Mosquitoes are pretty adaptable little creatures. Armed with receptors to detect the carbon dioxide plumes from animal breath and the odors of animal skin, the tiny bugs can track down the tastiest victims. But now, as Michelle Z. Donahue reports for National Geographic, researchers have found that mosquitoes seem to remember certain odors. And if these scents are associated with a near-death experience, like the swipe of a fly swatter, they will avoid the odor in the future.
The study, published last month in the journal Current Biology, suggests that the Aedes aegypti mosquito has the capacity to learn and remember (for short periods). Researchers at the University of Washington trained mosquitoes by pairing a mechanical shock created by a vortex mixer—a lab gadget usually used to mix vials of liquid—with the scent of certain animals, like rats or chickens. The mixer simulated the types of vibrations and feelings that might come from a good swatting. Each mosquito had 10 trials lasting 2 minutes each of the vibrating sessions.
As Joanna Klein reports for The New York Times, after 15 minutes, mosquitoes began to associate the scent of certain animals, including humans, with the shock. They began avoiding the odors associated with the simulated swat, choosing other hosts instead. In fact, the mosquitoes avoided the shock-related scents for at least 24 hours. But for some reason, they never learned to avoid the scent of chickens.
Christopher Potter, a researcher at Johns Hopkins Medicine who was not involved in the study tells George Dvorsky at Gizmodo that the results seem logical. “Yes, insects are extremely good at associating odors with whatever else might be going on when they smell something with that odor,” he says. He explains that the find is similar to classical memory experiments with fruit flies, which used electric shocks paired with puffs of different odors. “[T]he fly learned very quickly to avoid that particular odor," he says. "It is very plausible that different unpleasant aspects of the swat, such as the rushing feeling of the wind, the banging of the hand close to the mosquito, could then be remembered along with the smell of the person.”
The researchers also took a much deeper look into the brains of the insects. Previous research has shown that dopamine, a neurotransmitter, plays a key role in learning for other insects and animals. So in a second experiment, the team glued genetically modified mosquitoes that lack dopamine receptors to a device that allows them to fly in place. The researchers then exposed them to scents and the simulated swat while recording the activity of the neurons in the olfactory center of the insects' brains, which process scent information. Without the help of dopamine, the neurons fired less frequently with the scents, which suggests that the bugs would be less able to learn to avoid odors associated with swats.
So what does that mean in the age-old battle of man versus mosquito? “That learning ability makes them incredibly flexible,” UW neuroecologist and senior author Jeff Riffell tells Donahue. “It means they can learn associations about who is more defensive and who isn’t, and if we can prevent that, they’ll never learn and can be swatted away way more effectively.”
As Riffel tells Klein, the knowledge that mosquito learning relies on dopamine could lead to genetic modifications or insecticides that trigger the memory of avoidance, forcing the insects to avoid people. But any dopamine-based solution will not work on all mosquitoes. For instance, mosquitoes in the genus Culex, which transmit West Nile virus, have not shown the same ability to learn to avoid humans as Aedes. Because their main host is birds, they likely feed on humans when their preferred host is not available, Walter Leal, chemical ecologist at the University of California Davis, tells Donahue. Because of this, they likely don't have receptors for human scents and can’t learn to avoid people. In fact, different mosquitoes have different receptors for preferred hosts, which may be why Riffell’s little bloodsuckers didn't learn to avoid chickens.
The development of such dopamine-based mosquito deterrents would be a big step. “We’ve been using all these single-compound repellents like DEET for more than 60 years now, and we need to move on," Leal tells Donahue.
Until then, he suggests another strategy to take advantage of what we now know about the little blood suckers: vigorous swatting. It could teach the little hangers-on to leave you alone—at least for a little bit.