How Mosquitoes Stalk Victims Using Sense Of Smell

Mosquitoes pack a surprising aresenal of senses for tracking you down, a new study has found. The tiny biting, disease-spreading insects have evolved a triple menace of visual, olfactory, and thermal cues to zero in on humans.

You probably are aware that when an adult female mosquito wants a meal to feed her offspring, she looks for a host, frequently a human.

Certain insects, including mosquitoes, are attracted by the odor of the carbon dioxide (CO2) that humans and other animals breathe out. But mosquitoes can pick up other clues that signal a human is close at hand.

To spot a host, they use their vision, to detect body heat they can use thermal sensory information.

But, scientists wanted to know, how do mosquitoes blend this information together to map out the path to their next victim?

Researchers at Caltech released hungry, mated female mosquitoes into a wind tunnel in which different sensory cues could be manipulated independently. In one series of experiments, a high-concentration CO2 plume was injected into the tunnel, simulating the marker made by the breath of a human.

In control experiments, they introduced a plume consisting of background air with a low concentration of CO2. For each experiment, researchers released 20 mosquitoes into the wind tunnel and used video cameras and 3-D tracking software to follow their paths.

For the concentrated CO2 plume, mosquitos followed it within the tunnel as expected, but they showed no interest in a control plume consisting of background air.

First author Floris van Breugel, a postdoctoral scholar in Dickinson’s lab, says:

“In a previous experiment with fruit flies, we found that exposure to an attractive odor led the animals to be more attracted to visual features. This was a new finding for flies, and we suspected that mosquitoes would exhibit a similar behavior. That is, we predicted that when the mosquitoes were exposed to CO2, which is an indicator of a nearby host, they would also spend a lot of time hovering near high-contrast objects, such as a black object on a neutral background.”

It was expected to see the mosquitoes tracking a CO2 plume. But, van Breugel says:

“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.”

To test how a mosquito factors thermal information into its flight path, the researchers built two glass objects, coated with a clear chemical substance which made it possible to heat them to any desired temperature.

They heated one object to 37 degrees Celsius, which is about human body temperature, and let one remain at room temperature. They then placed them on the floor of the wind tunnel with and without CO2 plumes, and observed mosquito behavior.

They found that mosquitoes had a preference for the warm object. But contrary to the mosquitoes’ visual attraction to objects, that preference for warmth was not dependent on the presence of CO2.

“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,” van Breugel says.

Wit these reulats, the researchers were able to create a model of how the mosquito locates a host over different distances.

They believe:

  • From 10 to 50 meters away, a mosquito smells a host’s CO2 plume
  • As it flies closer, to within 5 to 15 meters, it begins to see the host
  • Guided by visual cues that draw it even closer, the mosquito can sense the host’s body heat, at a distance of less than a meter.

“Understanding how brains combine information from different senses to make appropriate decisions is one of the central challenges in neuroscience,” says Dickinson, the principal investigator of the study. “Our experiments suggest that female mosquitoes do this in a rather elegant way when searching for food. They only pay attention to visual features after they detect an odor that indicates the presence of a host nearby. This helps ensure that they don’t waste their time investigating false targets like rocks and vegetation. Our next challenge is to uncover the circuits in the brain that allow an odor to so profoundly change the way they respond to a visual image.”

Floris van Breugel, Jeff Riffell, Adrienne Fairhall, Michael H. Dickinson
Mosquitoes Use Vision to Associate Odor Plumes with Thermal Targets
Current Biology DOI: http://dx.doi.org/10.1016/j.cub.2015.06.046