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 found throughout much of the United States and Canada. For more information on budworm biology, view Forest Insect & Disease Leaflet 53: Western Spruce Budworm.
Recently, tree-ring records were used to reconstruct spatial and temporal patterns of western spruce budworm outbreaks in mixed conifer forests of southern Colorado. Reconstructions in 11 host stands showed a regionally synchronous pattern of at least 14 outbreaks during the past 350 years. The outbreaks appear to be associated with periods of increased rainfall.
More information on tree-ring studies can be found at A tree-ring reconstruction of western spruce budworm outbreaks in the San Juan Mountains.