I have seen at least five samples of red twig dogwoods here in the Extension office with completely defoliated twigs. The culprit is known as the Dogwood Sawfly. Dogwood Sawfly, Macremphytus tarsatus, is a significant pest to dogwood (Cornus) species. Because the Dogwood Sawfly takes on several forms while in the larval stage, it may not be easy to identify. The larvae strips plants at the end of season, normally just before late August.
The wasp-like adult sawfly lays eggs that hatch into larvae, the first instar of which is an almost translucent yellow. Look for groups of these larvae on the undersides of leaves that are being skeletonized. The second instar appears to be covered with a chalky powder, and the last instar is a one inch long creamy-yellow larva that has a shiny black head and black spots (see photo).
Normally damage is short lived, therefore control is not warranted.