Answer: Aphids and Japanese beetles are the two most common insect pests of crape myrtle trees but they feed on the foliage, not on the branches. The only thing I'm aware of that might make patterns in the trunk are birds (sapsuckers or woodpeckers). For these pests, there's not much you can do except scare them away when you see them.
Wish I had more information to share. Best wishes with your crape myrtles!
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