A newly identified crystalline polymorph (form II) of the insecticide deltamethrin, a leading compound for treating homes in areas where malaria is endemic, is significantly more efficient than the commercial version (form I), researchers report. The new form may continue to be effective even where malaria-transmitting mosquitoes have developed resistance. Michael D. Ward and Bart Kahr of New York University and coworkers found that heating deltamethrin to 110–120 °C and then cooling it to 25 °C results in a previously unknown crystal polymorph that consists of fine fibrils (Proc. Natl. Acad. Sci. U.S.A. 2020, DOI: 10.1073/pnas.2013390117). The time to immobilize 50% of mosquitoes is just 24 min with form II, compared with 282 min with form I. This difference is due to the kinetics involved when the mosquitoes take up the insecticide from the crystals. “The higher activity of form II implies that significantly less compound would be needed for the same level of prophylaxis, which in turn would reduce environmental impact and human exposure,” Kahr says via email. The researchers expect that users would be able to generate the more efficient polymorph in the field by melting and cooling the commercial insecticide. Alternatively, it may be possible to manufacture and stabilize form II, but such an approach has not yet been tested.