Is this sawfly larvae
Here is more information about them:
The dogwood sawfly is an occasional pest of dogwood. The larvae feed on the foliage of several species of dogwood. The larvae resemble caterpillars and are most often seen covered with a white powdery material.
Adult sawflies emerge during late spring and early summer. The female lays eggs on the undersides of the leaves. Upon hatching, the young larvae feed together and skeletonize the leaf. As they grow, they will eat all of the leaf except the midrib. After the second molt, the larvae become covered with a white powdery material. After their final molt they loose the powdery covering and change color. The mature larvae are yellowish with a shiny black head and black spots. These mature larvae will wander about in search of an overwintering site, generally in soft or decaying wood. There is one generation a year.
Control: is seldom needed. Hand picking is the easiest way to control this sawfly. However, if the infestation is heavy, they may be controlled with an insecticide registered for use on dogwood.
Here's a link for more info:
It is definitely not a dogwood. It is a Bush (multi stemmed) and it gets flat clusters of purple berries just like the elderberries I remember as a child. Is it possible that, that particular strain of sawfly could adapt to elderberry?
Red twig dogwoods are shrubs and also make flat clusters of bluish berries. The leaves in your photos are dogwood leaves. Elderberries have compound leaves. Here are some links:
http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=279363&isprof... (For Red twig dogwood info)
I have never seen dogwood sawfly larva on elderberry.