Lack of fitness cost in field-collected populations of <i>Musca domestica</i> with different susceptibilities to pyrethroids, neonicotinoids, and their combination

Abstract Background : The housefly is a significant sanitary pest affecting humans and domesticated animals. The application of synthetic insecticides remains the primary tool for managing housefly population outbreaks. However, insecticide resistance in housefly populations is a known major problem worldwide. Methods : Here, we used two field-collected populations of houseflies (POPUFLA and POPNEP) to assess the susceptibility of adults, larvae, and pupae stages to ten commercial formulations of insecticidal products. Then, we investigated the potential biochemical mechanisms underlying their differential susceptibilities using three detoxification enzyme inhibitors. Thereafter, we investigated fitness costs associated with such different susceptibilities to insecticides by constructing age-stage, two-sex, and fertility life tables. Results : Different toxicities of the tested insecticide formulations were found for larvae, pupae, and adult stages. Specifically, susceptibility levels to four insecticides (deltamethrin, lambda-cyhalothrin, thiamethoxam, and a mixture of thiamethoxam with lambda-cyhalothrin), representing two classes of insecticides (pyrethroids and neonicotinoids), varied between the two populations, suggesting resistance occurrence in POPNEP. Furthermore, near complete suppression of lambda-cyhalothrin resistance by piperonyl butoxide (PBO) suggested a CYP-mediated resistance; enhanced CYP and esterases activity is likely the main biochemical mechanism behind resistance to deltamethrin and the mixture of thiamethoxam with lambda-cyhalothrin, while partial reduction in resistance with PBO and diethyl maleate, and triphenyl phosphate implies that multiple mechanisms are responsible for resistance to thiamethoxam. Moreover, the analysis of different life-history traits, using an age-stage, two-sex, and fertility life tables, showed that none of the development, reproduction, and population parameters were affected, indicating that under laboratory conditions, no major fitness costs were found to be associated with the observed resistance. Conclusions : A better understanding of the resistance-related fitness cost can further improve resistance control strategies for M. domestica .