Insecticide susceptibility characterization and efficacy of insecticides for thermal space spray application against insecticide-resistant Aedes aegypti (L.) from Chanthaburi Province

Abstract

The objectives of this study were to investigate insecticide susceptibility of larvae and adults and levels of enzymes associated with insecticide detoxification in adults of Aedes aegypti population from Moo 2, Tambon Patthawi, Amphoe Makham, Chanthaburi Province. Additionally, the efficacy of eight insecticide formulations used for thermal fogging application were evaluated under simulated field conditions against a laboratory insecticide-susceptible strain (NIH) and an insecticide-resistant field population of Ae. aegypti using the World Health Organization (WHO) cage bioassay method. Susceptibility testing using standard WHO methodology, showed Chanthaburi population larvae were highly resistant (13.5% mortality) to temephos (0.012 mg/L). Aedes aegypti female adults were also highly resistant to five synthetic pyrethroids: cyfluthrin 0.15%, deltamethrin 0.05%, cypermethrin 0.05%, lambdacyhalothrin 0.03%, and permethrin 0.25%, with final 24-hr mortality ranging between 0-7.8% and one organophosphate, pirimiphos-methyl 0.21% with 0% mortality. Biochemical assays of Chanthaburi adult Ae. aegypti had increased non-specific esterases and mixed function oxidases (MFO). Significant differences in elevated esterases and MFO activity were found between field population and NIH strain (P<0.001). While there was no significant difference in mean acetylcholinesterase enzymatic activities between field and reference mosqutioes. For indoor space spraying of pyrethroid (7 formulations) and organophosphate (1 formulation) insecticides applied by a handheld thermal fogger at the maximum WHO-recommended dosage, low mortality (0-6.7%) were observed in Chanthaburi population after exposure to each pyrethroid while very high mortality (99.7%) was observed for pirimiphos-methyl (200g/ha), indicating complete susceptibility. Furthermore, the efficacy of pirimiphos-methyl using four serial doses: 20, 50, 75, and 100 g/ha resulted in 13.7, 41.7, 54.8, and 97.7% final mortality, respectively. No significant difference was found in mosquito mortality between pirimiphos-methyl 100 and 200 g/ha (P>0.05). This study indicates that pirimiphos-methyl at an operational dosage of 100g/ha can be considered a viable alternative to control Ae. aegypti, especially in the areas where mosquitoes have developed resistance to pyrethroid insecticides.