Why mosquitoes decide to prey on your feet and arms versus the similarly exposed human sitting next to you is a question science has yet to definitively answer. But thanks to new data we may be one step closer.
Female mosquitoes appear to choose their targets first by smell and then by using their eyes to zoom in for the kill once their prey is in close range, according to a new study published online July 18 in the journal Current Biology.
Previous research found mosquitoes need both scents and visual cues to find hosts — and that mosquitoes are attracted to smells we give off (like the carbon dioxide [CO2] we breathe out), but the new study sheds more light on how the bugs do that. The data suggest mosquitoes find hosts more easily if they smell CO2 first and follow their nose until they’re close enough to see their target; and that the insects’ sense of smell appears to play a bigger role in getting mosquitoes close to their target than their sense of sight.
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“Our breath is just loaded with CO2,” says the senior study author, Jeffrey Riffell, PhD, a biology professor at the University of Washington. Mosquitoes may be attracted to the smell from more than 100 feet away, he explains.
Data: Mosquitoes' Sense of Smell Appears to Be Stronger Than Their Sight
To test how mosquitoes might follow a trail of CO2 to their food, researchers focused on Aedes aegypti, a species that is sometimes called the yellow fever mosquito and that can also transmit dengue fever and other viruses, according to the World Health Organization.
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Researchers collected data from approximately 250 female mosquitoes (male mosquitoes do not feed on blood), tracking their behavior and recording in real time the mosquitoes' brains during a series of experiments conducted in a cylindrical arena about 7 inches in diameter. The scientists were able to measure the mosquitoes’ wing movements (using a special type of optical sensor) in response to different odors and visual stimuli.
One-second puffs of air containing 5 percent CO2 — human exhalations are typically 4.5 percent CO2 — prompted the mosquitoes to beat their wings faster.
The mosquitoes also beat their wings faster in response to the visual cue of a bar moving horizontally across a screen around the arena. The mosquitoes tried to move in the direction of the moving bar. But increase in wing-beating speed was more pronounced when mosquitoes smelled the puff of CO2 before they saw the moving bar, compared with just seeing the bar move.
The researchers repeated the experiments with a genetically modified strain of the Aedes aegypti mosquito whose central nervous system cells were designed to glow fluorescent green when they are actively firing.
The data revealed regions of the mosquitoes’ brains linked to visual cues lit up (and were active) when the mosquitoes saw the horizontal bar moving during the experiment, as well as when the puff of CO2 was released. The reverse wasn’t true, however. Seeing the bar before smelling CO2 didn’t alter activity in regions of the brain that control smell, Riffell explains. It shows that for these mosquitoes, he says: "Smell triggers vision, but vision does not trigger the sense of smell."
Data May Help Mosquito Control in the Future, but More Research Is Needed
The researchers hope the insights can be used to help develop new methods for mosquito control and lower the spread of mosquito-borne diseases in the future, they note in the study.
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The results offer new insight into how mosquitoes compensate for having relatively poor eyesight, and it’s possible that more research might one day yield new methods of mosquito control (and therefore lowering the spread of mosquito-borne disease), says Antoine Cribellier, a PhD candidate researching mosquito flight at the University of Wageningen in the Netherlands who wasn’t involved in the study.
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One such direction he and his colleagues are studying are ways to trap mosquitoes. Knowing more about mosquito host-seeking behavior will help those efforts, he says. Reducing the number of mosquitoes in a given area can also help, he adds. “An easy and very efficient way of doing that is to get rid of any standing water (like a rain bucket in your garden) where females can lay their eggs.”
Unfortunately, he adds, “this study doesn’t tell us much about ways for people to avoid mosquito bites.”
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To repel mosquitoes, it’s necessary to either mask the scent from the hosts (you and me) that attracts the insects to begin with, or to interfere with mosquitoes’ sensory systems, explains Joseph Conlon, a technical adviser for the American Mosquito Control Association, who was not involved in the new research.
“I don’t necessarily find any earthshaking data [from this research] that should influence personal protective measures.”
What experts do recommend to avoid mosquito bites is:
- Cover arms, legs, and other exposed skin
- Use insect repellents
- Stay indoors at dawn and dusk (when mosquitoes tend to be most active)
- Use a fan or find an area that is breezy (mosquitoes are weak fliers; a little airflow can help keep them away)
One limitation of the study is that mosquitoes were tethered in a controlled environment in a lab, and it’s possible their flight responses might be different in the open air. Also, the Aedes aegypti is a species of mosquito that is active during the day, and it’s possible results might differ for mosquitoes that come out at night, Conlon notes.