The western spruce budworm, Choristoneura occidentalis Freeman, is the most widely distributed and destructive defoliator of coniferous forests in Western North America. It is one of nearly a dozen Choristoneura species, subspecies, or forms, with a complexity of variation among populations found throughout much of the United States and Canada. It occurs in the Rocky Mountains from Arizona and New Mexico northward into Colorado, Utah, Wyoming, Montana, and Idaho; in the Pacific Northwest in Oregon and Washington; and in British Columbia and Alberta, Canada.
The first recorded western spruce budworm outbreak was in 1909 on the southeastern part of Vancouver Island in British Columbia, Canada. Since that year, infestations of this and related species frequently have been reported in western Canada.
The budworm was first reported in the United States in 1914 in Oregon; however, it was not recognized as a serious threat to western coniferous forests until 1922, when two outbreaks were reported near Priest Lake in northern Idaho. Subsequent widespread and destructive outbreaks in the Rocky Mountains and in the Pacific Northwest have caused top-killing, serious economic losses in tree growth, and some tree mortality primarily in regeneration, sapling, and pole-sized trees.