RESISTANCE DEVELOPMENT IN CULEX-QUINQUEFASCIATUS (DIPTERA, CULICIDAE)TO BACILLUS-SPHAERICUS

Larvae of laboratory and field-collected colonies of Culex quinquefasc iatus Say were subjected to selection pressure with a preparation of B acillus sphaericus Neide strain 2362 in the laboratory. Young fourth i nstars of every generation of both colonies were treated with the B. s phaericus 2362 preparation at a concentration that yielded 80% mortali ty (LC(80)). The survivors then were reared to continue to the next ge neration. The selected laboratory colony began developing resistance ( based on the resistance ratio [RR] at LC(50)) in F-20 with an RR of 8. 1. The magnitude of resistance increased gradually and reached 37-fold by F-80. In contrast, the selected, field-collected colony developed a slight degree of resistance as early as in F-5 (RR = 4.4). The resis tance level then fluctuated between 10 and 30-fold, with an RR of 27.4 in F-80. The unselected laboratory and field colonies, reared without any exposure to the B. sphaericus 2362 toxins, did not show any marke d change in their susceptibility to this microbial agent. The suscepti bility level of all successive generations of these unselected colonie s remained about the same as that of their parental generations.