Persistence of Bacillus Thuringiensis serovar. Israelensis toxic activity in the environment and interaction with natural substrates

The persistence of Bacillus thuringiensis serovar. israelensis (B.t.i.) toxic crystals was studied using environmental chambers filled with a commercial formulation of B.t.i. and natural substrates found in field treatment conditions. The chambers were kept near the bottom of a small oligotrophic lake. In cold waters, toxicity remained stable at least 21 days in the presence of lake water. The presence of solids substrates produced a rapid loss of toxicity. This was caused in part, by interactions between the substrates and the larvicidal crystals of B.t.i.. Toxicity reached a low point between day 43 and 69 and increased thereafter, except for the samples containing organic sediments. This increase in toxicity could be correlated with the behavior of the spore and viable cell counts of B.t.i., in the chambers. These counts indicated that in cold temperate climates, B.t.i. could sporulate and release new crystals.