add another variable—humidity—to the mix, to make the experiment even more true to life. Warm, moist air emenating from prey encourages mosquitoes to land, she notes, so it would benefit researchers to know whether humidity changes the kinds of scents they’re attracted to.
Ultimately, Giraldo says he hopes that understanding mosquitoes’ favorite and least favorite smells could be used to combat malaria. Favorite smells could be put in traps to lure mosquitoes away from people, for example, and aversive smells could be used as repellants. “They’re just amazing hunters,” Giraldo muses, “and we’re the prey.”