Abstract

Predation can be a major source of mortality in outbreaking pest insects and can help regulate the high densities of individuals at the peak of the outbreak. However, other mechanisms could be at play in the years of population crashing, contributing to the endemic levels of these populations. Forest tent caterpillars (Malacosoma disstria Hubner) are outbreaking pest species defoliating mixed wood boreal forests in eastern Canada, including Quebec. This species presents periodic population dynamics with peaks every 10 years and outbreaks lasting 1–3 years which can highly impact the health of its host tree. Different sources of mortality can play a role in the dynamics of these outbreaks such as extrinsic agents (natural enemies) and intrinsic ones (pathogens and maternal effect). Extrinsic mortality was considered in terms of natural enemies that can fly to reach the colony and those that have to walk to reach the prey. This study seeks to answer questions concerning the relative importance of these sources of mortality during and after the crash of an outbreak. We also investigated the difference between sites that had been defoliated during the last outbreak and those that were not to test for density dependence. For this experiment, we use a triad set-up of complete predator exclusion, partial exclusion and free colonies. We found that intrinsic mortality increases after the crash of the outbreak while only flighted mortality (related to parasitoids and other flying predators) did, and not walking mortality. There was no difference between the defoliated and control sites. These findings support our hypothesis that predation would increase after the outbreak and would lead to regulation of the population. However, we had underestimated the importance of intrinsic mortality for this regulation. This finding is important when thinking out potentially control agents to limit the spread of future outbreaks.