Effects of Bacillus thuringiensis var. Israelensis on nontarget benthic organisms in a lentic habitat and factors affecting the efficacy of the larvicide

Field and laboratory studies were conducted to determine the effects of Bacillus thuringiensis var. israelensis (Vectobac-G®) on waterfowl macroinvertebrate food resources at the Minnesota Valley National Wildlife Refuge, Bloomington, Minnesota. Laboratory toxicity tests were conducted to evaluate the influence of environmental variables (e.g., temperature and depth). The EC50 was 0.20 ppm for Chironomus riparius. Field studies evaluated the effects of three applications at 5.6 kg/ha on ben-thic communities during the spring and summer of 1989, and three experiments with one application of 5.6 kg/ha and 28.1 kg/ha during the spring and summer of 1990. Benthic invertebrates were sampled before each application of Vectobac-G and at 48 h after application. There was no reduction of the major taxa found in the ponds that could be accounted for by the treatments. However, results from laboratory toxicity tests conducted with field-collected Chironomidae indicated that these organisms were affected at 5.6 and 28.1 kg/ha. Further toxicity tests using Chironomus riparius determined that various environmental variables influenced the efficacy of Vectobac-G, including temperature, larval instar, water depth, and water surface area coverage by macrophytes. This study indicates that chironomids (a major waterfowl food source) are adversely affected by Vectobac-G under a controlled laboratory situation; however, environmental factors reduce the efficacy of the larvicide in the field.