Holes, frass and what made them
Wood-Boring Beetles and the Timbers They Attack
Woodworm is the general name given to damage caused by the larvae of wood-boring beetles. Different species attack different timbers, leave different exit holes and present very different levels of risk to a property.
This guide explains the beetles most commonly encountered in UK buildings, the evidence they leave behind and why old flight holes do not necessarily mean that an infestation remains active.
The first thing you measure
Exit Hole Size by Species
The size and shape of an exit hole can help narrow down which beetle may have affected the timber. Small round holes are commonly associated with furniture beetles and powderpost beetles, while the much larger oval holes left by house longhorn beetles are easier to distinguish.
However, hole size does not provide a diagnosis on its own. Several species leave similarly sized holes, and damage from an historic infestation can remain visible long after the beetles have disappeared.
A professional timber survey also considers the type and age of the timber, the appearance of the bore dust or frass, surrounding moisture conditions and whether there is evidence of recent activity.
Species by species
Which Beetle Attacks Which Timber?
Not every wood-boring beetle can infest every piece of timber. Some species attack softwood and hardwood sapwood, while others are associated with partially decayed oak, recently installed hardwood or bark remaining on roof timbers.
Correctly identifying the beetle helps establish whether the damage is likely to be active, whether the surrounding conditions continue to support an infestation and whether any action is necessary.
Common Furniture Beetle
Anobium punctatum
- Timbers attacked
- The sapwood of softwoods and European hardwoods. It can affect structural building timbers, floorboards, roof timbers and furniture.
- Exit hole
- Round and approximately 1 to 2mm in diameter.
- Frass
- Coarse and gritty, containing distinctive lemon-shaped pellets.
The common furniture beetle is the wood-boring insect most frequently encountered in British properties. Its larvae develop beneath the timber surface, where they can remain for several years before emerging as adult beetles.
Old exit holes are not proof of continuing activity. Fresh frass, newly formed holes and adult beetles found during the emergence season provide more useful evidence when assessing whether an infestation remains active.
Deathwatch Beetle
Xestobium rufovillosum
- Timbers attacked
- Partially decayed hardwoods, principally oak. It is most commonly associated with historic buildings, churches and older properties containing substantial hardwood timbers.
- Exit hole
- Round and approximately 3mm in diameter.
- Frass
- Contains distinctive bun-shaped pellets that can normally be seen without magnification.
Deathwatch beetles are closely associated with timber that has already been affected by fungal decay. Discovering evidence of this species should therefore prompt an investigation into both the infestation and the source of moisture affecting the wood.
Adults generally emerge during April, May and June. Where activity is suspected, inspecting the property during this period may provide clearer evidence.
House Longhorn Beetle
Hylotrupes bajulus
- Timbers attacked
- Softwood, particularly the sapwood of roof timbers.
- Exit hole
- Oval and considerably larger than most other woodworm holes, typically measuring approximately 6 to 10mm.
- Frass
- Cylindrical pellets packed tightly within the larval tunnels.
House longhorn beetle can cause serious damage because the larvae create extensive tunnels beneath the timber surface. In heavily affected roof timbers, only a thin outer layer of apparently sound wood may remain.
Within the UK, established infestations are principally associated with parts of Surrey and neighbouring areas. It is not considered a typical woodworm problem across the Midlands and North of England, although its distinctive damage remains important to recognise.
Powderpost Beetle
Lyctus brunneus and related species
- Timbers attacked
- The sapwood of wide-pored hardwoods, including oak, ash, elm and sweet chestnut. Infestations are generally associated with unseasoned, recently dried or relatively new timber.
- Exit hole
- Round and approximately 1 to 2mm in diameter.
- Frass
- Very fine and flour-like, falling freely from the exit holes.
Powderpost beetle larvae feed on starch contained within hardwood sapwood. Infestations may therefore be found in newer hardwood flooring, furniture, panelling and imported timber products.
As the timber ages, its available starch content reduces and conditions become less suitable for the beetle. The age and type of the affected wood are therefore particularly important when making an identification.
