Patterns of Long-Term Balsam Woolly Adelgid Infestations and Effects in Oregon and Washington

Abstract

Abstract The balsam woolly adelgid (Adelges piceae) is an introduced pest discovered damaging grand fir (Abies grandis) in Oregon's Willamette Valley around 1930. In 1955, the insect was found infesting and killing Pacific silver fir (A. amabilis) near Mt. St. Helens in Washington and subalpine fir (A. lasiocarpa) in the Cascade Range in Oregon. In the next 10 yr, the pest dispersed widely throughout western Oregon and Washington, causing significant tree mortality over thousands of acres in the Cascade Range. Observations on trend plots established 35 to 40 yr ago have shown somewhat consistent trends in the patterns of infestation and damage. Infestations and damage were most severe on the best sites and at the lowest elevations where the hosts grow. Tree damage was consistently most severe in the first decade of infestation, but the insect never seemed to disappear from a stand; infestations and tree killing were observed on some plots 40 yr after the initial infestation. An ecological problem with the balsam woolly adelgid is a pattern of attack that is gradually eliminating grand fir from low-elevation landscapes west of the Cascade Range. The adelgid is also removing subalpine fir as a pioneer tree species in many of the important mountain environments. West. J. Appl. For. 16(3):121–126.