Insect Damage to Timber
Rethinking control mechanisms
insecticides systematically used to control woodworm and other
beetle infestations are often not needed in historic buildings,
and in some cases make matters worse by killing the insects' principal
natural predators - spiders. This article examines the insects
that cause timber damage and the options for their control.
|Damage may be more extensive if, through neglect, the heartwood of the building’s timber has been modified by fungus. (All photos: Brian Ridout unless otherwise stated)
All of the
organisms that damage timber in buildings are part of the natural
process that takes dead wood to the forest floor, decomposes it
into humus and recycles the nutrients released back into trees.
Each stage in this process requires the correct environment and
if we replicate this in our buildings then the organisms belonging
to that part of the cycle will invade. A poorly maintained roof
is, after all, just an extension of the forest floor to a fungus.
All decay fungi require a great deal of moisture to initiate an
infestation and to maintain it but wood-boring beetles and their
larvae are more versatile. Some, like weevils, require high moisture contents and
wood that has been softened and modified by fungi. Weevils are
therefore a secondary problem because the wood must be partially
decayed already. Weevils will fly away if the wood dries and the
damage is usually easier to find than the beetles. Repairs are
all that are required and there is no necessity for chemical treatments.
The group of beetles which generally causes us the most problems
in buildings has, as its natural habitat, the dead parts of standing
trees. This is an environment that can be dry for significant
periods and the larvae are able to tolerate fairly low wood moisture
contents. These beetles belong to the world-wide family of Anobiidae,
and the two species that most concern us are the furniture beetle
(Anobium punctatum) and the deathwatch beetle (Xestobium rufovillosum).
In order to discuss them, however, we need to briefly consider
the anatomy and development of a tree trunk.
Tree trunks conduct
moisture and nutrients from the roots to the crown of the tree.
This function takes place in the outer part of the woody stem
called the sapwood. As the tree grows taller so the sapwood grows
wider from a band of cells just under the bark, but there is a
limit to the amount of sapwood that a tree needs, depending on
species and conditions where the tree is growing. The volume of
sapwood is generally proportional to the size of the leafy crown.
When the optimum sapwood content is reached, the inner sapwood
cells die as more are added to the outer edge so that the volume of sapwood remains approximately constant. The nutrients these
inner cells contain are absorbed and various metabolic products
are deposited in what is now heartwood. It is these metabolic
products that give the wood durability.
The larvae of furniture
beetle can easily attack the sapwood of our usual structural timbers.
Those of deathwatch beetles normally confine their attentions
to oak, but the heartwood of these timbers is extremely hard and
indigestible unless the chemistry of the timber has been modified.
Fortunately for this beetle, the dead part of a tree will always
contain fungus, and even a small amount of fungus will change
the chemistry of the wood so that the beetle larvae can exploit
These basic facts enable us to understand the damage we see
in buildings, and so to devise acceptable and efficient control
A trunk may be squared through the sapwood and then cut
into four posts through the heartwood. The resulting posts will
have two faces containing a little residual sapwood that the beetle
larvae might colonise and two faces of heartwood that will resist
attack. The beetle larvae might eat wood and the roof might be
made of wood but it does not mean that the insects can destroy
it. It is this damage in residual sapwood scattered amongst the
timbers that causes confusion. If a few rafters
in a Victorian softwood roof have beetle damage in their sapwood
edges then the inference is that most of the timber is immune
from attack and not that the roof is in danger of being eaten.
Check the pattern of damage and think about how the wood was converted.
How much sapwood is actually present?
|The internal structure of a tree trunk
Where softwood has been
used, the answer to this question is likely to depend on the age
of the building. Major fires in cities, fashion, and the cost of
oak all hastened the decline of timber framed building construction
during the 18th century and encouraged the use of European red
wood (Scot's pine) for structural timbers. This softwood had been
imported for centuries - mostly from Norway - but generally for
joinery. In the 1760s the huge pine forests that could supply
timber by river to the Baltic ports began to be exploited. Before
then, timber had never been a cost-effective cargo because its
volume was too great compared with its weight, and it had generally
been shipped as a part load. But the demand now became so great
that ships were adapted to carry only timber, and the trade continued
throughout the 19th century, with a reduction in volume during
the Napoleonic wars when the American trade was favoured by manipulating
The importance of this for us is that the trees felled
from these natural forests were hundreds of years old. The sapwood
growth on a tree is generally about 20 annual rings and this is
nothing compared with the two or three hundred years of heartwood
growth. The majority of the timber in our Georgian and Victorian
buildings is therefore resistant to furniture beetle attack and
the precautionary spray treatment of roofs (for example) in these
buildings is generally an unjustifiable use of pesticides.
situation changed after the First World War with trees grown in
plantations as a managed crop. These plantations were planted
where they would thrive best and thinned to maximise space and
therefore speed of growth. The stand of trees would be felled
after perhaps 50 years and the thinnings would be sold as soon
as they reached a useable thickness.
Now 20 years of sapwood growth
becomes significant because many sections of wood will have a
massive sapwood content. Furniture beetles can cause serious damage
to these timbers, and some form of biocide treatment, either pre-treatment
before use or spray treatment of an infestation if it develops,
may become necessary.
|The hewn face of this post has furniture beetle holes because there is residual sapwood.
