This is not a well-studied group of scales. Not much is known about their biology or management.
The oak eriococcids have one generation per year. The cottony sack you see now is the ovisac (egg sack). If you dig around in the cottony material you will see tiny yellowish eggs. Pretty soon crawlers will emerge from the cottony mass to find a spot to settle and feed. They also produce honeydew so these are some of the scales responsible for the tiny drops on your windshield if you park under willow oaks or other infested species. Heavily infested trees like some on campus make the sidewalk black with sooty mold.
Other felt scales of importance are the azalea bark scale, European elm scale, and the new crape myrtle bark scale. Control of felt scales has also not been studied well. However, like their aphid and mealybug relatives, imidacloprid and other neonicotinoids should provide some control. If you watch for the crawlers (soon!) you could make a dent with horticultural oil. The oak eriococcid sscales are one of the earliest scales to produce eggs in Raleigh. They are harbingers of all the other scale crawlers to come.