Overcoming Insecticide Resistance in Anopheles Mosquitoes by Using Faster-acting Solid Forms of Deltamethrin

Abstract

Background: Controlling malaria-transmitting Anopheles mosquitoes with pyrethroid insecticides is becoming increasingly challenging because of widespread resistance amongst vector populations. The development of new insecticides and insecticidal formulations is time consuming and costly, however. A more active crystalline form of deltamethrin, prepared by heating the commercial crystalline form, previously was reported to be 12-times faster acting against susceptible North American Anopheles quadrimaculatusmosquitoes. Herein we investigate whether heat-activated deltamethrin dispersed on chalk can overcome various resistance mechanisms amongst five West African Anophelesstrains and evaluate its long-term sustained lethality. Methods: The more active deltamethrin form was generated in a commercial dust containing deltamethrin by heating the material as purchased. Tarsal contact bioassays were conducted to investigate its efficacy, potency, and speed of action against resistant Anophelespopulations compared to the commercially available form of deltamethrin dust. Results: In all cases, D-Fense Dust heated to generate the more active form of deltamethrin was substantially more effective than the commercially available formulation. 100% of both Banfora and Kisumu populations were knocked down 10 minutes post-exposure with no recovery afterwards. Gaoua-ara and Tiefora strains exhibited 100% knockdown within 15 minutes, and the VK7 2014 strain exhibited 100% knockdown within 20 minutes. In all cases, 100% mortality was observed 24 hours post-exposure. Conversely, the commercial formulation (unheated) resulted in less than 4% mortality amongst VK7, Banfora, and Gaoua-ara populations by 24 hours, and Tiefora and Kisumu mosquitoes experienced 14 and 47% mortality by 24 hours, respectively. The heat-activated dust maintained comparable efficacy 6 months after heating. Conclusions: The heat-activated form of commercial deltamethrin D-Fense Dust outperformed the material as purchased, dramatically increasing efficacy against all tested pyrethroid-resistant strains. This increase in lethality was retained for six months of storage under ambient conditions in the laboratory. Higher energy forms of commonly used insecticides may be employed to overcome various resistance mechanisms seen in African Anopheles mosquitoes through more rapid uptake of insecticide molecules from their respective solid surfaces. That is, resistant mosquitoes can be killed with an insecticide to which they are resistant without altering the molecular composition of the insecticide.

Chlorfenapyr Crystal Polymorphism and Insecticidal Activity

“…Space group determination was performed using the XPREP utility included in the SHELXTL software package. 21 Using Olex2, 22 the structure was solved with SHELXT 17 using intrinsic phasing and refined with SHELXL 18 using least-squares minimization (full-matrix least-squares on F 2 ). Transmission electron microscopy (TEM) was used to search for new, metastable polymorphs.…”

“…10,11 The role of polymorphism in the action of pesticides and insecticides has become evident only recently. 12,13 An inverse correlation between lethality of contact insecticides and thermodynamic stability of crystal polymorph has been found in several contact insecticides, including DDT, 14 lindane, 15 deltamethrin, 16,17 and imidacloprid. 18 The use of a more lethal polymorph of a contact insecticide can overcome resistant organisms.…”