This photograph shows a tethered flying Aedes aegypti mosquito. Courtesy of Kiley Riffell (with permission).
Most of us surely don't think of mosquitoes as being especially adept at learning. But researchers reporting in Current Biology on January 25 now show that mosquitoes can learn to associate a particular odor with an unpleasant mechanical shock akin to being swatted. As a result, they'll avoid that scent the next time.
"Once mosquitoes learned odors in an aversive manner, those odors caused aversive responses on the same order as responses to DEET, which is one of the most effective mosquito repellents," says Jeffrey Riffell at the University of Washington, Seattle. "Moreover, mosquitoes remember the trained odors for days."
It had been clear that mosquitoes don't decide whom to bite at random. They show obvious preferences for some people over others. They are also known to alternate hosts seasonally, feeding on birds in the summer and mammals and birds during other parts of the year, for instance. Riffell and his colleagues wanted to find out more about how learning might influence mosquitoes' biting preferences.
As a first step, they trained mosquitoes by pairing the odor of a particular person or animal species (a rat versus a chicken) with a mechanical shock. For the mechanical shock, they used a vortexer machine to simulate the vibrations and accelerations a mosquito might experience when a person tried to swat them. The insects quickly learned the association between the host odor and the mechanical shock and used that information in deciding which direction to fly. (Interestingly, they couldn't learn to avoid the smell of a chicken.)
Learning in many animals, from honeybees to humans, depends on dopamine in the brain. Additional study showed that dopamine is also essential in mosquito learning. Genetically modified mosquitoes lacking dopamine receptors lost the ability to learn.
The researchers also glued mosquitoes to a custom 3D-printed holder that allowed the insects to still fly in place while the activity of neurons in the olfactory center of their brains was recorded. Those experiments showed that without dopamine, those neurons were less likely to fire. As a result, mosquitoes became less able to process and learn from odor information.
The findings may have important implications for mosquito control and the transmission of mosquito-borne diseases, according to the researchers.
"By understanding how mosquitoes are making decisions on whom to bite, and how learning influences those behaviors, we can better understand the genes and neuronal bases of the behaviors," Riffell says. "This could lead to more effective tools for mosquito control."
With this new understanding of how mosquitos learn to avoid certain hosts, the researchers say they are now exploring mosquitoes ability learn and remember favored hosts. "In both cases, we think dopamine is a critical component," Riffell says.