Positive and negative feedback interactions between forest tent caterpillars (Malacosoma disstria Hbn.) and trembling aspen (Populus tremuloides Michx.) may play an important role in the performance of this insect and perhaps even contribute to its cyclic population dynamics. Early spring larval feeding on opening buds did not induce strong rapid changes in the leaf quality of aspen; instead, some evidence for negative impact on caterpillar growth and development was found. Real and simulated insect defoliation in early summer (mid June) induced foliar changes in trembling aspen that had negative effects on larval performance. These changes were induced systemically throughout the tree and occurred a year following the damage. Significant interclonal variation in response to insect feeding damage was found, despite inherent clonal differences in leaf quality. Furthermore, true insect feeding appeared to elicit stronger induced defenses than mechanical wounding. This suggests that induced responses are indeed defensive. Whether or not they evolved as responses to herbivores in general or are co-evolutionarily linked specifically to M. disstria , remains to be determined.