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Warming up deltamethrin can help kill pesticide-resistant mosquitoes

A few minutes in the microwave made a common insecticide about 10 times more lethal to mosquitoes in lab experiments.

Deltamethrin toxin is used worldwide in household aerosols and bed nets to curb the spread of mosquito-borne diseases such as malaria, which kills more than 400,000 people each year, according to the World Health Organization. But “mosquitoes around the world show resistance to deltamethrin and (similar) compounds,” says Bart Kahr, a crystallographer at New York University who helped develop a more potent form of deltamethrin by heating it.

This form of deltamethrin may have a better chance of killing insecticide-resistant pests, Kahr and colleagues reported online on Oct. 12 at the Proceedings of the National Academy of Sciences. Malaria has been essentially eradicated in the United States, but more effective pesticides could be a blessing to regions like sub-Saharan Africa, where the disease is a major public health problem.

Kahr’s team increased the potency of the commercial deltamethrin powder spray by simply melting a bottle of it, either by heating it to 150 ° C in an oil bath for five minutes or by putting it in a 700-watt microwave for the same amount of time. While the microscopic deltamethrin crystals in the original spray have a haphazard structure, which looks like a lot of misaligned flakes, the molten deltamethrin crystals solidified into bursting forms when cooled to room temperature.

The deltamethrin crystals in the typical insecticide spray (left microscope image) contain "many individual leaflets that are helter-skelter-oriented," says New York University crystallographer Bart Kahr. In a new version of the spray, deltamethrin crystals are more star-shaped, with fibers growing from a single point (right).Jingxiang Yang

The chemical bonds between deltamethrin molecules in burst-shaped crystals are not as strong as those of the original microcrystalline structure. “Molecules are inherently less happy or settled in the arrangement,” Kahr says. So when a mosquito lands on a burst-shaped crystal dust, it should be easier for deltamethrin molecules to be absorbed into the insect’s body by its feet.

Researchers have tested the most potent version of deltamethrin in laboratory-raised mosquitoes of two species: Anopheles quadrimaculatus, which can spread malaria, and Aedes aegypti, which can transmit other life-threatening diseases, such as Zika and dengue (SN: 1/8/19). Forty mosquitoes of each species were released on Petri dishes coated with the original deltamethrin powder spray and another 40 on a plate covered with the new form of insecticide.

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That altered version of deltamethrin eliminated half of the exposed mosquitoes of A. quadrimaculatus in 24 minutes. In contrast, the original spray took almost five hours to remove half of the exposed anopheles, about 12 times as long. Similarly, the new spray took only 21 minutes to remove half of the exposed A. aegypti, while the original spray took three hours.

Although A. quadrimaculatus can carry the parasite that causes malaria, this species of mosquito is native to North America, where the disease is not a major public health crisis. To ensure that the new type of deltamethrin would be effective in the world’s malaria hotspots, “we need to do these experiments with species called gambiae and funestus, which are the species of African Anopheles mosquitoes,” says Kahr, as well as the six major malaria – Anopheles species extending into South Asia.

Janet Hemingway of the Liverpool School of Tropical Medicine in England says heat treatment of deltamethrin sprays "may increase their toxicity, but there are several obvious experiments we would have to do before we thought about adding it to the production system." . which studies insecticide resistance against mosquitoes.

First, researchers need to test the new version of the insecticide against pesticide-resistant mosquitoes. Mosquito resistance to deltamethrin, along with other chemicals in the class of synthetic pesticides known as pyrethroids, is a growing problem (SN: 29/06/12). “My prediction … is that (insects) would be highly resistant to both forms,” Hemingway says.

Researchers should also make sure that the most toxic form of deltamethrin is safe for people around it, says Hemingway, who did not participate in the study. "Summary: Interesting observation, but one that is a good distance from something that could be implemented."

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