Persistence of toxic activity and recycling of Bacillus thuringiensis var. israelensis in cold water:: Field experiments using diffusion chambers in a pond

The persistence of the larvicidal activity of Bacillus thuringiensis var: israelensis (Bti) was tested over five-month period in a low-temperature aquatic environment. Diffusion chambers filled with a suspension of Bti (100 mg l(-1)) and different experimental substrates (pond water, periphyton, sediments and vegetation), but without mosquito larvae, were placed near the bottom of a large pond and removed at various intervals to measure residual toxic activity to mosquito larvae, spore concentration and proteolytic activity. Within the pond water substrate, 50% of the initial toxicity I was still present after one month in cold water while with the periphyton substrate 30% remained in the liquid fraction after the same period of exposure. Within the vegetation substrate (blue-joint glass, Calamagrostis canadensis), an average of 30% of the initial toxicity was still present in the liquid fraction between day 84 to day 154. Solid fractions of vegetation became toxic very early and remained toxic for five months At the end (day 154), there was still 54% of the original toxic activity put in the chambers associated with the vegetation samples. In the absence of mosquito mosquito larvae spore recycling eas observed in the chambers especially with sediments and vegetation But spore recycling did not appear to play a major role in the observed persistence but rather rapid absorption onto vegetation substrates was responsible for the persistence of Bti in a cold climate.