The woolly aphid (Eriosoma lanigerum) is a serious pest of apple trees throughout the State, and infests both the tops and the roots of the trees. The greatest infestation on the roots occurs during extremes of heat or cold.

In the spring the aphids migrate from the roots to the tops of the trees and these top parts are infested during the growing period. In the autumn there is a considerable reduction in infestation, many of the new generation aphids having made their way to the roots. Where trees are heavily infested, colonies may persist on the above-ground parts of the trees throughout the winter.

Infestations cause knotted and gnarled lumps and scars on the older wood, usually coated over with the white woolly secretion produced by the aphids. Under this waxy secretion the dark-bodied aphids may be found sucking the sap from the trees.

As the young lateral shoots develop during the summer, the aphids migrate to the under-surfaces of these, and may cause very severe injury to this new wood and destruction of the buds. Heavily infested trees become "staggy" in appearance, with little fruiting or leaf-bearing wood.