You may see woolly aphids on many trees and shrubs in the landscape. The following is more information.
Woolly aphids (family Eriosomatidae) are found in the fall on many hardwood trees and shrub species including elm, silver maple, ash, alnus, alder, apple, pear, pine, spruce, hawthorn, and juneberry (Amelanchier). They are small (2-4 mm in length), pear shaped insects, and are often covered with white waxy strands. Woolly aphids generally have a primary host on which they overwinter, and a secondary host on which they spend much of the summer. They usually overwinter as eggs laid in bark of their primary host. The following spring, eggs hatch into females which give birth without mating. Each female can produce hundreds of offspring, so populations can grow rapidly. After one or two generations on the primary host, winged females are produced, and they fly to secondary hosts where they remain for the rest of the summer. Additional generations of aphids are produced until late summer or early fall when winged females fly to a primary host where they give birth to tiny male and female aphids that mate. Gravid females deposit a single large egg (or eggs) into protected locations in the bark and then die. While woolly aphids generally have two hosts, many species can sustain themselves on their secondary host alone. Woolly aphids feed on leaves, buds, twigs, and bark, but can also feed on the roots. Damage symptoms include twisted and curled leaves, yellowed foliage, poor plant growth, low plant vigor, and branch dieback. Natural enemies help keep these aphids from becoming a problem. In addition to the physical damage to the plant, accumulations of wax and shed skins can be very conspicuous on the leaves, twigs, and bark.
Information and photo from the October 9, 2009 edition of the TPM/IPM Weekly Report for Arborists, Landscape Managers & Nursery Managers from the University of Maryland Cooperative Extension http://www.ipmnet.umd.edu/09Oct09L.pdf