The seasonal abundance of woolly apple aphid (Eriosoma lanigerum Hausmann) was studied from 2005 to 2008 in commercial orchards in central Washington State. There was a single period of crawler migration on the tree trunks each year lasting from 7 to 16 wk. The peak period of crawler activity ranged from early June to late July, although peaks in June were more common. In one orchard, peak densities occurred 7 wk later in 2006 than in 2005. In all orchards where large numbers of crawlers were caught (up to 1,420 per band per week), more crawlers were caught in the lower band (migrating up from root colonies) than in the upper bands (migrating down from the aerial colonies). However, the peak period of crawler migration was similar for root and aerial colonies. In contrast, peaks in population densities of aerial colonies were sometimes bimodal, usually spanning the period from late May through mid-October; however, colonies were detected in mid-April in one orchard. There was no consistent relationship between the crawler density and aerial colony density within an orchard; this raises challenging questions regarding annual reinfestation of the aerial portions of the tree. Nymphs and adults formed 66 and 15%, respectively, of all stages found throughout the season. Alates were only found in September and October in orchards with fall populations, comprising up to 43% of individuals in aerial colonies. Overwintering survival was higher on the root colonies than in the aerial colonies in both years this was studied. Fruit infestation by aphids was relatively rare, and occurred only when aerial colonies were numerous.