Insect Damage to Timber: Rethinking control mechanisms
by Dr Brian Ridout
Originally published in The Building Conservation Directory 2005, page 163.

[Continued from page one]

Death watch 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 death watch 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.

Death watch beetles add a sexual twist to the problem. The beetles don’t feed, thus 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.

Controlling Deathwatch Beetles

The first fact to remember about death watch 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.

Deathwatch beetle larvae

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 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.

Deathwatch beetle caught in a spider's web

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 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.

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 beetles 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.

 

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