Development of Culex quinquefasciatus resistance to Bacillus sphaericus strain IAB59 needs long term selection pressure

Abstract

Abstract Culex quinquefasciatus may exhibit resistance to the biolarvicide Bacillus sphaericus (Bsp), when subjected to high selection pressure. The major insecticidal factor present in most Bsp products is the binary (Bin) toxin. The IAB59 strain, in particular, has attracted attention, since it shows another toxic factor, besides the Bin toxin, and this feature is likely to decrease the risk for selecting resistance. The aim of this study was to investigate whether it was possible to select a Cx. quinquefasciatus colony (CqRL2/IAB59) for high level resistance towards Bsp strain IAB59, under laboratory conditions. Selection performed by treating groups of about 15,000 third and fourth instar larvae from each generation with high concentrations of Bsp IAB59 yielded a 70% average pre-imaginal mortality. The CqRL2/IAB59 colony reached a high level of resistance (≈40,000-fold), after 72 generations. This colony also displayed a high level of cross-resistance to strain 2362, close to 70,000-fold, while larvae remained susceptible to Bti. Cross-resistance to strain 2362 was observed before the colony had attained a high level of resistance to strain IAB59. Data from the reciprocal crosses, between individuals from the susceptible and the resistant colonies, showed that resistance is inherited as a recessive trait, since the LC50 of Bsp IAB59 towards F1 larvae were similar to that of the susceptible parental colony. Data from this study showed that continuous selection pressure with Bsp IAB59 can select Culex resistant populations, although the evolution of resistance is much slower, than formerly observed for the commercial 2362 strain.