Mosquitoes Use Multiple Senses to Locate Human Prey

Posted on July 18, 2015

Three ways mosquitoes track humans

Researchers from Caltech and the University of Washington have been studying mosquitoes in a wind tunnel to determine exactly how mosquitoes locate a human being to bite. It has been known that mosquitoes are attracted to carbon dioxide exhaled by humans and this new study reaffirms previous findings. The researchers also found that mosquitoes also rely on vision and a thermal sensing ability when they get closer to a human body.

The researchers say their findings indicate that mosquitoes can sense CO2 from 10 ten to 50 meters away. They can switch to vision and spot a human when they are about 5 to 15 meters away. As they get much closer to a warm blooded mammal they can use a thermal sense to detect body heat. The body heat detection works from within 1 meter of a human being.

The wind tunnel experiments included placing hungry, mated female mosquitoes in a wind tunnel where different sensory cures could be controlled. In one experiment, the tunnel was flooded with a CO2 plume and a dark object was placed on the floor of the tunnel. The mosquitoes were attracted to the dark high-contrast object if there was CO2 in the tunnel but were not interested in it when there was no CO2 in the tunnel.

Floris van Breugel, a postdoctoral scholar in Dickinson's lab and first author of the study, says in a statement, "The new part that we found is that the CO2 plume increases the likelihood that they'll fly toward an object. This is particularly interesting because there's no CO2 down near that object-it's about 10 centimeters away. That means that they smell the CO2, then they leave the plume, and several seconds later they continue flying toward this little object. So you could think of it as a type of memory or lasting effect."

UW biologist Jeff Riffell, a co-author of the paper, says, "When we gave them the odor stimulus, all of the sudden they were attracted to this black dot. It's almost like the carbon dioxide gas turned on the visual stimulus for the mosquitoes to go to this black dot."

The researchers also tested the thermal detection abilities of mosquitoes. They put two glass objects that were coated with a clear chemical substance in the wind tunnel. The heated one of the objects to 37 degrees Celsius and left the other one at room temperature. The mosquitoes showed a preference for the warmer object and this preference was not dependent on CO2 being present in the tunnel.

Van Breugel says, "These experiments show that the attraction to a visual feature and the attraction to a warm object are separate. They are independent, and they don't have to happen in order, but they do often happen in this particular order because of the spatial arrangement of the stimuli: a mosquito can see a visual feature from much further away, so that happens first. Only when the mosquito gets closer does it detect an object's thermal signature."

This triple threat is what makes it so difficult for people to avoid getting bitten by mosquitoes. The authors of the study say the best technique to avoid the mosquitoes is to become "invisible or visually camouflaged" but even then they can still find you if they are close enough to track your heat signature.

The researchers say, "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. The independent and iterative nature of the sensory-motor reflexes renders mosquitoes' host seeking strategy annoyingly robust."

The research results were published here in a paper in Current Biology.

Image: Lance Hayashida/Caltech