Efficacy of Bacillus thuringiensis Serovar Israelensis against Bacillus sphaericus Resistant and Susceptible Larvae of Culex quinquefasciatus Say

A potential biological larvicides like Bacillus sphaericus (Bs) has been advocated in mosquito control operations. However, due to rapid development of resistance of mosquitoes to Bs toxin alternate mosquito control measures are needed. A strain of Culex quinquejasciatus Say was found highly resistant (resistance ratio (RR) = 6223 folds) in the field (Gandhinagar, Kochi, Kerala) during the year 1995 and was reared in the laboratory by subjecting to moderate selection pressure using Bs 2362 strain to maintain resistance. A susceptible mosquito colony of C. quinquejasciatus was also reared simultaneously. Bioassays were conducted with lyophilized bacterial culture of spore/crystal toxin of Bs 2362 with the inbred generations obtained from these colonies. We found that a high level of resistance to Bs (2362) toxin (RR at LC⁵⁰ = 249.2 and at LC⁹⁰ Bacillus thuringiensis serovar israelensis (BIi) (VeetoBac*). This observation suggest that Bli based biopesticide can be used for the management of Bs resistant mosquito control operations.