Development of a high level of resistance to Bacillus sphaericus in a field population of Culex quinquefasciatus from Kochi, India.

Field resistance to Bacillus sphaericus was observed in a population of Culex quinquefasciatus in Kochi, India, exposed to 35 rounds of spraying with a formulation of B. sphaericus 1593 M over a 2-year period. Larvae from the sprayed area gave LC50 and LC90 values that were 146 and 180 times greater than corresponding values for a susceptible strain from an unsprayed locality. When the resistant strain was colonized in the laboratory and subjected to moderate selection pressure at each generation, resistance rapidly increased and by the 18th generation it was 6,223 and 31,325 times greater at the LC50 and LC90 levels in comparison with the susceptible strain. There were no significant differences among 6 susceptible strains tested. Tests were repeated and validated using the standard primary powder SPH88, B. sphaericus 2362. No cross resistance was observed against B. thuringiensis H-14.