Resistance to Bacillus sphaericus involves different mechanisms in Culex pipiens (Diptera: Culicidae) larvae

Field Culex pipiens pipiens (L.) mosquitoes that were collected after a control failure with Spherimos in southern France developed high resistance (>10,000-fold) to Bacillus sphaericus crystal toxin after <8 generations of laboratory selection. We show that this resistance is encoded by a single major recessive gene on linkage group I at 22.1 recombination units from the sex locus, and that it is not associated with any loss,of binding affinity between brush border membrane fractions and the B. sphaericus radiolabeled toxin. Thus, in Southern France, resistance differs from the high B. sphaericus resistance developed after laboratory selection of Californian C. p. quinquefasciatus. This demonstrates that at least 2 different mechanisms may confer high levels of resistance to B. sphaericus crystal toxin in mosquitoes of the C. pipiens complex. These results have important implications for mosquito control strategies.