The secret weapon mosquitoes are using to win the war against humans: evading insecticides

Humans have been fighting a tiny animal for more than a century in an arms race that we had been winning. But things went wrong a decade ago. In 2015 cases of malaria —one of the diseases spread by mosquitoes—hit a historic low. Since then, however, figures have plateaued. Part of this inflection is explained by climate change, which has carried virus‑carrying mosquitoes to places where they were never seen before. But an unexpected advantage has also consolidated. The insecticides used since the 1970s to control mosquitoes have become less effective. These insects are evolving to evade them. Two recent studies explain how.

Researchers in India examined resistance mechanisms to α‑cypermethrin, a widely used insecticide. In the study area, mosquitoes showed a mortality rate of 97.9% when exposed to the insecticide—right at the World Health Organization’s threshold, below 98%, for testing whether resistance is emerging. That was the approach taken by a team led by Sarita Kumar, professor in the Department of Zoology at the University of New Delhi. “Mosquitoes that survived exposure to the insecticide showed an increase in detoxifying enzyme activity,” Kumar said in a message exchange. Her analysis, published this week in the journal Frontiers in Tropical Diseases, reveals the mechanism by which mosquitoes are developing immunity.

“When an insecticide enters a mosquito’s body, it activates a cellular alarm system. That triggers a cascade of responses in the insect’s cells and ramps up production of defensive proteins,” Kumar explained. The problem with using the same class of insecticide every year is that the most resistant individuals survive, reproduce and pass that trait to new generations. A 3% survival rate can grow exponentially in just a few years. And this is happening not only in India but worldwide. Previous studies in countries such as Mexico, Ecuador, Peru and Colombia have reported varying levels of resistance to α‑cypermethrin. Mosquitoes are adapting to our insecticides; they are learning to dodge them. And only now are we beginning to understand how.

“Mosquitoes have short life cycles and large populations, which allows beneficial traits to spread rapidly when the same insecticides are used repeatedly,” Kumar said. They have an astonishing capacity to adapt and, because their survival depends on humans, they quickly adjust to our changes. The most striking example of that ability exists underground. In the little more than 160 years since the London Underground opened, a new blood‑feeding mosquito species has emerged. The new species was named Culex pipiens molestus. Its closest relatives, the common mosquitoes or Culex pipiens, overwinter in cold weather and prefer bird blood to human blood. But molestus breeds year‑round because underground temperatures remain warm, and it has specialized in feeding on human blood. A similar process has been observed in the subway systems of New York and Moscow.

The mosquito mutates to adapt to our habits, and the next change is happening before our eyes, in our gardens. Another recent study from the University of North Carolina monitored the local population of the tiger mosquito from 2016 to 2024 and observed how they began to develop mutations that made them resistant to pyrethroids, another class of insecticide. The first resistant individuals did not appear until 2018. By 2024, 39% of the population carried a genetic mutation that helped negate the insecticide’s effects. One of the study’s most curious details was the difference in that percentage between mosquitoes collected in wealthy neighborhoods and those from poorer areas. In more affluent neighborhoods—where residents were presumably more able to afford garden fumigation—resistance was markedly higher than in more humble areas. “What is most striking about this study is not so much the speed of evolution as the mosaic pattern observed,” said Martha Burford Reiskind, a biologist at the University of California and the study’s lead author, in a message exchange.

Burford says this information should make us more cautious in how we use insecticides. “One of the main implications of this study is that homeowners who spray to enjoy their gardens and reduce daytime biting insects could be contributing to the development of resistance,” she said. “The problem is that we need populations of mosquitoes susceptible to insecticides in case an outbreak occurs.”

It’s not only insecticides that are beginning to fail; there are also signs that repellents could lose effectiveness against this troublesome insect. A study published in May in the Journal of Experimental Biology suggests that, much like Pavlov’s dogs learned to associate a bell with food, mosquitoes can learn that the presence of repellents signals an opportunity to feed.

In the experiment, more than 60% of trained insects preferred skin treated with DEET, a common repellent, over untreated skin. The insects relied on prior experiences, remembering that that odor predicted fresh blood. The study’s authors emphasized that these results occur only under specific laboratory conditions. It is unlikely that wild mosquitoes would change their responses based on prior experience, because a single insect typically encounters different repellents during its short life.

The experiment, however, helps explain the remarkable capacity for adaptation and learning these insects possess. It sheds light on the powerful strategies they can develop to continue this quiet interspecies war we have been waging for centuries.

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