Excerpt from the Popular Garden Series magabook Orcharding with permission from its publisher, BowTie magazines, a division of BowTie Inc. Purchase Orcharding here.
Aphids are part of a group that includes leaf-hoppers, psylla, stink bugs and scale. These insects have a gradual (or incomplete) metamorphosis, which means that this group of insects has three stages: eggs, nymph and adult. A nymph resembles the adult and feeds in the same way, but is smaller and lacks wings. These insects feed by sucking, and both nymphs and adults inflict damage. Aphid pests of fruit trees fall into two main categories: foliage and root. Foliar-feeding aphids include green apple aphid, spirea aphid, green peach aphid and rosy apple aphid. The root aphid of concern is the wooly apple aphid.
Aphids are soft-bodied and vary in size (average around 1/16-inch long) and color (yellow to green to black). Some species of aphid, including the wooly apple aphid, are covered with wooly looking, waxy material. Aphids can be distinguished by the presence of two small, horn-like structures on the end of the abdomen called cornicles. During the summer, most of the aphid population is female and gives birth to live offspring without mating (up to 12 per day). Aphid populations can grow very quickly. A sexual form develops in the fall, mates and lays eggs that overwinter. Wooly apple aphids overwinter as adults on roots. Winged forms are produced in response to high populations and will disperse to start new colonies.
Foliar aphid feeding causes varying degrees of leaf distortion and curling. The other damaging effect of aphids is their honeydew production. Honeydew grows a black fungus called sooty mold that, when heavy, can stress a tree by blocking sunlight.
Low to moderate foliar aphid populations are not usually a problem. Aphid populations are effectively controlled by natural enemies: ladybugs, syrphid flies, soldier beetles, lacewings, parasitic wasps and many others. It is important, however, that ants be controlled if they are present. Ants will tend aphids, collecting the honeydew for food and protecting the aphids from natural enemies in exchange. High aphid populations should be reduced without impacting benefsicials. Aphids can be knocked off trees with a strong stream of water or controlled with insecticidal soaps. Dormant sprays will envelop, wet and smother aphid eggs.
Wooly apple aphid is a more serious pest that can kill young trees if the infestation is severe. Wooly apple aphids move up and down the tree and feed on roots, causing galls that compromise root function and cause stunting. Aerial colonies, found mostly in rough bark and cracks, cause cankering and may reduce fruit buds.
Additionally, wooly apple aphid can spread a disease called perennial canker fungus and infest fruit in severe infestations. The control of aerial colonies is the same as for foliar aphids, but on roots the aphids are hard to control. The recommendation is to plant resistant rootstock like M111 or M106. These rootstocks were developed specifically for resistance to wooly apple aphid.