I have a sticky cotton wool-like growth on the branches of my apple tree. What is causing it and will it harm my tree? This is woolly aphid (eriosoma lanigerium) which can infect apples, cotoneasters, sorbus and several other trees and shrubs. The young overwinter in cracks or under loose bark. They emerge in spring and multiply quickly around wounds or new shoots and can cause knobbly galls on the trunk and make trees more susceptible to disease. They can be sprayed with an approved insecticide or physically removed with a brush and soapy water. Check regularly for reinfection and treat immediately. Hanging feeders in trees to encourage insect eating birds and underplanting with flowers to attract beneficial insects will help prevent future infestations of woolly aphid and many other pests.