About wood pests
Pest is the name given to microorganisms and insects that reduce the value of the wood in the living tree or later during processing and use.
From a biological point of view, the so-called wood pests are part of the natural eco system. They break down dead plant matter, releasing nutrients which are then taken up by young plants. They are, therefore, indispensable for the global carbon cycle. Unfortunately, it makes no difference to fungi whether the wood they attack is a dead branch on the forest floor or a palisade post in your garden. Some fungus and insect species flourish particularly well under the conditions created by humans, making them troublesome wood pests.
To grow, fungi need plenty of moisture and, just like insects, are very sensitive to temperatures. Wooden objects and structures are therefore susceptible to pest attack to varying degrees, depending on their function. Wood with ground contact is particularly at risk, as it provides ideal living conditions for some fungi and is also open to attack from bacteria living in the soil. Wood in houses is not at risk from fungal attack, unless there are some constructional defects, which allow the wood to be affected by moisture after its installation. Such sources of moisture can include roof leaks, a lack of moisture seals or condensation.
Insects, like the common furniture or house longhorn beetles, also attack dry wood used in internal applications. In contrast to fungi, which, due to their spores, are practically everywhere, infestation by wood-destroying insects is in a limited area initially.
DIN 68800 describes the extent to which wood is at risk from infestation by wood pests in various different applications. For many uses, especially when wood is exposed to the elements, preventive treatment with chemical wood preservatives is imperative, depending on the type of wood used.
Wood and wood-based materials can be ruined by wood-destroying fungi. In and on the wood, the fungi form thread-like networks of hypha, referred to in their entirety as mycelia.
As a rule, such fungi only flourish on wood or wood-based materials with a high moisture content. The fungi normally need a temperature range of between +3°C and +40°C and a minimum moisture level of over 15% to develop properly.
Wood-destroying fungi are divided into three main groups; brown rot, white rot and soft rot types.
- What are brown rot fungi?
Brown rot fungi mainly degrade the wood’s carbohydrates, i.e. the cellulose and hemicelluloses. The infested wood turns brown in colour because the dark brown lignin in the wood remains as a residue. The wood loses density, volume and strength, shrinks noticeably when it dries and shows longitudinal and lateral cracks, which give it a characteristic “checkerboard” appearance (so-called “cubic” rot). In the final stage, the wood can be crumbled between the fingers to a brown powder. Brown rot fungi are mainly found in coniferous wood, though some can be found on hardwood or on both hardwood and softwood. They occur in both living and weakened trees, as well as in the heart and sapwood of processed timbers. The brown rot fungi include dry rot, wet rot, antrodia sinuosa and bracket fungi.
- What are white rot fungi?
White rot fungi degrade both the lignin and the cellulose in the wood cell walls. While this is going on, the wood structure remains largely unchanged for quite a long time, unlike brown rot infestation. The wood loses density and strength and turns brighter in colour. It often maintains its fibrous structure, even into the final stages, with cube-like crumbling scarcely occurring. On wood infested with white rot, dark coloured lines can occasionally be seen that separate the areas affected from areas of healthy wood or wood infested with other fungus species. White rot mainly tends to attack coniferous wood. White rot fungi include wet rot, root rot, the Turkey tail, the Tinder or Hoof fungus and also the asterostroma cervicolor fungi.
- What are soft rot fungi?
Soft rot fungi degrade the carbohydrates in the wood cell wall. They mainly occur in permanently moist wood that is constantly exposed to water or damp, moist soil. Soft rot fungi attack the wood from the surface, turning the outer layers a dark, greyish-black colour. The infested areas are soft and greasy and crack across the grain when they dry. This discoloured, infested surface layer distinguishes itself clearly from the healthy wood lying beneath it. Soft rot can attack all coniferous and hardwoods. They belong to the acomycetes or deuteromycetes (Fungi imperfecti) groups and do not form a large fruiting body. Other typical white rot fungi belong to the chaetomium, phialophora und humicola species.
Insects can cause serious damage in wood. The most commonly found wood destroyers are beetles, wasps, ants and termites. If insect infestation has occurred, determination of the species and knowledge of the biological course this insect takes are vitally important for achieving effective treatment and cure.
- What are dampwood and drywood insects?
Wood pests can be divided into two categories, dampwood or drywood insects, depending on their biological preferences.
Damp or greenwood insects only lay their eggs on living trees or freshly cut wood. The duration of the larvae development cycle in the wood means that the adult insects can emerge after the wood has been worked and installed in its final application, though any new infestation by dampwood insects at this point is not possible. Drywood insects, on the other hand, lay their eggs on air-dry wood or wood with low sap content. The larvae infest the wood and the adult insects immediately re-infest the damaged wood or attack new timbers in the building interior. Damp or greenwood insects include wood breeding bark beetles, like the stripped ambrosia beetle, wood wasps, wood borers, or freshwood long horn beetles like the violet longhorn beetle. The drywood category includes drywood long horn beetles, like the house longhorn beetle, the furniture beetle, powder post beetles, wood boring beetles and termites.
Wood staining fungi can cause bluish, brownish or other shades of discolouration, often limited to sapwood. As a rule, the fungi settle first in the rays. This is because they can’t actually degrade the substance of the wood and live on carbohydrates and fats which are stored in the sapwood rays. Although they don’t destroy the wood, permanent staining can greatly reduce its value.
- What kind of staining occurs frequently?
Stains caused by sapstain fungi and mould are particularly common in coniferous timbers. The sapstain fungi can also grow into the wood so that the stain penetrates deeply. This makes it impossible to plane the stain away mechanically. Mould only discolours wood on the surface through its spores, but often leaves mould stains after their removal. Special anti-sapstain preservatives, that should be applied directly after the timber is cut, provide protection against wood-staining.
- Is staining always caused by fungi?
Not all stains are caused by microorganisms. Dirt and dust particles on the wood’s surface can be mistaken for mould. Cut wood can react to iron particles, also resulting in stains. Such stains occur in wood types that contain tanning agents, like oak and sweet chestnut, but also Douglas fir, when they come into contact with iron or water with a high iron content during storage or processing. The phenolic content in the wood reacts with the iron ions and forms dark colourings. Besides, wood can also suffer discolouration in the living tree, as is the case with red heartwood formation in beech trees.