A previously untreated field population of Culex quinquefasciatus Say, collected near Bakersfield, CA, was subjected to intensive laboratory selection with the bacterial insecticide Bacillus sphaericus Neide (strain 2362) at a level producing 95% mortality. Resistance rapidly appeared and resistance levels increased such that fourth instars of generation 12 were able to survive a concentration of B. sphaericus that was 7,000 times higher than the median lethal concentration (LC50) of the susceptible reference colony. Similar resistance levels were detected in first instars. Cross-resistance in the selected colony was detected toward B. sphaericus strains 1593 and 2297, but little or no cross-resistance was observed toward B. sphaericus strains IAB59 or ISPC5 (= WHO 2173). Cross-resistance also was not detected toward the bacterial insecticide Bacillus thuringiensis subsp. israelensis, toward a recombinant strain expressing both B. thuringiensis subsp. israelensis and B. sphaericus (strain 1593) toxins, toward individual or multiple toxins from B. thuringiensis subsp. israelensis, or toward conventional synthetic insecticides. Genetic analysis revealed that B. sphaericus resistance was inherited as a recessive trait and controlled by a single major locus. These data are discussed in relation to cases of field resistance toward this biopesticide in the Cx. pipiens (L.) complex.

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