Mosquitoes may seem like small, unassuming insects. But, in reality these tiny blood suckers spread a number of dangerous diseases including malaria and dengue, making them one of the deadliest animals on the planet. From 2010-2019, over half a million people died each year from malaria alone. That doesn’t include the death toll of any other diseases from the unfortunately long list of potentially life-threatening illnesses mosquitoes can vector.

Given their public health impacts, managing mosquito populations is critical. Particularly those species that spread malaria. But mosquitoes are quick to develop resistance to many of the insecticides commonly used for control. This is particularly true when it comes to pyrethroids. However, one group of researchers may have found an interesting way to “breathe new life” so to speak back into a commonly used active ingredient that malaria-transmitting mosquitoes are known to have some resistance too.

 

 

Deltamethrin is widely used pyrethroid to control many dangerous mosquito species including Anopheles quadrimaculatus¸ a common malaria-vector in the US. Due to the long-time use of deltamethrin, resistance among many mosquito species is well documented. Trying to develop new active ingredients to overcome the resistance issue can be incredibly expensive and time consuming. So, rather than try to “reinvent the chemistry wheel,” researchers may investigate ways to enhance the effectiveness of an existing active ingredient. Which is exactly what Jessica Carson and her colleagues were able to do.

In a previous study published in 2020, Bart Kahr and colleagues determined that heating deltamethrin in an oven changed its crystalline structure enough to increase the knockdown time and potency of the insecticide by a factor of up to 12 compared to using the original unheated form. An important note here was that the researchers used susceptible mosquitoes in this study, meaning the mosquitoes exposed to the heated deltamethrin were already known to be sensitive to the insecticide.

In a continuation of this work, Kahr teamed up with Jessica Carson and the other authors of the most recent 2023 study that is the focus of this summary to determine how heating Deltamethrin would impact the effectiveness of the insecticide when exposed to DELTAMETHIRN RESISTANT mosquitoes. In this study they used a commercially available, powdered formulation of deltamethrin. And, because entomologists are wild and crazy folks, they decided to literally throw the stuff in the microwave to see if that achieved the same result as heating the powder to a prescribed temperature in an oven. Turns out, the microwave worked just as well! The researchers ultimately exposed the heated deltamethrin to five different strains of resistant Anopheles mosquitoes and found that the heated insecticide killed the resistant mosquitoes.

On the one hand, these results are incredibly exciting because this could pave the way for an easy-to-use technique to enhance the effectiveness of deltamethrin (and maybe even other insecticides) to resistant mosquitoes. Or even other resistant pest groups!

Butthere are a handful of important caveats and disclaimers here to run through. Some by the authors, and some by me personally. First, the researchers used a controlled laboratory environment to microwave their insecticide. This wasn’t your microwave in the breakroom that your colleague just used to heat up their burrito. They had a dedicated machine used for this purpose alone. Second, they followed a handful of other safety protocols when conducting this study so…basically, don’t go thinking you can just start zapping your products in the microwave to fix your next tough-to-control pest issue.

Another consideration here is that many current commercially uses for deltamethrin to control mosquitoes, such as cotton bed nets impregnated with the AI, may not lend well to heat treatments. Though, the researchers did suggest some methods of application that could work around these considerations that seem to be promising for the near future. At any rate, this is an exciting discovery and will seemingly lead to new important developments in the fight against insecticide-resistant mosquitoes and perhaps other pests as well.

Article by Mike Bentley, PhD, BCE

References

Carson, J., Erriah, B., Herodotou, S. et al. Overcoming insecticide resistance in Anopheles mosquitoes by using faster-acting solid forms of deltamethrin. Malar J 22, 129 (2023). https://doi.org/10.1186/s12936-023-04554-x 

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