The sawn face is heartwood that the beetles could not attack. (Photo: John Fletcher)
||The amount of sapwood in these plantation grown timbers is highlighted by blue-stain fungus.
beetles can be a more insidious problem. These insects live in
hardwood trees and are particularly common in willow trees along
the banks of rivers. They thrive in oak building timbers although
they will sometimes attack old softwood particularly, for some
unknown reason, in the Channel Islands. The insects can easily
destroy sapwood and flight holes throughout one or more faces
of a timber that spans a room or roof are generally attributable
to that form of attack. Damage will generally be superficial because
the beetles would not have been able to colonise the underlying
heartwood. The damage will usually be historical and rarely requires
The real problem, as mentioned earlier, occurs when
fungi have chemically modified the heartwood. Partially decayed
wall plates and bearings in damp walls make an ideal home for
the beetles because the environment is moist and stable. Effective
treatment may be impossible without causing more damage than the
beetles could because much of the infested timber will be inaccessible.
What is to be done?
Both furniture beetles and deathwatch beetles
require a little more water than is generally found in a dry and
well maintained building. The affects of drying and wetting are
accumulative and opposite. The drier the wood, the longer it takes
the larvae to grow; the adult beetles are smaller and they lay
Deathwatch beetles add a sexual twist to the problem.
The beetles don't feed so that the basic nutrient resources that
a female beetle has for egg laying and dispersal are those she
accrued as a larva. The male beetle provides significant additional
nutrients when they mate as part of the spermatophore package
passed between them, and she is weighing him when he climbs on
her back. If he has not fed well and is small then she will shrug
him off and continue to look for a mate. A butch beetle dies of
exhaustion, but presumably dies happy - a weedy beetle skulks
in the corner and dies of boredom.
|The deathwatch beetle larva eats timber. The beetle does not feed.
||The adult deathwatch beetle is usually 4-6mm long and is the reproductive stage of the
insect’s life cycle.
fact to remember about deathwatch beetles in your building is
that they have probably been there for centuries and will continue
long after you have gone. Beetle damage in oak timbers is a slow
process and if we make it slower by good maintenance then the
beetle population may eventually decline to extinction.
fact is that natural predation will help you. Spiders are a significant
predator and will help to keep the beetle population under control.
They will speed the decline of a beetle population in a well-maintained
The beetles fly to light and some form of light trap
may help to deplete a population. The place in which it is used
must be dark, so that there is no competing light source, and
the air temperature must rise above about 17°C during the emergence
season (April to June) so that the beetles will fly. This must
be discussed with English Nature if there are any indications
that bats use the space.
Beetle holes do not disappear when the
beetles have gone so it is sometimes necessary to confirm active
infestation if remedial works are planned. This is generally easy
with beetle damage in sapwood because the hole will look clean
and have sharp edges, usually with bore dust trickling from them. Infestation
deep within modified heartwood is more difficult to detect, particularly
because the beetles will not necessarily bite their own emergence
holes if plenty of other holes are available. This problem may
be overcome by clogging the suspected holes with furniture polish
or by covering a group of holes tightly with paper or card. Any
emerging beetles will make a hole that should be visible, so that
the extent and magnitude of a problem can be assessed. Unnecessary
pesticide treatments must be avoided.
Sometimes a building cannot
be dried enough to eradicate the beetles or a localised population
will have built up unnoticed. A few scattered beetles in a building
need not cause much concern, but dozens of beetles below a beam
end might indicate the need for some form of treatment if the
infested timber is accessible.
Insecticides formulated as a paste
can be effective - either applied to the surface or caulked into
pre-drilled holes - but the formulations may only be obtainable
by a remedial company. Surface
spray treatments are generally ineffective because they barely
penetrate the surface of the timber and the beetles' natural behaviour
does not bring it into much contact with the insecticide. Contact
insecticides might also kill the natural predators.
|Spiders are natural predators which help control
for entire buildings are available and the continental experience
is that they are effective. They are also likely to be expensive
but they may be the only way to eradicate a heavy and widespread
infestation without causing considerable damage to the building.
Two other beetles are worth a mention. The first is the House
Longhorn Beetle (Hylotrupes bajulus). This is a large insect that
produces oval emergence holes that are packed with little cylindrical
pellets. The beetles restrict their activities to the sapwood
of 20th century softwood, although there is now some evidence
that they will attack older softwood.
The beetle larvae can cause
considerable damage but infestation has generally been restricted
to the south west of London, possibly because they need a high
temperature before the beetles will fly. Old damage is, however,
frequently found elsewhere, thus indicating a wider distribution
in the past, and infested timber is sometimes imported. This is
an insect that might become more widespread because of climate
The second is the Lyctus or powderpost beetle. There are
several species that are rather difficult to tell apart. These
beetles live in the sapwood of oak. The beetles breed rapidly
so that many cylindrical pellets may be present and the round
emergence holes resemble those of the furniture beetle.
and has always been, a pest of newly installed oak. Timbers with
an exploded sapwood surface are frequently found in old buildings
and the damage will have occurred during a few decades after the
timbers were installed. Our main interest with these beetles is
that they seem to have become more common at the present time.
Beetle infestation within a few months of a new oak construction
will be Lyctus beetle in the sapwood and not furniture beetle.
The problem can be avoided by using oak with minimal sapwood content.
The beetle infestation will cease after a few years but spray
treatment may be necessary if an infestation is heavy.
- Brian Ridout,
Timber Decay in Buildings: The Conservation Approach to Treatment, E & FN Spon, London, 2000
- AF Bravery, RW Berry, JK Carey, and DE Cooper, Recognising Wood Rot and
Insect Damage in Buildings, Building Research Establishment,