Small holes found in in tree trunks are often blamed on wood boring insects. While insects are truly the culprits in some cases, many holes are the result of a different animal all together.
Arrangement of the holes on a tree is often a good indicator of what pest is responsible. For example, if the holes are arranged in neat rows or columns, they can usually be attributed to birds rather than insects.
Sapsuckers, (a type of woodpecker) can be considered pests because they attack living trees. These birds feed largely on sap that they harvest by drilling a line of shallow holes in the bark of the trees. The resulting "sap wells" fill with sap and also attract and trap visiting insects. Both are food sources for the birds.
Sapsuckers injure ornamental and fruit trees, especially if the trees are young. In most cases, however, the damage that they make is neither permanent nor fatal to the tree.