We have many types of oak trees on our property. Some of the oaks have limbs that look as though they are being attacked by something, possibly a borer. The bark of the limb will be stripped off and eventually break off from the tree. The remaining stump sometimes has a hole in it and the remaining bark almost looks like it has been cut in a circle around the base of the limb. I am enclosing a couple of pictures to illustrate. Does anyone know what this might be? I cannot find the precise problem described on the internet anywhere.