We investigated whether changes in chestnut oak (Quercus prinus L.) leaf quality caused by gypsy moth (Lymantria dispar L.) defoliation affected leaf breakdown rates in southern Appalachian streams of differing disturbance history. Breakdown rates of second-flush leaves produced after defoliation were compared to those of natural spring-flush leaves shed in autumn. Second-flush leaves broke down significantly faster than spring-flush leaves in three of the six streams tested. Initial fiber content and the ratio of fiber to protein were significantly higher in spring-flush leaves than in second-flush leaves, showing that initial differences in internal leaf constituents could explain the faster breakdown rates of second-flush leaves. Using changes in leaf toughness through time as a measure of microbial conditioning we found that the faster-decaying second-flush leaves also softened at a faster rate than the spring-flush leaves. In addition, both types of leaves incubated in three streams draining a recovering 14-y-old clear-cut catchment broke down significantly faster than leaves incubated in three streams draining a reference catchment. We attributed this increase in leaf breakdown to significantly higher abundance and density of leaf-shredding insects and greater microbial conditioning in leaf packs in the streams of the recovering clear-cut catchment. Overall, our results show that insect defoliation accelerates detritus processing in southern Appalachian streams and that this acceleration may be especially important in previously disturbed streams in which leaves are already processed faster.