Such fluctuations influence the rate of tree mortality thereby affecting control programs and potential salvage operations. Unfortunately, these fluctuations usually can not be reliably predicted. Fire suppression and the retention of overmature spruce stands render forests more susceptible to attack. Spectacular increases in outbreaks have also coincided with unusually heavy production of staminate flowers on spruces and balsam. Impacts of repeated defoliation are similar to those caused by other forest defoliators and are most like the damage caused by the western spruce budworm.
Dispersal occurs during all the active stages of the budworm.
During emergence from the eggs and hibernacula, windborne dispersal of small larvae is common and can result in the extension of outbreaks. The distance small larvae can be transported by air currents is not well known but is probably limited to a few kilometres. Moth dispersal, particularly along prevailing winds, is likely the most significant dispersal mechanism. The rapidity with which the size of outbreaks has increased would indicate that, at least at certain times, population increases occur over large areas rather than as a result of spread from discrete centres. Natural mortality factors that limit the size, intensity, and duration of outbreaks include predators, parasites, pathogens, starvation, and weather effects.