Scale
Scale insects on oak twig
Scale is an insect of the aphid family, and like most aphids, they feed by sucking the juices or nutrients out of the foliage and sometimes twigs. Scales get their name from the waxy covering that adult females secrete and hide under while they feed and lay eggs.
Scale insects spend the winter as eggs hidden under the scale attached to twigs. In the spring, the eggs hatch into nymphs or "crawlers" that have legs and many move to other parts of the plant or onto new plants. The crawlers grow through several stages until the adult female settles in, secrete their covering, and lay their eggs. The adult females are immobile once they have secreted their covering.
There are many species of scales infesting a wide variety of plants. Most scales can be identified by first identifying the species of the host plant and then the size, color, and shape of the scale covering.