I suspect that the critters you have seen are barklice, more properly named Psocids (pronounced "SOSS-sids"). They are also called tree cattle because of their habit of living as a group on the bark of hardwoods, particularly oaks. Large numbers of adults and nymphs are occasionally seen on woody shrubs and trees, living underneath a fine silken webbing. These insects do not harm the tree; they feed on lichens, fungi, and dead animal and plant matter. Because they do not harm the tree, nothing needs to be done. However, if the appearance bothers you, a strong hose-directed stream of water will wash them and the webbing off the plant.
If you are not sure of this identification, take a specimen to your local Extension agent for positive ID and management suggestions.
