This appears to be an infestation of Crapemyrtle Bark Scale, a relatively new insect pest that prior to a few years ago only occurred in the southern U.S. The lined page provides information about this particular scale species (what is known so far, at least, since it's still being studied in Maryland) and links to our general scale information page for management options.

Yes, cutting the shrubs back will not only immediately remove the bulk of the scale insects, but will make what's left of the plants much easier to treat if you opt to use insecticides. (Especially if you use a low-toxicity option like horticultural oil that requires thorough contact with the insects to work.) Be prepared for successful management of an established scale population to potentially take more than one year, and if there are infested crapemyrtles nearby (say, in that part of the neighborhood), the plants might get recolonized some day in the future by crawlers blowing in on the wind or hitching a ride on wildlife feathers/fur, so be vigilant for new populations and try to physically rub them off the bark (gently enough to not damage the plant) if found at that point.

Miri