Hold your breath: carbon dioxide triggers exploratory behaviour in mosquitoes to help find hosts

A female mosquito feeding on a hapless victim.
A female mosquito feeding on a hapless victim.

Smack!

The tranquility of a lakeside sunset, disturbed by my attempts to stop bloodthirsty mosquitos from eating me as supper. I don’t know why mosquitoes think I’m a more appetizing meal than my camping companions but thanks to a new study, I now have a better understanding of how they hone in on targets such as myself.

Mosquitoes rely on a number of different cues to find their hosts. These include the heat and scents we emit, the humidity generated when our sweat evaporates and the carbon dioxide that we breathe out. What is less well known is how these different cues interact with and influence one another. For example, does sensing one cue help a mosquito pick up on other cues? That’s the question Dr. Michael Dickinson at the California Institute of Technology tried to answer. Together with colleagues at the University of Washington, his team showed that carbon dioxide triggers mosquitoes to explore visual elements in their environment, which in turn guides them to potential hosts.

To study the host-seeking behaviour of mosquitoes, the researchers released female mosquitoes into a wind tunnel and used a 3D tracking system to follow their flight movements. Concentrated carbon dioxide was pumped into the wind tunnel through a tube at one end and not far from this, the researchers placed a spot on the floor. This spot contrasted sharply with the rest of the floor and served as a proxy for the visual features of a host.

When clean air was pumped into the wind tunnel, the mosquitoes spent a long time exploring the ceiling and walls of the wind tunnel but did not approach the spot on the floor. When the clean air was replaced by carbon dioxide, the mosquitoes became a lot more active and spent the majority of their time exploring the spot on the floor. The researchers also repeated this experiment with male mosquitoes and found that males did not respond to carbon dioxide—they spent an equal amount of time exploring the dark spot on the floor with clean air or carbon dioxide. Given that male mosquitoes feed only on flower nectar and fruit juices, it’s not surprising that they do not respond to the carbon dioxide cues indicative of a blood meal. These results are the first to demonstrate that for female mosquitoes, carbon dioxide activates an exploration of visual elements in their environment.

The researchers also wanted to study how sensing carbon dioxide influenced the mosquitoes’ response to other sensory cues including heat and humidity. For these experiments, they modified the wind tunnel setup so that instead of a single spot on the floor, there were two identical transparent glass objects. The two objects could be each heated to a different temperature and covered with a special filter that would make it appear dark to mosquitoes. These features allowed the researchers to independently manipulate the thermal and visual cues the mosquitoes would encounter.

When both object were dark but at different temperatures, mosquitoes preferred the warmer object, regardless of whether or not carbon dioxide was being pumped into the wind tunnel. While carbon dioxide seems to directly impact mosquitoes’ attraction to visual elements, it does not seem to affect their response to thermal cues.

To study humidity, the researchers put a wet paper tissue on the glass object and heated it up to create evaporating moisture. The mosquitoes responded much more strongly to a warm humid object than to a warm dry object. This behaviour could help these clever insects distinguish warm inanimate objects like dark rocks that emanate heat from the sun from live animals such as myself that sweat profusely and increase the humidity around them.

Mosquito nets used to be worn as veils in the early 20th century. Maybe it's time for mosquito netting to make a fashion comeback. (Credit: Wellcome Library. CC BY 4.0)
Mosquito nets used to be worn as veils in the early 20th century. Maybe it’s time for mosquito netting to make a fashion comeback. (Credit: Wellcome Library. CC BY 4.0)

Their results allowed the researchers to propose the following model for how mosquitoes zero in on their victims: detecting the attractive aroma of carbon dioxide triggers mosquitoes to become more active and curious, seeking out contrasting visual elements in their environment. Exploring these visual features brings the mosquitoes closer to their potential hosts. At this point, heat, humidity and other smells can be used to decide if the target is food. In choosing a spot to land, mosquitoes likely rely more on skin odours (which might direct them to your feet or arms) than on carbon dioxide (which would lead them to your mouth).

The researchers acknowledged one main caveat with their results. In their experiments, they pumped a continuous stream of carbon dioxide into the wind tunnel. Under natural conditions, mosquitoes are more likely to encounter intermittent puffs of carbon dioxide that are dispersed by the wind. However the researchers showed that even a brief exposure to carbon dioxide is enough to trigger the visual-seeking, exploratory behaviour, suggesting that their conclusions will likely still hold under more natural conditions.

Now that we have a better sense of how the different sensory cues impact one another, it will be important to dissect the molecular mechanisms through which these phenomena occur. Why does sensing carbon dioxide make mosquitoes more attune to visual cues but not thermal ones? How is that information processed and transmitted? How can we manipulate these sensory inputs to reduce the burden of mosquito-borne illnesses like malaria and dengue fever? And on a more personal note, how can I use this information to protect myself from becoming an all-you-can-eat buffet for mosquitoes?

In what is probably the most honest and clear concluding paragraph I have ever read in a scientific paper, the researchers addressed my question:

“For a human hoping to avoid being bitten by a mosquito, our results underscore a number of unfortunate realities. Even if it were possible to hold one’s breath indefinitely, another human breathing nearby, or several meters upwind, would create a CO2 plume that could lead mosquitoes close enough to you that they may lock on to your visual signature. The strongest defense is therefore to become invisible, or at least visually camouflaged. Even in this case, however, mosquitoes could still locate you by tracking the heat signature of your body provided they get close enough. The independent and iterative nature of the sensory-motor reflexes renders mosquitoes’ host seeking strategy annoyingly robust.”

In other words, I’m screwed.

Reference: van Breugel, F., Riffell, J., Fairhall, A., & Dickinson, M. (2015). Mosquitoes Use Vision to Associate Odor Plumes with Thermal Targets Current Biology DOI: 10.1016/j.cub.2015.06.046

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