Mosquitocidal Toxin from Bacillus sphaericus Induces Stronger Delayed Effects Than Binary Toxin on Culex quinquefasciatus (Diptera: Culicidae)

We studied the toxicity and delayed effects of a mosquitocidal toxin (Mtx1) and a binary toxin (Bin) produced in Escherchia coli E-TH21 and Bacillus thuringiensis B-CW1, respectively, on Culex quinquefasciatus (Diptera: Culicidae). Bioassay results showed that both E-TH21 powder and B-CW1 sporulated culture were highly toxic against susceptible Cx. quinquefasciatus, with LC50 values of 0.65 and 1.70 mg/liter against third and fourth instars at 48 h, respectively. After initial 48-h exposure of larvae to different concentrations of Mtx1 and Bin, significant continued mortality could be observed in larval, pupal, and emergence stages of Cx. quinquefasciatus. Importantly, the Mtx1 could induce higher cumulative larval and preadult mortalities than Bin toxin on the target mosquito. This finding is important for understanding the mode of action of Mtx1 and Bin toxins and for developing a new bioassay procedure for evaluation of toxicity of Bacillus sphaericus Neide, some strains of which produce Mtx1 and Bin, in the laboratory and field.