Microbial agents Bacillus thuringiensis ssp. israelensis and Bacillus sphaericus suppress eutrophication, enhance water quality, and control mosquitoes in microcosms

Abstract

Bacillus thuringiensis ssp. israelensis de Barjac (Bti) and Bacillus sphaericus Neide are promising microbial mosquito larvicides currently employed in mosquito control programs. During the screening and evaluation of new formulations of these control agents, it was noted that productivity of some algal species was noticeably suppressed, and that certain water quality parameters were enhanced by the suppression of primary productivity with these agents. Quantitative studies on these aspects were conducted in microcosms where Culex mosquitoes bred naturally. Treatments with VectoBac G (Bti) at the high dosage of 48.1 kg/ha, B. sphaericus water dispersible granules at 3.1 kg/ha, and Bti water dispersible granules at 0.6 kg/ha yielded good control of immature mosquito populations. Two species of unicellular algae, Closterium sp. and Chlorella sp., existed in both the controls and treatments. Growth of these algae was suppressed by the microbial agents applied. Suppressed algal productivity and photosynthesis resulted in lower water turbidity and oxygen concentrations in the treatments than in the controls, especially during the hot season. Water in treatments was discernibly clearer than in the controls, which was further borne out by differences in the optical density readings.