Malaria control is under threat by the development of vector resistance to pyrethroids in long-lasting insecticidal nets, which has prompted calls for a return to the notorious crystalline contact insecticide DDT. A faster acting difluoro congener, DFDT, was developed in Germany during World War II, but in 1945 Allied inspectors dismissed its superior performance and reduced toxicity to mammals. It vanished from public health considerations. Herein, we report the discovery of amorphous and crystalline forms of DFDT and a mono-fluorinated chiral congener, MFDT. These solid forms were evaluated against Drosophila as well as Anopheles and Aedes mosquitoes, the former identified as disease vectors for malaria and the latter for Zika, yellow fever, dengue, and chikungunya. Contact insecticides are transmitted to the insect when its feet contact the solid surface of the insecticide, resulting in absorption of the active agent. Crystalline DFDT and MFDT were much faster killers than DDT, and their amorphous forms were even faster. The speed of action (a.k.a. knockdown time), which is critical to mitigating vector resistance, depends inversely on the thermodynamic stability of the solid form. Furthermore, one enantiomer of the chiral MFDT exhibits faster knockdown speeds than the other, demonstrating chiral discrimination during the uptake of the insecticide or when binding at the sodium channel, the presumed destination of the neurotoxin. These observations demonstrate an unambiguous link between thermodynamic stability and knockdown time for important disease vectors, suggesting that manipulation of the solid-state chemistry of contact insecticides, demonstrated here for DFDT and MFDT, is a viable strategy for mitigating insect-borne diseases, with an accompanying benefit of reducing environmental impact.