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 o f Bacillus thuringiensis var. israelensis (Vectobac G(R)) on waterfowl macroinvertebrate food resources at the Minnesota Valley National Wil dlife Refuge,Bloomington, Minnesota. Laboratory toxicity tests were co nducted to evaluate the influence of environmental variables (e.g., te mperature and depth). The EC50 was 0.20 ppm for Chironomus riparius. F ield studies evaluated the effects of three applications at 5.6 kg/ha on benthic 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 t he spring and summer of 1990. Benthic invertebrates were sampled befor e 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 ac counted for by the treatments. However, results from laboratory toxici ty tests conducted with field-collected Chironomidae indicated that th ese organisms were affected at 5.6 and 28.1 kg/ha. Further toxicity te sts using Chironomus riparius determined that various environmental va riables influenced the efficacy of Vectobac-G, including temperature, larval instar, water depth, and water surface area coverage by macroph ytes. This study indicates that chironomids (a major waterfowl food so urce) are adversely affected by Vectobac-G under a controlled laborato ry situation; however, environmental factors reduce the efficacy of th e larvicide in the field.