CONTROLLING DEATHWATCH BEETLES

CONTROLLING DEATHWATCH BEETLES

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.

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 up 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. 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 holes 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 the 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 structural degradation of 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 southwest 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