Defoliation of juvenile western hemlock by western blackheaded budworm in Pacific coastal forests

An outbreak of the western blackheaded budworm [Acleris gloverana (Walsingham)] defoliated 170,000 ha of western hemlock [Tsuga heterophylla (Raf.) Sarg.] on the Queen Charlotte Islands, British Columbia, Canada, between 1996 and 2001. Defoliation was first noted in mature forests in the southern portion of the archipelago, and was observed occurring progressively northward in subsequent years. Much of the defoliation occurred in juvenile stands with trees less than 40 years old. Many of these stands had been spaced to reduce tree density. A two-stage, stratified survey was used to compare defoliation in spaced and unspaced stands in two age classes: younger than 25 years old, and from 28 to 39 years old. After 1 year of defoliation, trees in spaced stands sustained approximately 20% more defoliation than those in unspaced stands. Neither age class nor its interaction with spacing affected overall defoliation. Defoliation was most intense in the top third of the tree crown. Top-stripping (complete defoliation of leading shoot in crown) occurred in most stands, but was more frequent and most severe in the youngest, spaced stands. The tallest trees within a stand were most severely defoliated. The implications of these patterns of susceptibility for management of western hemlock in Pacific coastal forests are discussed.