Insect Damage to Timber: Rethinking
control mechanisms
by Dr Brian Ridout
Originally published in The Building Conservation Directory 2005, page
163.
The 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 responsible and the options for their control.
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.
Deathwatch beetle damage
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 it.
These basic facts enable us to understand the damage we see in buildings,
and so to devise acceptable and efficient control methods.
Figure 1 shows how 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 taxes.
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.
The 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.
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