Bottom-up and top-down forces regulate spruce budworm (Lepidoptera: Tortricidae) biological performance on regenerating white spruce

Abstract

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. The spruce budworm is the most important outbreaking conifer defoliator in Eastern Canada, and we aim to better understand budworm herbivory on young trees, given its importance for post-outbreak forest succession. We conducted a 4-year field survey in Northwestern Qu´ebec, Canada, to compare plant phenology, budworm density, defoliation rates, predator populations, and parasitism between the 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, affecting synchronization with budworm emergence from diapause. Field survey results also included slightly higher budworm density and lower larval parasitism in plantations, but no significant difference in current-year growth defoliation. The bottom-up experiment showed slightly better budworm biological performance, indicated by higher pupal mass, in plantations. The top-down experiment suggested slightly higher predation and parasitism in the natural forests. Together, our results show how mechanisms controlling insect defoliator populations are context-dependent. In plantations both bottom-up and top-down pressures on the spruce budworm appear slightly eased, leading to marginally better biological performance and higher population density of this forest pest. However, differences are so minor that no impact on defoliation is observed.