Efficacy and efficiency of new Bacillus thuringiensis var. israelensis and Bacillus sphaericus formulations against Afrotropical anophelines in Western Kenya

We evaluated the efficacy of new water-dispersible granular (WDG) formulations of Bacillus thuringienis var. israelensis (Bti; VectoBac®) and B. sphaericus (Bs; VectoLex®, Valent BioScience Corp., Illinois, USA) for the control of larval Anopheles gambiae sensu lato Giles mosquitoes in a malaria-endemic area around Lake Victoria, Western Kenya. WDG and powder formulations were compared in laboratory bioassays and followed by efficiency and residual effect assessments of both WDG formulations in open field experiments. LC50 and LC95 values for the Bti/Bs strains and their formulations show high susceptibility of A. gambiae sensu stricto under laboratory conditions. The larvae proved more susceptible to Bs than to Bti and the WDG formulations were slightly superior to the powder formulations. High efficiency was also shown in the open field trials, and a minimum dosage of 200 g/ha Bti WDG, representing the LC95 of the laboratory tests, was sufficient to fully suppress emergence of mosquitoes when applied at weekly intervals. Bti WDG did not show a residual effect, irrespective of the concentration applied. The Bs WDG formulation, however, showed significant larval reductions up to 11 days post-treatment at application doses of either 1 or 5 kg/ha. We conclude that the main malaria vector in our study area is highly susceptible to these microbial control agents. Minimum effective dosages to achieve elimination of the larval population in a given habitat are extremely low and environmental impact is negligible. Microbial products for larval control have therefore great potential within Integrated Vector Management programmes and may augment control efforts against adult vector stages, such as the use of insecticide-treated bednets, in many parts of Africa.