Cost-Effective Monitoring of Spruce Budworm Larvae

The spruce budworm, <i>Choristoneura fumiferana</i> Clem. (Lepidoptera: Tortricidae), is the most severe defoliator of balsam fir (<i>Abies balsamea</i>) in eastern Canada and northeast U.S.A. A large budworm outbreak is currently underway in the province of Québec, with 10.5 million hectares defoliated in 2023, up 14.1% from the year before. Populations of budworms are controlled using management guidelines of Forest Protection Strategy (FPS); the approach aims at killing defoliating larvae with aerial application of microbial insecticide, with the objective to limit defoliation and prevent tree mortality. The decision to treat/not treat a given forest block is based, in part, on local density of overwintering second instars (<i>L2</i>) collected at ca. 600 sampling points each year across the entire province at a cost of CAD 350 for three branches per site; the threshold for FPS (T_FPS) corresponds to 20 <i>L2</i>/branch. Aerial defoliation maps also guide management decisions because FPS generally target areas within or in the vicinity of defoliated forest stands. Budworm abundance rapidly declines with distance to aerial defoliation (in km), to the extent that larval density rarely attains T_FPS outside the core range of FPS (>15 km from defoliation). A cost-effective monitoring approach is proposed whereby forest blocks outside the core range of FPS are sampled every second year (as opposed to every year), representing a potential economy of CAD 40,000 annually.