Technical Q&A 28 : Wood-boring Insects
Beetles can be a threat to old buildings but, as Douglas Kent, SPAB Technical
Secretary explains, sound timbers and old woodworm holes are sprayed unnecessarily
when a building changes hands.
Q. What are wood-boring insects?
A. They are species, including certain beetles (‘woodworm’), that feed on wood
and may seriously damage building timbers. In the UK, they include the furniture
beetle (Anobium punctatum), closely-related death watch beetle (Xestobium rufovillosum)
and other, more minor, decay insects such as the house longhorn beetle (Hylotrupes
bajulus), which is found in some southern areas. It is the beetle larvae (grubs)
that burrow into wood. Eventually the larvae pupate and adult beetles emerge to
mate via flight holes or fissures. Females lay eggs in crevices, maybe old flight
holes, to repeat the cycle.
Termites, another form of wood-boring insect, could gradually become more prevalent
in this country due to global warming.
Q. Why do insects attack timber?
A. Wood-boring insects, like timber-decaying fungi, only establish themselves where
dampness exists. Beetle damage in buildings is usually confined to sapwood, although
heartwood is vulnerable too if fungus is present. Sometimes active death watch
infestation is inadvertently reported in dry timbers where dust is dislodged from
old flight holes, perhaps by building work.
Good preventative maintenance and moisture monitoring can avert dampness and,
therefore, ensuing damage caused by wood-borers. However, over-reliance should
not be placed on surface readings from electrical moisture meters. False readings
may occur, for instance, because of surface deposits or past chemical treatments.
Q. How do I recognise active insect infestation?
A. Much beetle attack found in buildings is extinct. Active outbreaks are identified
by holes with sharp rather than round edges, and the interiors and bore dust (‘frass’)
are not dark but the colour of freshly-cut timber. Furniture beetles target both
softwoods and hardwoods, producing 1-2mm wide holes and lemon-shaped frass. In
contrast, death watch beetles favour oak and may be recognised by 2-3mm diameter
holes and bun-shaped pellets. Signs may be less obvious with smaller death watch
outbreaks as adults sometimes emerge from cracks instead of holes. The distinctive
sound death watch beetles make tapping their heads on timber in the springtime
during courtship may, nevertheless, reveal their presence.
Where signs of beetle activity are ambiguous, this can be resolved by monitoring
for a year. Water-soluble glue is used to cover a group of holes with acid-free
tissue paper, through which beetles will punch their way if present. Alternatively,
holes can be clogged with wax polish.
Q. What’s the solution for insect attack?
A. Because dry timber is immune to attack, the first measure for successfully arresting
its decay is to eliminate all causes of dampness and promote drying to reduce
its moisture content to below about 15 per cent – for example, by controlling
condensation. Major damage could necessitate conservative timber repairs, but
avoid automatic wholesale replacement.
Secondary measures may be required, particularly where infestation is extensive,
timber lacks durability or it is hard to cut moisture levels sufficiently. Action
could entail targeted chemical treatment - but not as a substitute for promoting
drying, or general precaution for extinct or non-existent outbreaks merely to
obtain a guarantee. Alternatively, heat treatments for entire buildings are now
possible. In some situations, traps using pheromones or ultraviolet light may also help
reduce furniture or death watch beetle populations respectively, without preservatives.
Where advice is sought, this should be from an independent chartered surveyor
or consultant, not a remedial treatment contractor.
Q. Is trimming back beetle-damaged timber a good idea?
A. Often not. Unfortunately, degraded surface material continues to be removed (‘defrassed’)
without good reason, by cutting, scraping or sandblasting. The intention is commonly
to allow chemical treatment of the underlying heartwood, but this is frequently
not at risk and accepts preservatives much less readily than the sapwood removed.
Serious mutilation can result.
Similarly, sanding can harm the historic value of timbers and, in addition, expose
a ragged mess of beetle runs.
Further reading
Is Timber Treatment Always Necessary? An Introduction for Homeowners, SPAB Information Sheet 14 (1999)
Timber Treatment: A Warning About the Defrassing of Timbers, SPAB Information Sheet 2 (1990)
Timber Decay in Buildings: The Conservation Approach to Treatment, by Brian Ridout, Spon Press (2000)
© Douglas Kent 2007