When you fail to land a fatal blow, you might wonder how much your swatting at buzzing mosquitoes really is doing to deter them. But new research indicates that swipes that don't find their target may not be a complete waste of time, as mosquitoes will associate the mechanical shock of a swat with that person's odor, teaching the insects that person is not someone to mess with.

If you've ever been eaten alive by mosquitoes while your friends get away unscathed, then the fact that the insects prefer the blood of some over others won't come as a surprise. Using this knowledge as a starting point, scientists at the University of Washington (UW) set out to learn more about mosquitoes' feeding habits, and how their learnings might influence their decision-making when dinnertime arrives.

This meant setting up an experiment where the critters would be subjected to mechanical shocks, just like the vibrations and sudden accelerations they experience when a person takes a swipe at them. These shocks were paired with particular odor, such as those of a rat, a chicken or a particular person, and delivered with a vortex mixer. The researchers found the bugs were fast to associate specific odors with the feelings of unwelcomeness, and the effect lasted for days at a 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," said senior author Jeff Riffell, a UW professor of biology. "Moreover, mosquitoes remember the trained odors for days."

A separate experiment uncovered one of the mechanisms behind this learned behavior. The neurotransmitter dopamine plays a central role in the reward humans feel from food, drugs and sex, and therefore the formation of habits. The researchers were able to demonstrate that it plays a role in the learnings of mosquitoes, too.

The team glued mosquitoes down inside a purpose-built "arena," in a way that enabled them to still "fly" while being stuck in place. Some were regular old mosquitoes, and others were genetically modified with CRISPR to suppress or eliminate the dopamine receptors. The team used sensors to monitor the activity of neurons in the olfactory center that controls the insect's sense of smell and found that in those without dopamine, these neurons were less likely to fire. As a result, those insects were less able to take lessons from the odor information.

These findings can have important ramifications, and not just for making summer barbecues more comfortable. There is no animal on the planet that kills more people than the humble mosquito, so scientists are always looking for new insights into how and why they go about their blood-sucking business. The UW researchers now think that dopamine plays a central role in how mosquitoes learn to love certain hosts and to avoid others.

"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," said Riffell. "This could lead to more effective tools for mosquito control."

The research was published in the journal Current Biology.

Source: University of Washington

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