After they hatch, the larvae feed beneath the bark. Some species bore into the wood, others remain under the bark. Adult flatheaded borers emerge in early spring and summer and lay their eggs in cracks and wounds in the bark. The larvae first feed beneath the bark then bore into the heartwood. Because they're generally more numerous, roundheaded borers often do more damage than their flatheaded cousins.
Borers make holes as large or larger than a pencil.
The hole is usually packed with excelsior-like material called frass. The frass can sometimes be found strewn around the trunk of the tree. A discharge of sap from the tunnel opening can be seen as a discoloring of the surrounding bark. In comparison, flatheaded borer damage usually consists of shallow, long-winding oval galleries beneath the bark, usually on the sunny side of the tree. Once borers have entered a tree, control measures are difficult.