Culex quinquefasciatus field populations subjected to treatment with Bacillus sphaericus did not display high resistance levels

A major challenge for achieving successful mosquito control is that of overcoming insecticide resistance. Bacillus sphaericus is one of the most effective biolarvicides used to control Culex species and monitoring of larval susceptibility is essential for avoiding the selection for resistance. Culex quinquefasciatus populations in two urban areas in Brazil, Agua Fria and Pinheiros River, were subjected to B. sphaericus treatments over a 3-year period and their susceptibility investigated. Larval samples were submitted to in vivo toxicity assays and in vitro analysis of the receptor Cqm1, the target site of the binary (Bin) toxin, which is the major insecticidal factor in B. sphaericus. Bioassays were performed three times during the Agua Fria Program and showed an increase from 3- to 7-fold, while for the Pinheiros River area a 5-fold increase was found at the two latest evaluations. Binding assays using the 125I-Bin toxin and midgut microvilli samples from Agua Fria and the Pinheiros River revealed that the number of receptors and their binding affinity for the Bin toxin are compatible with the profile of susceptible populations. Data indicated that the field-treated populations showed only a slight decrease in in vivo susceptibility and that the toxin receptor was present and functional in larval preparations. Monitoring should be continued to avoid the selection for high levels of resistance and to ensure the sustainability of B. sphaericus-based vector control programs.