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

The persistence of the larvicidal activity of Bacillus thuringiensis var: i sraelensis (Bti) was tested over five-month period in a low-temperature aqu atic environment. Diffusion chambers filled with a suspension of Bti (100 m g l(-1)) and different experimental substrates (pond water, periphyton, sed iments and vegetation), but without mosquito larvae, were placed near the b ottom of a large pond and removed at various intervals to measure residual toxic activity to mosquito larvae, spore concentration and proteolytic acti vity. Within the pond water substrate, 50% of the initial toxicity I was st ill present after one month in cold water while with the periphyton substra te 30% remained in the liquid fraction after the same period of exposure. W ithin 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 t oxic very early and remained toxic for five months At the end (day 154), th ere was still 54% of the original toxic activity put in the chambers associ ated with the vegetation samples. In the absence of mosquito mosquito larva e spore recycling eas observed in the chambers especially with sediments an d vegetation But spore recycling did not appear to play a major role in the observed persistence but rather rapid absorption onto vegetation substrate s was responsible for the persistence of Bti in a cold climate.