What created this cocoon on a beach plum? - Cooperative Extension: Garden and Yard - University of Maine Cooperative Extension

Question: I recently discovered this growth/cocoon on a young beach plum. It is only about 5/8″ long and at first assumed it to be some kind of cocoon given the perfectly round opening, only it appears to the be the same woody material as growing on the beach plum (or perhaps very well camouflaged). Any read more