Borer insects often drill into the main trunk or big branches off the main trunk. Inspect the highest area damaged looking for holes made by the borer as well as some ooze coming out of the holes. The ooze may have fallen down into a lower branch or leaves. Borers usually deposit eggs in the Fall and the larvae feeds off the tree, exiting some time after being born.

I had a Chinese Pistache Tree affected by borers last year and the top 20% of the tree did not leaf out but the tree survived. Does not look too bad a year afterwards.

Luis