With Malaise traps, we monitored the flight of adult Plecoptera andTrichoptera following emergence from headwater streams in the FernowExperimental Forest, WV, during the second year after application ofdiflubenzuron. We placed five traps at various distances from eachstream during May through September of 1991, 1992, and 1993. Wecollected pretreatment samples during the first year. In May 1992,diflubenzuron was applied to two watersheds, and the other twowatersheds were used as untreated references. The 1992 study tested theeffects of diflubenzuron that fell directly into the streams or werewashed into the stream during the first year. For 1993, we tested thehypothesis that diflubenzuron affected adult flight following emergenceduring the year following abscission and possible ingestion of thetreated leaves. The analysis compared the regressions of the number ofadults caught in each trap versus distance of the trap from the streamamong years and between treatments for each species. The flight of thestonefly Leuctra ferruginea (Walker) was reduced in thetreatment watersheds compared with the reference watersheds during theyear following abscission of the treated leaves. Adult flight of otherspecies did not decrease in the treatment watersheds during 1993. Theseresults show a relatively small effect of diflubenzuron on theseaquatic insects; however, our study involved only a single applicationof diflubenzuron. Additional research may be needed to predict thepossible effects of multiple applications of diflubenzuron over severalyears as often occurs during actual efforts to suppress gypsy moth,Lymantria dispar (L.).