A microcosm test was used to evaluate the survival and effects of Bacillus thuringiensis var.israelensis (Bti) in aquatic systems. Two commercial formulations (Vectobac from Abbott; Mosquito Attack from Reuters Laboratories) and a laboratory preparation of Bti spores were tested. In all cases, Bti spores persisted in the microcosms, showing no change in abundance over six weeks of observation. The Bti preparations killed larval mosquitoes (Aedes aegypti) and midges (Tanytarsus sp.) in microcosms within 24 to 48 h. Other effects were limited to minor increases in primary production and community respiration (as measured by changes in pH and dissolved oxygen). These effects appeared to be a response to the loss of grazing insect species, the decomposition of dead insects, and the addition of a biodegradable compound (the formulation) with the formulated products. Bioassays of samples from the microcosms against Aedes aegypti larvae showed that Bti lost its toxicity within seven days.

The microcosm procedure proved to be a useful method for evaluating the survival and effects of microbial spores. The results obtained with this procedure were similar to those reported from field tests of Bti.