Heating deltamethrin can kill pesticide-resistant mosquitoes

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A few minutes in the microwave made a common insecticide used in laboratory experiments for mosquitoes about ten times more lethal.

The toxin deltamethrin is used in home sprays and bed nets around the world to help curb the spread of mosquito-borne diseases such as malaria. According to the World Health Organization, over 400,000 people die every year. 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 through heating.

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

Kahr’s team increased the effectiveness of commercial deltamethrin dust spray simply by melting a vial – either by heating it to 150 ° Celsius for five minutes in an oil bath or by simultaneously immersing it in a 700-watt microwave. While the microscopic deltamethrin crystals in the original spray have a random structure that looks like a jumble of misaligned flakes, the melted deltamethrin crystals solidified into starburst shapes upon cooling to room temperature.

The deltamethrin crystals in a typical insecticide spray (left microscope picture) contain “many individual leaflets that are oriented in a helter-skelter fashion,” says crystallographer Bart Kahr of New York University. In a new version of the spray, delta methrin crystals are shaped more like starbursts, with fibers growing out of a single point (right).Jingxiang Yang

The chemical bonds between deltamethrin molecules in the starburst-shaped crystals are not as strong as in the original micro-crystal structure. “The molecules are inherently less fortunate or less fortunate,” says Kahr. When a mosquito lands on a dust of starburst-shaped crystals, deltamethrin molecules should be more easily absorbed into the insect’s body through its feet.

The researchers tested the more effective version of deltamethrin on laboratory-grown 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 in petri dishes coated with the original deltamethrin dust spray and another 40 in a dish covered with the new form of the insecticide.

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This modified version of deltamethrin eliminated about half of the exposed A. quadrimaculatus mosquitoes within 24 minutes. In contrast, it took nearly five hours for the original spray to knock out half of the exposed Anopheles – about twelve times as long. Likewise, it only took 21 minutes for the new spray to knock out half of the exposed A. aegypti, while the original spray lasted over 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 is effective in the world’s malaria hotspots, “we need to conduct these experiments with species called the Gambiae and Funestus, which are the African Anopheles mosquito species,” says Kahr and the six main malaria species Distribution of Anopheles species in South Asia.

Heat treatment for deltamethrin sprays “could increase their toxicity, but there are some obvious experiments that we would need to do before we even think about adding this to the production system,” says Janet Hemingway of the Liverpool School of Tropical Medicine in England. Who studies mosquito insecticide resistance?

First, the researchers have to test the new version of the insecticide against pesticide-resistant mosquitoes. Resistance of mosquitoes to deltamethrin, as well as other chemicals in the class of synthetic pesticides known as pyrethroids, is a growing problem (SN: 6/29/12). “My prediction … is that [the insects] would be very resistant to both forms, ”says Hemingway.

Researchers also need to make sure that the more toxic form of deltamethrin is safe for humans, says Hemingway, who was not involved in the study. “Conclusion – interesting observation, but one that is far from anything that could be implemented.”

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