Mosquitoes Are Adapting to Evade Pesticides, New Examine Reveals

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It’s easy to assume that you can take out mosquitoes effortlessly with a little pesticide, but a surprising new study found that the little blood-suckers are smarter at evading this protective measure than most people realize.

The study, which was published in the journal Scientific Reports, discovered that female mosquitoes learn to avoid pesticides after a single exposure that doesn’t kill them.

For the study, researchers exposed female Aedes aegypti and Culex quinquefasciatus mosquitoes (which spread dengue, Zika virus, and West Nile fever) to non-lethal doses of common pesticides malathion, propoxur, deltamethrin, permethrin, and lambda-cyhalothrin. The scientists then tested how the mosquitoes reacted when they were exposed to the same pesticide again in the future.

The researchers discovered that the mosquitoes that had been exposed avoided moving through a pesticide-treated net to reach a food source at a higher rate than those who hadn’t been exposed. The difference was pretty impressive: Only 15.4% of Aedes aegypti and 12.1% of Culex quinquefasciatus mosquitoes that were previously exposed to pesticides passed through the net to get food, compared to 57.7% of Aedes aegypti and 54.4% of Culex quinquefasciatus mosquitoes that weren’ t previously exposed.

As a result, the mosquitoes that had been exposed to pesticides in the past were more than twice as likely to survive a repeat attempt to fend them off with pesticides than those who hadn’t seen the pesticides before.

Study co-author Frederic Tripet, Ph.D., a professor in medical and molecular entomology at Keele University, says that he and his fellow researchers were building off of a previous study they did that showed mosquitoes are capable of associating memories with certain objects , cues, and smells. He says the latest research shows that “we have underestimated behavioral changes and learning capabilities in mosquitoes.”

“We now formally know that this is a problem,” Tripet says. “It’s really a change in our understanding of mosquito intelligence.”

Why might mosquitoes learn to avoid pesticides?

While Tripet’s work didn’t specifically look at why the previously exposed mosquitoes avoided pesticides, he says that learning behaviors like this are just part of animal evolution.

“It’s our understanding that mosquitoes have developed this recently,” he says. “Learning has been part of their response to pesticides since we started using them.”

Nancy Troyano, Ph.D., a board-certified entomologist and director of operations education and training for Ehrlich Pest Control, agrees. “Insects, like so many other living organisms, are capable of learning what is harmful to them if they were able to survive an initial encounter,” he says. “This avoidance behavior based on prior exposure has also been demonstrated in other insects, such as cockroaches.”

Mosquitoes that learn to avoid pesticides that don’t kill them will go on to survive longer, and breed more, raising the odds that you’ll face even more of these smart mosquitoes in the future, Tripet says.

What does this mean for fending off mosquito bites?

Tripet says his findings are a “game-changer” for trying to figure out better pesticides. “We should definitely count on mosquitoes learning, unless the pesticides are so good they kill the mosquito on the first encounter,” he says.

Mosquitoes that are able to fend off pesticides on the first round will either move on to the nearest person or animal or “just bite you later” if you’re using pesticides on your clothing, a mosquito net, or around your home, Tripet says .

If you use repellent on your skin, though, it’s more likely they’ll go elsewhere to feed, says Ben Hottel, technical services manager at Orkin.

If you’re applying pesticides around your home, Troyano says it’s important to follow the directions carefully to ensure that the mosquitoes are taken out the first time. “Misapplication of pesticides is a common way to cause sub-lethal exposures—primarily due to wrong dilution of pesticide concentrates and the over/under application of the chemical,” she says.

But pesticides are just one way to get rid of mosquitoes. “It is also important to use an integrated approach to controlling mosquitoes,” Troyano says. She also recommends taking steps to eliminate mosquito breeding sites by getting rid of standing water around your home, and getting cutting back tall grass and weeds, which can harbor mosquitoes.

The study findings don’t mean that pesticides are useless against mosquitoes, Hottel says. “If you use pesticides properly, there is data to show that they will be effective,” he says. Hottel also notes that the study was conducted in a lab, and that the results might be different in the wild. “Ultimately, the goal is to reduce people getting bitter in their yards when you use these pesticides outside,” he says. That still can happen, whether you deter the mosquitoes or kill them, he adds.

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