Evolution of resistance toward Bacillus sphaericus or a mixture of B. sphaericus + Cyt1A from Bacillus thuringiensis, in the mosquito, Culex quinquefasciatus (Diptera: Culicidae)

The 2362 strain of Bacillus sphaericus (Bs) Neide is a highly mosquitocidal bacterium used in commercial bacterial larvicides primarily to control mosquitoes of the genus Culex. Unfortunately, Bs is at high risk for selecting resistance in mosquito populations, because its binary toxin apparently only binds to a single receptor type on midgut microvilli. A potential key strategy for delaying resistance to insecticidal proteins is to use mixtures of toxins that act at different targets within the insect, especially mixtures that interact synergistically. We tested this hypothesis for delaying the phenotypic expression of resistance by exposing Culex quinquefasciatus Say larvae to Bs alone or in combination with Cyt1A from Bacillus thuringiensis subsp. israelensis. Two laboratory lines of Cx. quinquefasciatus, one sensitive to Bs and the other containing Bs resistance alleles, were subjected to intensive selection pressure for 20 generations with either Bs 2362 or a 3:1 mixture of Bs 2362 + Cyt1A. At the end of the study, the sensitive line had evolved >1000-fold resistance when selected with Bs alone, whereas the parallel line selected with Bs + Cyt1A exhibited only low resistance toward this mixture (RR95, 1.4). Similar results were observed in the lines containing Bs resistance alleles. Both lines selected with Bs + Cyt1A exhibited substantial resistance to Bs in the absence of Cyt1A. Although selection with Bs + Cyt1A did not prevent the underlying evolution of resistance to Bs, these results suggest that a mixture of Bs with other endotoxins, particularly one like Bs + Cyt1A in which the components interact synergistically, will provide longer lasting and more effective mosquito control than Bs alone.