Woodworm in beams: old holes or an active infestation?
Exit holes remain visible after beetles have left the timber. The holes alone do not tell you whether larvae are still present in the beam today. Record the signs first, and have insect activity, moisture and structural condition assessed separately.
The short answer
Old exit holes alone are not a sufficient reason for treatment. New bore dust, changes around holes and beetles found nearby can help an investigation. A short period without fresh signs does not rule out activity. If load-bearing beams may be weakened, a structural assessment takes priority.
In this article
What do holes and bore dust tell you?
The term ‘woodworm’ does not establish which insect made the signs or whether it is still active. Several wood-boring beetles can cause damage; their larvae live inside the timber. An inspection should therefore consider the likely species, current activity and the condition of the beam. Monumentenwacht recommends first looking for signs and monitoring their development before assessing whether treatment is needed. [1]
Fresh-looking bore dust or sharply defined holes may indicate activity. However, vibration or building work can also dislodge old dust. Do not assess a single pile of dust in isolation from the rest of the investigation. [2]
Three questions a timber inspection should answer
| Question | Possible evidence | What this does not establish |
|---|---|---|
| Which insect may have made the signs? | A beetle specimen, the shape and size of holes, bore dust and timber species. | Hole size alone is not enough to identify the insect. |
| Is there activity now? | New exit holes documented over time, insects found or suitable monitoring. | Old damage and dislodged bore dust do not prove a current infestation. |
| Is the timber still suitable for its function? | Assessment of remaining timber, joints, moisture and loads by a suitably qualified building or structural professional. | A pest treatment report or the absence of new beetles does not establish load-bearing capacity. |
Monumentenwacht and SPAB both recommend investigating the problem first. Only then can the need for intervention and its appropriate extent be assessed. [1] [2]
Ask for observations and conclusions to be recorded separately. ‘There is bore dust on beam B3’ is an observation; ‘active infestation has been confirmed on B3’ needs supporting evidence. Ask for uncertainties and inaccessible parts of beams to be stated explicitly, so a later inspector can see what was actually examined.
Take photographs and record changes
| Question | What should you record? |
|---|---|
| Where is the damage? | Number the beam or piece of furniture and take an overview photograph. |
| What does it look like? | Take close-up photographs with a scale beside the hole. |
| What has changed? | Record new dust, any beetles found and the date of discovery. |
| What happened in the room? | Mention building work, moving items or cleaning before the dust appeared. |
| Is there moisture or distortion? | Photograph accessible signs of leaks and describe visible changes. |
Keep previous inspection reports and information about earlier treatments. If a former occupant says the timber was treated, ask whether a report is still available. Record what they tell you and which documents can be provided.
Why one check may not be enough
Beetle emergence varies through the year. Monumentenwacht describes deathwatch beetles as emerging mainly in spring, and common furniture beetles in the warmer summer months. It identifies detection paper as one tool for monitoring activity. [3]
Ask a specialist which monitoring method suits your timber, when it should be checked and how the findings will be recorded. Do not sand, pierce or attach paper to valuable historic timber yourself before discussing the investigation with a specialist.
Which approach fits the findings?
The choice depends on species, activity, timber condition and function. If there is a moisture problem, its cause may need to be addressed first. For old signs without confirmed activity, targeted monitoring may be more appropriate than immediate treatment. Where activity is confirmed, the inspection should establish which areas need intervention. Weakened structural elements also need a building or structural assessment: insect treatment does not restore lost strength.
Moisture readings also need interpretation. SPAB warns that previous surface treatments can affect electrical moisture-meter readings. Ask the report to state what was measured, which instrument was used and how the reading was assessed. A percentage without an explanation of the measurement is not enough to judge timber condition. [2]
Questions before accepting a proposal: what problem does each measure address, why is the proposed extent necessary, how will valuable timber be preserved and when will the result be checked? For combined work, ask who is responsible for the moisture source, insect control and structural repairs. Different specialists may be needed.
What should you discuss with the inspector?
Ask the inspector to support the conclusions and proposal: which species was identified, what evidence shows activity and which areas need intervention? If beams are sagging, pieces of timber are coming loose or load-bearing capacity is uncertain, arrange a structural assessment first.
When contacting us, explain whether the timber is a load-bearing beam, flooring or furniture, describe the signs and mention any previous investigation.
Further information: inspection and treatment of woodworm and longhorn beetles.
Do you live or work in Limburg? See our information about pest control in Limburg for help and contact options in your area.
Sources and notes
Sources checked on 18 September 2026. This checklist helps you prepare for an inspection. Photographs can support an inspection, but cannot establish the remaining load-bearing capacity of timber.
