Connecting the dots: Elucidating the relationship between spruce budworm population dynamics and defoliation

Spruce budworm (Choristoneura fumiferana) outbreaks are a major disturbance in northeastern American forests. Monitoring, forecasting, and mitigating outbreak risks require information on how local population densities translate to defoliation. Given landscape-scale heterogeneity, forest structure, and local population dynamics, one expects the relationship between population densities and defoliation to vary spatially and temporally. We analyzed 17 years of larval density data from over 1,000 locations in Québec, Canada, to investigate how larval densities and environmental context translate into observable defoliation. We found a positive latitudinal gradient and a positive effect of hardwood species on insect population growth rates. Further, we identified a 3-year cumulative effect of larval densities on defoliation, with a 2-year lag having the strongest influence. On average, a density of 15 larvae per branch corresponded to a 29% probability of defoliation. However, this probability varied widely (13-52%) with the proportions of balsam fir and black spruce. Our study elucidates the relationship between SBW populations and defoliation, highlighting the importance of cumulative larval density effects and environmental context in defining defoliation risk.