Variations in the response to Bacillus sphaericus toxins in different strains of Anopheles stephensi Liston

Bacillus sphaericus has a potential of use as a larvicide in water storage practices but no such study has been done against Anopheles stephensi. Baseline susceptibility status of eleven wild and three mutant strains of An. stephensi to Spherix, a formulation of B. sphaericus, strain B-101 (serotype H5a, 5b) was determined to find the variations in the degree of their response to B. sphaericus toxins and to study the possibility of development of resistance. The LC50 and LC90 values of different strains of An. stephensi to B. sphaericus formulation varied from 0.088 to 1.42 mg/l and 0.314 to 10.98 mg/l, respectively. Among all the strains tested, Sarojini Nagar, Delhi strain of An. stephensi (a wild type strain) was least susceptible. Laboratory selection of Sarojini Nagar strain of An. stephensi with B. sphaericus at a concentration of 4 mg/l resulted in the development of a high degree of resistance (LC50 > 1600 mg/l) to B. sphaericus within four generations.