Changes in the populations ofBacillus thuringiensis H-14 andBacillus sphaericus applied to vector breeding sites

Summary A dark brown mutant ofBacillus thuringiensis H-14 (BT-MB24) and a macrofibre mutant ofBacillus sphaericus H5 ab (BS-MS3) were applied to mosquito breeding habitats such as rain-water pools, casuarina garden pits, paddy fields and cesspits. The population of these bacilli in the water and soil of these habitats was monitored. The results show that the population of the two bacteria recovered from the water decrease with simultaneous increase in the soil populations within one hour of application: by day 19, the bacilli are virtually absent from the water. Only a few cells remain viable in the water for up to 200 days and in the soil for up to 270 days. Even though both these bacilli persist in the waters for a long time at about 100 CFU mL^–1 they do not cause any significant reduction in the mosquito larval density.The authors form part of the research team at the Vector Control Research Centre of the Indian Council of Medical Research, Indira Nagar, Pondicherry.