A 35 min exposure of a 3.10 ppm (124,000 sp/ml) suspension of Bacillus thuringiensis var. israelensis (Bti) resulted in only a 25% reduction in S. vittatum larvae 24 hr following treatment 312 m downstream from treatment application. Increased exposure time to 70 min with 1.55 ppm (41,000 sp/ml) suspension of Bti resulted in a posttreatment reduction in the number of S. vittatum larvae along the entire 312 m test stream of 50-70% at 24 hr, 35-80% at 48 hr and 25-78% at 72 hr.