From SCIENCE MAGAZINE:
Antimalaria pesticide may have backfired, helping mosquitoes find love Poison appears to have caused mutation that helps insects home in on mates in noisy cities
4 AUG 20261:00 PM ETBYANIRBAN MUKHOPADHYAY
It’s a classic case of evolution in action. Beginning in the 1950s, malaria control programs sprayed a powerful insecticide known as dieldrin on the inside walls of homes, where Anopheles mosquitoes—the insects that spread the disease—often rested. Within a few years the bugs had developed a genetic mutation that made them impervious to the poison.
Dieldrin was eventually abandoned as resistance spread and concerns mounted over its toxicity. But the story didn’t end there.
Decades later, the mosquitoes have held onto one of the mutations that helped them survive the pesticide, despite the fact it makes them less active. A new study may explain why: The mutation appears to make Anopheles mosquitoes more sensitive to sound, which may make it easier for them to home in on mates in noisy African cities, researchers report in a preprint posted late last month on bioRxiv. What once helped the insects avoid death may now be helping them find mates.
It’s “a fascinating new hypothesis,” says Francesco Baldini, a vector biologist at the University of Glasgow who was not involved with the work. The study, he says, “connects insecticide resistance, behavior, and sensory ecology in a way that has not really been considered before.”
The find emerged from two seemingly unrelated observations. Marta Andrés, a neurobiologist at the Spanish National Research Council, had found in earlier lab experiments that Rdl, the gene in which dieldrin-resistance mutations occur, dampens the activity of mosquito auditory neurons and their responses to sound. Blocking Rdl’s activity made those neurons more responsive. Then, during conversations with Diego Ayala, a vector ecologist at the University of Montpellier, Andrés learned that field observations had shown the Rdl variant was distributed differently between urban and rural populations across sub-Saharan Africa.
Male mosquitoes find mates by homing in on the faint whine of female wing beats. Andrés’s team wondered whether the mutation might sharpen that ability.
In the lab, Andrés, Ayala, and colleagues played sounds mimicking female flight tones. At frequencies of 300–550 hertz, the range corresponding to female wing beats, males with the pesticide resistance mutation were significantly more likely than males without it to fly toward and land on the speaker, suggesting greater sensitivity to those sounds.
But detecting females is not the same as mating with them. So the researchers placed 25 males in small cages with 25 females, then kept them for 48 hours in one of two incubators—one whose machinery was humming at about 70 decibels, the other filled with continuous white noise at about 93 decibels to simulate intense urban noise. Dieldrin-resistant males’ mating success was about the same under both conditions—about 40%—but susceptible males did much worse in the noisier incubator, with their mating success dropping from about 42% to 33%.
The finding is intriguing, yet puzzling, says Martin Göpfert, a neurobiologist at the Georg August University of Göttingen who was not involved with the study. If resistant mosquitoes hear better, he says, he would expect them to avoid noisy environments rather than thrive there. “If I would have very sensitive hearing, I wouldn’t go in a loud disco, yeah?”
The puzzle may reflect how little is known about mosquito hearing, says Joerg Albert, a neurobiologist at University College London who also was not involved with the study. The insects detect air-particle movement near a sound source, not the pressure waves that humans hear, he notes, making it difficult to define “noise” from conventional sound measurements. Truly understanding mosquito acoustics, he says, will require direct experiments on the hearing organs in the mosquitoes’ antennae, not just behavioral observations.
The researchers also tested their hypothesis in the field. Across seven urban sites in Bangui, the capital of the Central African Republic, and four nearby villages, the frequency of the mutation increased with environmental noise. Females lacking the mutation were also less likely to have mated at noisy urban sites than in nearby rural villages.
Still, the field data remain difficult to interpret because urban habitats differ from rural ones in many ways besides noise, says Lauren Cator, an evolutionary biologist at Imperial College London who was not part of the work. Insecticide exposure, pollution, and differences in temperature and humidity may also help maintain the mutation at higher levels in cities, she notes. The study did not directly test whether the mutation alters female hearing or mate choice, Cator points out, making that part of the proposed mechanism more speculative.
Irrespective of the factors that explain the mutation’s persistence, Göpfert believes studying resistance mutations that linger long after a pesticide is gone is important as it could have profound consequences. “You use an insecticide for a while, a chemical, and you really change the genetics, and that means the physiology of an animal,” he says. “It really shifts the balance in the ecosystem.”
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