Resistance development to diflubenzuron and Bacillus thuringiensis subsp. israelensis of Culex pipiens F. pipiens and Culex pipiens F. molestus (Diptera: Culicidae) and its effects on their winter survival

The Culex pipiens mosquito includes two biotypes named pipiens and molestus that exhibit substantial differences in their biology including overwintering. Diflubenzuron (DFB) and Bacillus thuringiensis subsp. israelensis (Bti) are the most widely used larvicides for controlling Cx. pipiens populations. 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. The aim of the present study was to explore the resistance development of pipiens and molestus biotypes to both larvicides and its potential fitness costs on their winter survival. Colonies of both forms of Cx. pipiens were selected for three successive generations by applying fixed doses corresponding to IE₈₀ (Inhibition of adult Emergence) and LC₈₀ (Lethal Concentration) for DFB and Bti respectively. At the end of this process, the resistance levels and the winter survival of the selected populations relative to controls (not selected colonies) were determined. Contrary to Bti, selection with DFB induced different levels of resistance between the two forms of Cx. pipiens. The selected populations of Cx. pipiens molestus to both larvicides exhibited a high fitness cost in terms of reduced winter larval survival rates relative to control. On the other hand, selection with both DFB and Bti had no apparent effects on Cx. pipiens pipiens female winter survival rates. Our findings are expected to contribute on the better understanding of the resistance development and evolution of Cx. pipiens different biotypes to DFB and Bti.