Woolly Aphids normally come in clusters, yet rarely seem to do much damage other than the scarred tissue that they leave behind on the stem. Woolly Aphids are normally seen on stems rather than on foliage like other Aphids. The waxy coat they cover themselves with are a good method of defence - both from the spray-happy gardener and other predators.
Woolly Aphids are often found on fruit trees, so the damage they do to stems and bark can give rise to secondary problems, such as cancer able to gain access for the spores through the damaged tissue.
Because of the waxy coating, some insecticide can simply wash off the Aphids. However a drench in early spring with Bifenthrin has a short term effect, but later on in the season the 'wash-off' effect seems to intensify.
Winter oil wash is a good - but messy - control. Provado - which acts with a contact and systemic action gives good control through most of the season.
Organic treatments include the cutting off of twigs and growths affected by woolly aphids and burning.