TRANSPORT OF BACILLUS-THURINGIENSIS VAR ISRAELENSIS AND ITS EFFECT ONDRIFT 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 th e Susquehanna River, Pennsylvania. We quantified (1) the passage and r etention of B.t.i. spores through a 1.5-km reach and (2) changes in bl ack fly and nontarget macroinvertebrate abundances in the drift and be nthos. B.t.i. exposure in the treatment riffle was 1.16 x 10(5) min-sp ores/mL or 16 min-mg/L in 1989 and 5.20 x 10(4) min-spores/mL or 4.4 m in-mg/L in 1990. Retention of B.t.i. within the reach was low. While b lack fly drift from the treatment riffle increased markedly, B.t.i. ap peared to have little effect on drift densities of most nontarget macr oinvertebrates during the 48-h period following application. Similarly , benthic densities of black flies in the treatment riffle decreased s ignificantly 1 d after the B.t.i. application whereas benthic densitie s of most nontarget taxa in the treatment riffle and depositional zone appeared unaffected 1, 7, and 28 d following B.t.i. application. Resp onses potentially related to B.t.i. exposure were observed for the aqu atic lepidopteran Petrophila and the chironomid Polypedilum. These res ults suggest that the application of B.t.i. on the Susquehanna River m arkedly reduces black fly populations, but does not negatively affect most nontarget macroinvertebrates.