Synergistic activity of Cyt1A from Bacillus thuringiensis subsp. israelensis with Bacillus sphaericus B101 H5a5b against Bacillus sphaericus B101 H5a5b-resistant strains of Anopheles stephensi Liston (Diptera: Culicidae).

Integrated vector control is an effective and essential part of any successful vector control program. Increasing insecticide resistance requires strategies to prolong the use of highly effective vector control compounds. Synergistic activity between current effective pesticides is a powerful tool and is one such strategy. In the current study, Cyt1A from Bacillus thurigiensis subsp. israelensis and Bacillus sphaericus B101H5aH5b Bs were evaluated for the synergistic activity against B. sphaericus-resistant strains of Anopheles stephensi. The combinations of Cyt1A and B. sphaericus were found to act synergistically and were greatly enhanced at different ratios. A ratio of 1:4 of Cyt1A and Bs was 13,200-fold more toxic at LC(90) against the Bs-resistant strains of A. stephensi than was Bs alone and this high level of activity resulted from synergism between Cyt1A toxin and Bs. Our results therefore suggest that Cyt1A may enhance toxicity by facilitating the binding or insertion of the binary toxins to the microvillar membrane.