If there are a lot of them, then you need to scrub the tree with Winter Wash( a natural plant based insecticide) or with old fashioned soap and water. Or if you have a power spray Karcher type thing then give the tree the once over with soap or insectice in it. Use a setting which does not damage the tree, but does remove the wool.
You may need to repeat this treatment a number of times to get rid of them all.