The holes have been there for a number of years and never seem to go away. What sort of insect causes them and will it cause problems for my trees?
The rows of holes you are describing aren’t created by an insect, they are actually made by a bird. Woodpeckers create these rows of small holes in the bark of a tree, and then return on a regular basis to eat any insects that have taken up residence in those holes. It’s not an ideal situation to have open wounds in the bark of a tree since it is a defensive layer to various pathogens, but chances are that a vigorous tree can survive woodpecker damage if it isn’t too extensive.