Aphids are present year-round on the roots. Females in the aerial colonies may give birth to crawlers at any time in spring, summer, or fall. Newborn nymphs are very important in the distribution of woolly apple aphid. They spread either through some mechanical agent or directly by crawling. Birds and insects can also transport aphids.
Crawlers are generally more abundant in spring and fall.
They are not able to work into and through the soil. In orchards the swaying back and forth of trees by wind and the presence of organic matter, clods, stones, and other factors may provide pathways to the roots. Crawlers begin to infest the roots early in the season. Their downward motion begins any time the crawlers are numerous, especially in early summer and fall. Infestations by aerial colonies are not a true indication of root infestations, since trees can have aerial infestations over a season without their roots becoming infested.