Some apple varieties, such as Northern Spy, are resistant to this pest. Elm trees in the vicinity of orchards increase the migration of the aphid to apple trees. Infested nursery stock is also a source for spreading aphids. Injury caused by the woolly apple aphid consists of gall-like formations and swollen enlargements on roots. Once started, these galls increase in size from year to year as a result of aphid feeding.
Galls form favorable places for fungi to attack.
Aboveground colonies of aphids may develop around leaf axils on sprouts or on new growth, particularly at abrasions or cuts, and they prevent injured bark from healing. They are often found on the crowns of trees just above the roots. They may also develop in large knots on roots and underground parts of the trunk. Infested trees often have many short fibrous roots. The underground forms are more damaging, while the aboveground forms cause little damage, especially on larger trees.