Cross-resistance to Bacillus sphaericus strains in Culex quinquefasciatus resistant to B. sphaericus 1593M.

Bacillus sphaericus 1593M resistant larvae of Culex quinquefasciatus were reared in the laboratory since 1995. Resistance in the larvae was monitored by subjecting selection pressure using B. sphaericus 1593M at every generation. Bioassays were conducted with different strains of B. sphaericus (Bs 2297, Bs 2362 and Bs IAB 59) and confirmed cross-resistance in the present study. The level ranged between 27.3 to 18.2 fold in comparison with susceptible larvae. But Bacillus thuringiensis var israelensis strains (Bti PG14 and Bti 426) did not show any cross-resistance in the larvae and it emphasized a need to study the mode of action of B. sphaericus toxin that induces cross-resistance in the larval strain.