Insecticide Immunity Solved From Inside Your Kitchen

A common mosquito landing on a person's fingers

    Insecticide resistance in bugs is becoming an increasingly prevalent issue as insecticide usage increases. From keeping bugs out of homes to preventing deadly diseases, the widespread use of insecticides has been on the rise over the past century. This increased usage has been incredibly good at stopping bugs from damaging people, property, and wildlife. The downside of using so many for so long is that bugs are starting to become resistant and even immune to certain types. Many researchers in the industry are dedicated to creating new insecticides to help curb this problem, but some crystallographers have found a possible solution to this daunting problem by using a simple home appliance; the microwave.

    Science News interviewed Bart Kahr, a researcher at New York University that was working on crystal growth experiments with colleagues. Over time, they found out that multiple insecticides, including DDT and deltamethrin, worked well for the proposed experiment because of their crystal structures. Both of these compounds have more than one crystal structure, which makes them interesting to experiment with. This happens when the crystals are heated, which causes the compounds to rearrange their crystal structures. While testing the effectiveness of the deltamethrin after heating, they found that the new composition worked faster at killing mosquitoes than the original. After telling some entomologists about this discovery, the team partnered up with the bug experts to test the efficacy of this new structure on mosquitoes that were already resistant to deltamethrin. The result was a 100% success rate in killing the mosquitoes. 

Line Structure of Deltamethrin

    This works because of the way that deltamethrin crystallizes. When left to its own devices at room temperature, crystals of this molecule will form in the way that scientists are used to, known as the form I structure. This is the same structure that is available as a consumer pesticide. When the form I crystals are melted and then cooled back to a solid, the crystals form differently than normal because of the alternative pathway to reaching the crystalline form. This results in a polymorph, known simply as form II, of the crystalline structure. They initially used an oven to melt the form I crystals to the target 120 °C, but later decided to try a microwave oven to see if they could get the same results. The common household appliance was just as successful at melting the form I crystals and later crystallizing into the form II crystals at 25 °C. Despite the marvel of the experiment's success, the scientists involved in the study advised against doing this with the same microwave that you heat your food with.

    The fact that this experiment was a success can spell good things for the future of disease prevention. This is by far the most important use of insecticides across the world, as many nations are still significantly impacted by malaria. Since this disease is mainly spread by mosquitoes, it's important to have insecticides around human civilizations to prevent the spread of this horrible disease. It's important not to get your hopes up about the situation, though. While this discovery is definitely important and very impactful, there are a lot of drawbacks. The form II crystals can only be used while in a solid state. If they get dissolved in water, they will revert back to the form I structure. This means that all liquid based sprays are out of the question for this new method. Most bug nets that contain deltamethrin have the compound inserted when the net is being made, and if they wanted to turn those crystals into the form II structure, it would require evenly heating the entire net at once. Since these nets get quite big, it's hard to ensure the possibility of this happening. Nonetheless, the discovery itself is important in understanding how insecticides affect bugs and understanding what steps can be taken in the future to help combat the increase in insecticide resistance.

References: 

Saey, T. H. (2023, May 21). Microwaving an insecticide restores its mosquito-killing power. Science News. Retrieved April 2, 2024, from https://www.sciencenews.org/article/mosquito-microwave-insecticide-spray

Yang, J., Erriah, B., Hu, C. T., Reiter, E., Zhu, X., López-Mejías, V., Carmona-Sepúlveda, I. P., Ward, M. D., & Kahr, B. (2023, January 6). A deltamethrin crystal polymorph for more effective malaria control. PNAS. Retrieved April 2, 2024, from https://www.pnas.org/doi/full/10.1073/pnas.2013390117#sec-1