FUNCTIONAL-EFFECTS OF THE BACTERIAL INSECTICIDE BACILLUS-THURINGIENSIS VAR KURSTAKI ON AQUATIC MICROBIAL COMMUNITIES

Epilithic microbial communities were colonized on leaf disks and expos ed to commercial preparations of Bacillus thuringiensis var. kurstaki (Btk) in aquatic microcosms. Responses in terms of microbial respirati on, bacterial cell density, protozoan density, and microbial decomposi tion activity were measured. Test concentrations for treatments with D ipel 64AF and Dipel 8AF in microcosms were the expected environmental concentration (EEC) of 20 IU/ml, 100X the EEC, and 1000X the EEC. Bact erial cell density in the biofilm of leaf disks was significantly incr eased at concentrations as low as the EEC. There were no concomitant a lterations in protozoan density. Microbial respiration was significant ly increased, and decomposition activity was significantly decreased, but only at the artificially high concentration of 1000X the EEC. This effect was attributed to the spore-crystal component rather than form ulation ingredients. Microbial decomposition of leaf material was also determined in outdoor stream channels treated at concentrations rangi ng from the EEC to 100X the EEC. Although there tended to be reduced d ecomposition activity in treated channels, there were no significant d ifferences in mass loss of leaf material between treated and control c hannels. Various regression, classification, and ordination procedures were applied to the experimental data, and none indicated significant treatment effects. These results from laboratory and controlled field experiments indicate that contamination of watercourses with Btk is u nlikely to result in significant adverse effects on microbial communit y function in terms of detrital decomposition. (C) 1996 Academic Press , Inc.