Now let's take a closer look...
Upon closer inspection we see the top die back is being caused by coalescing sap sucker wounds. The yellow belly sap sucker drills holes in over 1000 species of trees. The purpose of this behavior is to feed on the high sugar content tree sap, and any insects that may get caught in it. Sap suckers form two kinds of holes. The first is a round hole that extends far into the tree and is not enlarged. These are the holes that are commonly associated with species like sugar maple. The second is a square hole that is shallow, but which needs to be be constantly maintained for sap flow. It's this square hole that I've seen kill whole branches of trees over the years, most notably holly species.
There aren't any good ways to prevent sap suckers from enjoying your trees. Repellent sprays on the market seem to not be effective. For high value trees I've seen burlap being wrapped around trunks during times of sap sucker activity. This provided some modicum of protection, but can be unsightly and labor intensive. In the interest of political correctness, I'll refrain from any BB-gun jokes.
