This thesis aims to investigate different trophic interactions related to the outbreaking cycles of the forest tent caterpillar (Malacosoma disstria Hübner, Lepidopter:Lasiocampidae), both top-down and bottom-up, in the two types of forests, the boreal and the temperate. Throughout field studies, we determined that forest tent caterpillar early-instar larvae were more susceptible to mortality due to pathogens and maternal effects in years after the outbreak than to predators. These results suggest delayed density dependence and contribute to low endemic levels between outbreak peaks. We also investigated the overwintering mortality of forest tent caterpillars at the egg stage. While larger egg masses tended to promote survival, this factor was not the only significant predictor. Mortality was also related to average winter temperature variation and cold spells. With increasingly unpredictable
climate patterns, both factors could cause high mortality levels during the winter. Finally, with the high amount of organic material released during the outbreaks, we investigated the impact on potential predators, such as ants. We observed a shift in ant communities in the boreal forest but not in temperate forests, suggesting that disturbances caused by the forest tent caterpillar can alter less ecologically complex and redundant ecosystems. Forest tent caterpillars are important disturbance agents and participate in multiple trophic interactions during outbreaks, cementing the importance of a better understanding of their population dynamics.