What does very fine wood frass beneath structural wood and pinhole openings in the wood surface indicate?

The presence of very fine wood frass beneath structural wood, along with pinhole openings in the wood surface, is indicative of powderpost beetle activity. Powderpost beetles are known for creating small holes in wood as they exit after larval development. The frass consists of fine wood particles they produce while feeding on the wood, which is specifically indicative of their infestations.

This fine frass is usually much smaller and more powdery compared to other types of wood damage. In contrast, termite activity typically results in larger, more fibrous frass and mud tubes rather than the very fine dust. Damp wood damage may lead to softening or decay, which is also not associated with frass. Woodpecker damage would generally involve larger holes as the birds peck at the wood, typically seen as distinctive large cavities rather than the small pinholes characteristic of powderpost beetles. Thus, when you observe very fine frass and small pinholes, it is clear evidence of powderpost beetles being the cause of the damage.