Short-Term Selection to Diflubenzuron and Bacillus thuringiensis Var. Israelensis Differentially Affects the Winter Survival of Culex pipiens f. Pipiens and Culex pipiens f. Molestus (Diptera: Culicidae)

Simple Summary In Europe, Culex pipiens (Diptera: Culicidae) mosquito, the prime vector of West Nile virus, consists of two forms, named pipiens and molestus, that exhibit substantial differences in their biology, including overwintering behavior. Diflubenzuron (DFB) and Bacillus thuringiensis var. israelensis (Bti) are among the most widely used larvicides which pose major concerns for resistance development. In temperate areas, winter represents a very challenging period for the survival of many insects, including mosquitoes, and therefore potential fitness costs associated with insecticide selection may reduce their overwintering success. In this context, we investigated how short-term selection of Cx. pipiens f. pipiens and molestus forms to DFB and Bti affect their overwintering success. Our findings revealed that selection to both larvicides induced a high fitness cost in terms of reduced winter survival of Cx. pipiens f. molestus but not of pipiens form, suggesting potential differences in the persistence of the selected individuals in the wild from year to year. The Culex pipiens (Diptera: Culicidae) mosquito is of high medical importance as it is considered the prime vector of West Nile virus. In Europe, this species consists of two forms, named pipiens and molestus, that exhibit substantial differences in their overwintering biology. Diflubenzuron (DFB) and Bacillus thuringiensis var. israelensis (Bti) are two of the most used larvicides in mosquito control, including that of Culex pipiens. The high dependency on these two larvicides poses major concerns for resistance development. The evolution and stability of resistance to insecticides has been associated with fitness costs that may be manifested under stressful conditions, such as the winter period. This study investigated how short-term selection of pipiens and molestus forms to both larvicides affect their overwintering success. Larvae from each form were subjected to the same selective pressure (80% mortality) for three successive generations with DFB and Bti. At the end of this process, the winter survival between the selected populations and the controls (colonies without selection) was determined for each form. Selection to both larvicides significantly reduced the winter survival rates of molestus but not of pipiens form, indicating potential differences in the persistence of the selected individuals from year to year between the two forms.