Top-down and bottom-up trophic interactions related to FTC outbreaks during low-density population density periods in temperate and boreal mixedwood forests in Quebec

The forest tent caterpillar (FTC) is a cyclical outbreaking pest species defoliating mixed wood boreal forests in eastern Canada. The mechanisms behind this periodicity are still not well understood, especially at the early instar and egg stages. I conducted three studies on some of these mechanisms, comparing sites that were defoliated during the last outbreak with ones that were not afflicted, to investigate the impact of disturbances on FTC population dynamics. First, I examined the importance of various sources of early-instar mortality during and after the crash of an outbreak, testing for density dependence and distinguishing between intrinsic mortality and mortality caused by natural enemies. Mortality from flighted natural enemies and intrinsic sources increased after the outbreak but did not contribute to the population crash. However, they helped maintain low endemic levels between outbreaks and showed density dependence. Next, I explored the impact of FTC defoliation on ant communities, some of which attack larvae. This disturbance lowered ant species richness and increased evenness, leading to higher homogeneity. These changes were driven by canopy openness and did not result in higher mortality of the FTC larvae. Finally, I studied the overwintering mortality of FTC, showing that mortality was higher in southern parts of the distribution likely due to higher variations in temperature. My PhD explores the roles of different sources of mortality at different FTC developmental stages and provides arguments for considering them all in pest management, especially when dealing with highly cyclical outbreaking species.