David; I can see why you've asked this question, the Internet has little information on "Wooly Apple aphids" ... Add another "L" and search "Woolly Apple Aphids" and the info's all over the place 
Though I did find the following. But first ... I had to hit my books!
From http://www.garden.org/regional/report/a ... nders/1511 ---I found:
"Spray Woolly Aphids. If you observe an accumulation of white, fuzzy deposits on branches and pruning wounds of crab apple, hawthorn, cherry, and pear trees, you have an invasion of woolly aphids. These fascinating insects will produce a protective, white covering that renders insecticides ineffective. One of the most effective ways to control the lint-like accumulations is to wash them off with a forceful spray of water."
From The Organic Gardner's Handbook of Natural Insect and Disease Control: "Aphids (Family Aphididae)
Damage: "Nymphs and adults suck plant sap from most small fruits, vegetables, ornamentals, and fruit and shade trees. Their feeding causes leaf, bud, and flower distortions; severely infested leaves and flowers drop. Fruit that forms on infested branches are misshapen and stunted. Aphids secrete sticky honeydew that supports growth of sooty mold on leaves and fruit. Feeding can spread viral diseases."
Control: spray frequently with a strong stream of water to knock aphids off; attract native predators and parasites by planting pollen and nectar plants; release purchased aphid midges, lady beetles, lacewings, or parasitic wasps; use homemade garlic, quasssia, or tomato-leaf sprays; spray insecticidal soap; as a last resort, spray pyrethrum, sabadilla, nicotine, or rotenone. Spray dormant oil to kill overwintering eggs, and plant flowering groundcovers in home orchards to attract predators and parasites.
From The Ortho Home Gardener's Problem Solver:
Analysis (Damage): "These common insects do little damage in small numbers. However, they are extremely prolific and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when the aphid sucks the juices from the leaves and buds." ... "Aphids transmit plant diseases such as mosaics and viral yellows."
"As soon as aphids appear, spray with an insecticide containing acephate (ORTHENE), diazinon, or malathion. Clean up plant debris in the fall.
Also try: http://ohioline.osu.edu/hyg-fact/2000/2208.html and http://www.ipm.ucdavis.edu/PMG/r4301711.html and http://www.uky.edu/Ag/Entomology/entfac ... /ef219.htm
David:
According to IPM for Apples & Pears (UC Davis):
M111 and M106 rootstocks are resistant to wooly aphids.
The aphids attack the trunk, enabling bacteria to invade which causes burrs (big lumps on the tree). They can also attack the roots, which is extremely difficult to control. Root colonies cause swollen nodular masses of gall tissue, stunting the tree.
There may be heavy infestations following applications of pyrethroid insecticides, which kill their natural enemies. The parasite Aphelinus mali can completely control aerial colonies
Many years ago, I found a big matt of wooly aphids on my Ashmead's Kernel Apple. They were feasting away on a large pruning scar. I squashed 'em, blew 'em off the trunk with a hose. And then, for good measure, I sprayed the whole tree thoroughly, with the ultimate aphid killer, warm water with a little dishwashing detergent in it.
The aphids never returned.
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