Bottom-up and top-down forces regulate spruce budworm biological performance on white spruce regeneration

Young plantation trees are often highly vulnerable to insect herbivory in ways that are difficult to predict as underlying mechanisms linked to plant traits and natural enemy pressure interact in context-dependent ways. We compared bottom-up and top-down forces acting on spruce budworm ( Choristoneura fumiferana ) on young white spruce ( Picea glauca ) trees in plantations vs in natural regeneration under hardwood canopy. Recognized as the most important outbreaking conifers defoliator in Eastern Canada, we aim to better understand how its herbivory on young trees can affect post-outbreak forest succession. We conducted a 4-year field survey in Northwestern Québec, Canada, to compare plant phenology, budworm density, defoliation rates, predator populations, and parasitism between two habitats. We also designed manipulative experiments with sentinel larvae to assess bottom-up and top-down forces in these habitats. The field survey showed earlier budburst phenology in plantation trees, which improves synchronization with a model (BioSIM) predicted timing of budworm emergence from diapause. The field survey showed higher budworm density and lower larval parasitism in plantations, but no significant difference in current-year growth defoliation during the initial outbreak phase. The bottom-up experiment showed slightly better budworm biological performance, indicated by higher pupal mass, in plantations. The top-down experiment showed greater predator and parasitoid pressure in the understory. Together, our results show how mechanisms controlling insect defoliator populations are context-dependent. In plantations both bottom-up and top-down forces on the spruce budworm are relaxed in these open habitats, leading to better biological performance and higher population density of this forest pest.