Woolly aphids live on several different trees and shrubs. The name describes what is peculiar about this group: The body of the aphid is covered with a white fluffy wax that resembles wool. In late summer you may notice colonies of woolly aphids clustered on the twigs and shoots of hawthorn and crabapple trees. Infestations are sporadic and vary from trees to tree, variety to variety and place to place.
Woolly aphids on hawthorn and crabapples feed on sap from the plant but are more alarming than damaging, especially late in the season. Earlier in the season there were woolly aphids of another species on the leaves and shoots of maple trees. In most cases the sap loss from aphid feeding is not significant to the plant and control is not practical. In some cases infested leaves may droop or shrivel and drop prematurely. This does not reduce the vigor of healthy trees.
Parasites, predators and even heavy rainfall will help reduce the populations. If you believe the natural population controls need your help you can use a forceful stream of water from the garden hose to dislodge the aphids or prune and remove selected, heavily infested stems and water sprouts. Spraying with insecticide is rarely justified.