Some Factors Affecting the Pathogenicity of Bacillus thuringiensis Berliner Against Blackflies 1

Abstract

Several factors that might affect the efficacy of Bacillus thuringiensis against Simulium vittatum larvae were investigated in the laboratory utilizing a conical-flushing rearing unit and the jar method for bioassay. S. vittatum and S. argus were considerably more susceptible to B. thuringiensis than the other species tested. There was a strong positive correlation between temperature and mortality (r = 0.97). The range of mortality for 4°–24°C was 23–95%. The midgut pH ranged from 8.2–11.4, sufficiently high for θ-endotoxin activity. When spore and crystal fractions were bioassayed separately, only the crystal fraction produced significant mortality. No additive action of the spore and crystal was apparent, activity was solely attributed to the parasporal crystalline inclusion bodies. Spore preparations autoclaved and not autoclaved, containing the β -exotoxin produced a low level of mortality at 10 ppm for 24 h. Spore suspensions of B. thuringiensis stored at 2° and 19°C remained active for 167 and 28 days, respectively, after which a sharp decline in activity was observed.