Answer: If the holes have a pattern such as a line or a square, they are from either woodpeckers or sap suckers. Any insect you see are secondary (they are attracted to the holes and to the sap that is exuded from the injury). When beetles attack a tree the holes are random, not in a straight line. Woodpeckers are looking for insects beneath the tree bark but they also rap on trees to attract mates; sap suckers feed on sap which they get from pecking holes through the bark. There's not much you can do to keep these birds from damaging your trees and, in the case of insects, you may be able to control them but you'll have to capture a few first and have them identified so you'll know what you're dealing with. Typically, healthy trees can tolerate some bird and some insect damage. However, stressed or sick trees often cannot overcome the damage. I'd try to collect some of the insects you see and have them identified, either through your local cooperative extension office, Master Gardener Clinic, or well-staffed nursery.
Best wishes with your trees!
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