Resistance Development in Culex quinquefasciatus (Diptera: Culicidae) to Bacillus sphaericus

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Abstract
Larvae of laboratory and field-collected colonies of Culex quinquefasciatus Say were subjected to selection pressure with a preparation of Bacillus sphaericus Neide strain 2362 in the laboratory. Young fourth instars of every generation of both colonies were treated with the B. sphaericus 2362 preparation at a concentration that yielded 80% mortality (LC80). The survivors then were reared to continue to the next generation. The selected laboratory colony began developing resistance (based on the resistance ratio [RR] at LC50) in F20 with an RR of 8.1. The magnitude of resistance increased gradually and reached 37-fold by F80. In contrast, the selected, field-collected colony developed a slight degree of resistance as early as in F5 (RR = 4.4). The resistance level then fluctuated between 10 and 30-fold, with an RR of 27.4 in F80. The unselected laboratory and field colonies, reared without any exposure to the B. sphaericus 2362 toxins, did not show any marked change in their susceptibility to this microbial agent. The susceptibility level of all successive generations of these unselected colonies remained about the same as that of their parental generation.

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