Wood-Boring Weevil
Euophryum confine
- Timbers attacked
- Damp and partially decayed timber. It is frequently discovered behind skirting boards, within wall plates and in wood already affected by wet rot.
- Exit hole
- Small and generally around 1mm, with a ragged appearance rather than a clean circular edge.
- Frass
- Fine material accompanied by distinctive surface channels or striations within the affected wood.
Wood-boring weevils are usually a consequence of persistent damp and timber decay rather than a separate cause of deterioration. Their presence indicates that the moisture source and condition of the timber require investigation.
Drying the area and addressing the conditions supporting fungal decay will normally make the timber unsuitable for continued weevil activity.
Bark Borer
Ernobius mollis
- Timbers attacked
- Bark remaining on softwood, most commonly along the waney edges of roof rafters.
- Exit hole
- Round and approximately 1.5 to 2mm, with the holes confined to the bark-covered edge.
- Frass
- Loose and similar in colour to the surrounding bark.
Bark borer damage is frequently mistaken for common furniture beetle activity. However, this species attacks the remaining bark rather than the sound timber underneath.
Where the holes and damage are confined to the bark, the structural timber is not normally at risk and insecticidal treatment should not be required.
Identification table
Telling Woodworm Apart
The word “woodworm” can refer to several different beetle species. Identifying the likely insect requires more than measuring a hole, particularly where similar-sized holes have been left in different types of timber.
The table below provides a useful comparison, but any conclusion should also consider whether the evidence is fresh, where it has been found and whether the timber conditions remain suitable for infestation.
| Species | Timber | Exit hole | Frass |
|---|---|---|---|
| Common furniture beetle | Softwood and hardwood sapwood | Round, 1–2mm | Coarse, gritty and lemon-shaped |
| Deathwatch beetle | Decayed hardwood, principally oak | Round, approximately 3mm | Distinctive bun-shaped pellets |
| House longhorn beetle | Softwood roof timbers | Oval, approximately 6–10mm | Cylindrical pellets packed within tunnels |
| Powderpost beetle | Recently dried hardwood sapwood | Round, 1–2mm | Fine and flour-like |
| Wood-boring weevil | Damp, partially decayed timber | Ragged, approximately 1mm | Fine, with surface channels |
| Bark borer | Bark remaining on softwood | Round, 1.5–2mm | Loose and bark-coloured |
An exit hole confirms that an adult beetle has emerged from the timber at some point. It does not establish when the hole was created or whether larvae remain active elsewhere in the wood.
Warning signs
Signs of an Active Woodworm Infestation
Woodworm holes can remain visible for decades, including in properties where the infestation has already died out or received previous treatment. The key question is not simply whether holes are present, but whether there is reliable evidence of current activity.
Possible warning signs include:
- Fresh, clean exit holes with pale edges
- New deposits of frass beneath or around the timber
- Adult beetles found near affected wood or on window ledges
- Live larvae discovered within damaged timber
- Crumbling edges around flight holes
- Fresh tunnels exposed by damage or investigation
- Weakened floorboards, joists or roof timbers
- Increasing evidence during the spring and summer emergence period
Dust beneath a timber does not automatically confirm active woodworm. Historic frass can be disturbed by vibration, building work, cleaning, spiders or movement within the structure. The location, condition and freshness of the evidence all need to be considered.
Professional investigation
What Does a Woodworm Survey Include?
A woodworm survey examines the accessible affected timber and the evidence surrounding it. The surveyor will consider the size and shape of the holes, the characteristics of any frass, the timber species, its age and condition, and whether adult beetles or fresh emergence signs are present.
The inspection may include accessible floorboards, joists, roof timbers, wall plates, structural woodwork and joinery. Moisture and fungal decay will also be considered where the suspected beetle species depends on damp or deteriorating timber.
The purpose is to identify the likely species, assess whether the infestation appears active or historic and establish the visible extent and significance of any timber damage. You will then receive a written report explaining the findings and any further investigation or action considered necessary.
Data note
Questions
Woodworm FAQs
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Tell us where the affected timber is located and what you have noticed. We will recommend the appropriate survey, confirm what the inspection will cover and provide the cost before anything is booked.
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