Efficacy of a floating sustained release formulation of Bacillus thuringiensis ssp. israelensis in controlling Culex quinquefasciatus larvae in polluted water habitats

Larvicidal efficacy and residual activity of a floating sustained release formulation of Bacillus thuringiensis ssp. israelensis (Bti) was tested in polluted water habitats against Culex quinquefasciatus, the vector of bancroftian filariasis, breeding in cesspools and cesspits. A total of 25 habitats having 65 m2 water surface area with C. quinquefasciatus immatures were treated with the formulation at the rate of 15 kg/ha. For comparison, in another area, 18 habitats with a water surface area of 70 m2 were kept untreated. Evaluation was continued up to 179 days with three rounds of Bti application. The mean number of egg rafts did not differ significantly between treated and untreated habitats during the trial period indicating same level of recruitment. However, abundance of larvae and pupae was significantly lower in Bti treated habitats compared to that in untreated ones. The activity of the formulation, with 2 weeks delay in onset compared to treatment, caused >80% reduction in pupal abundance for a period of 30–34 days. Considering this, it is suggested that monthly application of Bti formulation will considerably reduce the recruitment of pupae of C. quinquefasciatus.