Cross‐resistance between strains of Bacillus sphaericus but not B. thuringiensis israelensis in colonies of the mosquito Culex quinquefasciatus

Two colonies of Culex quinquefasciatus Say (Diptera: Culicidae) were selected with Bacillus sphaericus strains C3-41 and IAB59 in the laboratory for 13 and 18 generations; they attained 145,000- and 48.3-fold resistance, respectively, in comparison with a susceptible laboratory colony (SLCq) and showed very high levels of cross-resistance (8500- to 145,000-fold) to B. sphaericus strains C3-41, 1593, 2297 and 2362. They were relatively susceptible to B. sphaericus strains LP1-G and 47-6B (only 0.8- to 2.8-fold tolerance), with 24.8- to 48.3-fold cross-resistance to strain IAB59. B. sphaericus-resistant mosquito colonies remained highly susceptible to B. thuringiensis israelensis, suggesting that B.t.i. would be of value in the management of B. sphaericus-resistant Cx. quinquefasciatus colonies. The demonstration of low or no cross-resistance of two selected resistant Cx. quinquefasciatus colonies to IAB59, LP1-G and 47-6B strains of B. sphaericus and the finding of a major 49 kDa protein in these strains suggest that there is likely to be another mosquitocidal factor in the three strains.