Insect Damage to Timber
Rethinking control mechanisms

Brian Ridout

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 that cause timber damage and the options for their control.

Carved wooden detail riddled with wood boring insect holes
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 it.

These basic facts enable us to understand the damage we see in buildings, and so to devise acceptable and efficient control methods.

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?

Diagram showing internal structure of tree trunk
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.

Timber post with emergence holes Stack of timber with blue-stain fungus visible in end grain
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.

Deathwatch 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 treatment.

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 fewer eggs.

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.

Deathwatch beetle larva Adult deathwatch beetle
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.


CONTROLLING DEATHWATCH BEETLES

The first 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.

The second 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 building.

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 17C 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.

Spider and web
Spiders are natural predators which help control deathwatch beetles.

Heat treatments 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 change.

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.

This is, 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.

 

Recommended Reading

  • 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, Watford, 2003

 

This article is reproduced from The Building Conservation Directory, 2005

Author

DR BRIAN RIDOUT MA PhD AIWSc is Managing Director of Ridout Associates, timber decay and damp consultants, and he is commissioned by English Heritage to advise on timber decay. In addition to a PhD in entomology at Birkbeck College, University of London, Brian Ridout lectures widely and is the author of several articles and books on the subject.

Further information

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