Talking about the wind tunnels, Riffell said, “What’s great about this wind tunnel is that it provided a nice control of wind conditions and the environment these mosquitoes are flying around in”.
Past studies had shown that the insect’s sense of smell may play a crucial role in finding food, but Riffel and his fellow researchers wanted to know precisely which of the senses is more important during hunting. Mosquitoes are guided by visual cues that draw them close to the victim to the level that they can sense their prey’s body heat and ultimately decide to leave their mark. As the bloodsucking insects evolve to resist our best pesticides, mosquito control may shift more to understanding how the mosquitoes find a tasty – and unsuspecting – human host.
Researchers reached the insightful conclusions after tracking mosquito behavior under a variety of circumstances, by watching them in a wind tunnel which served as a safe and controlled environment.
The wind tunnels were built without special features, but had a dark spot on the floor.
What happens is pretty straightforward: the mosquito can smell a host’s carbon dioxide (CO2) from 10 to 50 meters away, which prompts it to fly closer. They found that in the presence of the carbon dioxide plumes, the mosquitoes were attracted to the dark high-contrast object.
“When we gave them the odor stimulus, all of the sudden they were attracted to this black dot”, said Riffell. Under regular conditions, the mosquitoes were uninterested in the dot. These results might mean that mosquitoes control or “gate” their sensory systems. “That is, we predicted that when the mosquitoes were exposed to CO 2, 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”.
“Even if it were possible to hold one’s breath indefinitely”, the authors note toward the end of the paper, “another human breathing nearby, or several meters upwind, would create a carbon dioxide plume that could lead mosquitoes close enough to you that they may lock on to your visual signature”. If this theory is correct, the scents picked up by the mosquito’s nose may determine whether or not it engages other sensory systems in the search, especially vision. The mosquito also tracks our body heat, to make sure our blood is still warm, and uses its eyes to make sure we definitely are what the other sensors said we are. Riffell and his colleagues worked to find what those triggers are.
Now the team is recording how certain parts of the mosquito brain respond to other odors, hoping to find those that repel the bugs instead of attracting them. “That may be how they discriminate among potential blood hosts”. They found that mosquitoes showed a preference for the warm object.
Riffell explained that Carbon dioxide is a signal that a warm-blooded host is around which a mosquito can sniff from a 30 foot distance.
The work provides researchers with exciting new information about insect behavior and may even help companies design better mosquito traps in the future.
The experiments in the Current Biology paper began in the UW biology department before professor Michael Dickinson, the paper’s senior author, relocated to Caltech.
This research was funded by the National Institutes of Health.