Transport of Bacillus thuringiensis var. israelensis and Its Effect on Drift and Benthic Densities of Nontarget Macroinvertebrates in the Susquehanna River, Northern Pennsylvania

The microbial pesticide Bacillus thuringiensis var. israelensis (B.t.i.) was applied aerially to suppress larval black fly populations in the Susquehanna River, Pennsylvania. We quantified (1) the passage and retention of B.t.i. spores through a 1.5-km reach and (2) changes in black fly and nontarget macroinvertebrate abundances in the drift and benthos. B.t.i. exposure in the treatment riffle was 1.16 × 105 min-spores/mL or 16 min-mg/L in 1989 and 5.20 × 104 min-spores/mL or 4.4 min-mg/L in 1990. Retention of B.t.i. within the reach was low. While black fly drift from the treatment riffle increased markedly, B.t.i. appeared to have little effect on drift densities of most nontarget macroinvertebrates during the 48-h period following application. Similarly, benthic densities of black flies in the treatment riffle decreased significantly 1 d after the B.t.i. application whereas benthic densities of most nontarget taxa in the treatment riffle and depositional zone appeared unaffected 1, 7, and 28 d following B.t.i. application. Responses potentially related to B.t.i. exposure were observed for the aquatic lepidopteran Petrophila and the chironomid Polypedilum. These results suggest that the application of B.t.i. on the Susquehanna River markedly reduces black fly populations, but does not negatively affect most nontarget macroinvertebrates.