The woolly apple aphid, known as Eriosoma lanigerum, is one type of aphid that damages apple trees. Though not the most common aphid to attack apple, it can be a minor or major pest depending on the degree of infestation. The aphids eat young wood, green tender growth and leaves when present. They can also overwinter and feed on the tree roots. Their activity causes foliage to yellow, roots to be truncated and starved of nutrients weakening the tree and making it susceptible to secondary infections. Reducing tender growth, introducing biological predators and judicious use of insecticide products can manage and minimize the aphids' presence and damaging impacts.
Prune away all sucker growth that crops up at the lower tree trunk in the spring. In the summer, prune back all upright water sprouts that appear in the canopy of the tree. This will deprive woolly apple aphids of fresh tender growth on which they feed and breed.
Introduce Aphelinus mali wasps or lady beetles as a biological control measure. The wasps are parasites of the woolly aphids and will lay eggs in the aphids killing them. The lady beetles, more commonly known as lady bugs, will eat the aphids. Containers of the beneficial insects can be opened and set in the shaded crotch of the tree in the spring or when woolly aphid populations are found.
Spray with an insecticidal soap or an insecticide product containing diazinon. Spray according to the product label dosing directions beginning in the late spring and continuing through the middle of summer. Spray all of the foliage until wet including the underside of leaves and make repeat applications as needed in keeping with the recommendations on the product label.