Appendix A: Wood-Boring Insects

Appendix A: Wood-boring Insects Timber does not contain the seeds of its own destruction for it will last indefinitely unless attacked by an outside agent such as wood-rotting and sap-stain fungi (discussed in appendix B). Most other damage to timber is caused by insects and of these, a few species of wood-boring beetles are by far the most serious pests in temperate regions. In the sub-tropics and tropics, termites, popularly called white ants, can be devastatingly destructive, and in salt and brackish water several species of crustaceans can be extremely destructive. Not all insects associated with timber cause damage, for example, the land form of wood-louse, found under any piece of wood that has been left in contact with the ground, in sheds, or in the open for any length of time, causes no significant damage. In temperate climates the timber user is concerned with a small number of beetles which, like all insects, pass through several stages of development from egg to adult beetle, called a life-cycle. The adult beetles mate, and the female lays eggs. These hatch into grubs, become larvae, which go through a resting stage, followed by pupation, when metamorphosis occurs, to eventually emerge as an adult beetle. The damage beetles do is frequently referred to as 'worm' in timber. Except for the group known as Ambrosia beetles, the damage is done by the feeding larvae, which tunnel in timber, feeding on cell-wall substance or the cell content. With Ambrosia beetles it is the adult that does the tunnelling, introducing a fungus, known as Ambrosia fungus, on which the larvae feed. The different types of insects, their means of identification, and methods of control, are discussed fully in various pamphlets and bulletins. ERE Digest 307: 'Identifying damage by wood-boring insects' and the publication Timber pests and their control' produced by TRADA and the British Wood Preserving Association, are excellent publications on the subject of beetle infestation. 142 Appendix A: Wood-boring Insects 143 Beetle identification Basically, the size and shape of the exit holes, and the nature and colour of the bore dust (frass) provide the means of identifying the different pests. Unfortunately for identification purposes, it is by no means certain that adult beetles, or the remains of such beetles, will be found even when there is continuing active attack. Some beetles attack timber in the forest or while it is seasoning after conversion, whereas others only attack dry timber, or timber that has first been decayed by wood-rotting fungi. AMBROSIA BEETLES. This group does a considerable amount of damage to tropical timbers in particular, although there are a few species in this country that are occasionally found in oak and other hardwoods. The popular names for these beetles is 'pin-hole' or 'shot-hole' borers. The adult beetle tunnels spirally at right-angles to the grain, into living trees and newly felled logs, laying the eggs in specially constructed egg-chambers. At the time of egg-laying, the adult introduces the Ambrosia fungus into the galleries, upon which the larvae feed when hatched. Provided the extent of pin-hole and shot-hole borer damage infestation is not excessive, the timber can safely be used as there is no risk of spread of attack and still less risk of infections of other timber in the vicinity. This is because as the converted timber dries, the fungus also dies, and the larvae cannot complete their life-cycle for want of food. It is important to recognise such wormy timber for what it is, since there is no need to apply an insecticidal treatment to timber containing this form of beetle attack, see figure 23. Among carcassing timbers, hemlock is the one species likely to contain pin-worm damage, and since hemlock is quite extensively used in modern estate development, the surveyor should be able to recognise the holes for what they are, so that he can assure his client that the attack is of no practical significance, and calls for no remedial measures. LONGHORN BEETLES. A large number of species belong to this group, the majority of which are tropical whose larvae usually die when the timber is brought into temperate climates. Exceptionally, an adult beetle may emerge a few years later, but it would be unable to mate and re-infest timber in a temperate climate. There is one temperate-region species, the house longhorn beetle (Hylotrupes bajulus L.) figure 24(i). This is a serious pest in parts of northern Europe and in some districts infestation has to be notified, and appropriate remedial measures are compulsory. The house longhorn beetle which is 10-20 mm (approximately 1/2_3/4 inch) long, attacks only the sapwood of softwoods. The larvae tunnel just beneath the surface, completely