Woodworm is not one insect
Several beetle species can attack building timber, and they do not all favour the same type or condition of wood. Correct identification can therefore change the treatment decision.
Exit holes are evidence of past emergence
The familiar small holes show where adult beetles left the timber. They do not, by themselves, prove that larvae are still active inside.
Look for fresh evidence
Fresh frass, newly appearing holes, adult beetles and continuing damage are more useful signs of current activity than old dark holes that have been present for years.
Timber condition still matters
Some insect attack is mainly confined to sapwood and may have little structural significance. Other infestations, particularly where timber is already damp or decayed, can be more serious.
Useful technical sources
- Historic England — Timber Treatments and Pest Control
- Historic England — Timber Decay in Buildings and its Treatment
These links provide background guidance. A particular building still needs its own inspection.