Management and life cycle information can be found on our Crapemyrtle Bark Scale and Introduction to Scale Insects pages; they include manual removal and various approaches to insecticide use. The timing will depend on which part of the life cycle is being targeted (generally, this is the crawler stage, which is discussed in the first page linked above). Fortunately, we are seeing increasing levels of predation of this scale by the larvae of our native ladybug species, so intervention might not be necessary if scale populations aren't very high. (They don't look too bad in the photos.)

Miri