destroying the sap wood behind a shell of appar ently sound wood. The life-cycle is a long one and, in some cases, can be upwards of ten years. The result of such a long life-cycle is that serious damage may result before the first flight holes bring the attack to light. The frass contains pellets that resemble the flints of petrol lighters when examined with a pocket lens, see figure 24(ii). The flight holes made by the adult beetles as they emerge are 6-10 mm (approximately 1/4 to 1/2 inch) long and oval in section. The finding of frass or oval flight holes is not proof of house longhorn infestation, and still less that there is continuing activity. Other beetles produce similar frass and oval flight holes. Active house longhorn beetle infestation is generally found in the Home Counties. Even in these areas no active infestation has been found in any building more than fifty years of age, unless new timber has recently been used for repairs in such buildings. The reason for this appears to be that the habitat requirements of the house longhorn beetle are critical. The larvae only hatch if temperatures are sufficiently high and because of the long life-cycle, it is probable that re-infestation often cannot occur because 144 Structural Surveying Figure 23 Pin-hole damage (Crown copyright: Reproduced by permission of the BRE Princes Risborough Laboratory) a sufficient number of male and female beetles have not emerged at about the same time, and during a spell when temperatures are adequate for the eggs to hatch. Although attack is not confined to roof timbers, it is most frequently found in rafters, purlins and ceiling joists within the roof void, the particularly vulnerable timbers being rafters on either side of a chimney stack containing flues in regular use. Exit holes in the lead flashings to dormer windows have been found indicating that the adult's jaws are powerful. If the surface of carcassing timber is seen to be bulging outwards as if the wood were covered with blisters, attack should be suspected and the timber should be probed, when a shower of typical frass is likely to spill out. This may be the only indication that attack is in progress if a life-cycle has not been completed and there are no characteristic oval exit holes. When attack is found the surveyor must ascertain whether or not the infestation is still active. If the attack is active, drastic remedial measures are called for. The attacked sapwood should be cut back to sound wood and the whole roof void should be given an insecticidal Appendix A: Wood-boring Insects 145 m M Figure 24 (i) House longhorn beetle and (ii) frass (Crown copyright: Reproduced by permission of the BRE Princes Risborough Laboratory) treatment. If the proportion of sapwood is high, the damage may sometimes be so serious as to call for complete replacement of roof timbers. This was found to be necessary in a large number of council properties in the Camberley area. In several districts in Berkshire, Hampshire and Surrey, the 1976 Building Regulations called for the special pretreatment of all roof timbers in new buildings; this is not the case in the 1985 Regulations which give no specific advice on the measures to be taken to protect the timber. In endemic areas pressure-treated timber, or timber treated with boron by the dif fusion process, is much to be preferred to a dip or brush application with organic solvent preservatives. BRE Information Paper IP 12/82 'House Longhorn Beetle Survey' provides information of the damage caused by the beetle and gives advice on preservative treatments. POWDER-POST BEETLES. There are two families of these, the Bostrychidae and the Lyctidae. The former are mainly tropical species whereas the latter are both tropical and temperate- region species. The larvae of these two families do not live on cell-wall substance, but on the starch content of the sapwood of certain timbers. Unlike the sap-stain fungi however, the larvae have to destroy the cell-wall substance to get at the stored starch. The egg-laying habitats of the two families of powder- post beetles differ, as do their demands in regard to the degree of dryness of wood favoured for egg-laying. The Bostrychid beetles bore into wood, constructing aY-shaped egg tunnel, which is kept free from dust and in which the female lays her eggs. When the eggs hatch, the larvae tunnel into the wood longitudinally, packing the gallery system with fine, flour-like dust. Bostryhid beetles will attack timber soon after conversion, while boards, planks, and scantlings are still in stick. In practice the only appropriate remedial measure is to remove the sapwood. The galleries of the Bostrychid beetles are up to 5 mm (approximately 31I6 inch) in diameter.

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