Biological attacks - Mould and Fungi

Fungi

These non-photosynthetic organisms obtain their nutrients from various sources, including wood. Their dependence on organic matter makes them a major threat to the natural appearance and integrity of wood. Their structure is microscopic and not visible to the naked eye. Fungi develop inside the wood and sometimes on its surface. Different types of fungi attack wood both externally and internally, with varying degrees of aggressiveness. The most common type of fungal decay is rot, which typically occurs when the moisture content is extremely high — between 20% and 30%.

Brown Rot

This is the most common type of fungal decay. The wood becomes darker and cracks, forming a squared pattern. A white membrane is often visible stretching from the affected area towards moist soil. Wood affected by brown rot loses its mechanical strength and must be replaced. It’s also essential to eliminate any indirect sources of moisture.

White Rot

In addition to feeding on cellulose, white rot fungi can partially degrade lignin. This causes the wood to appear lighter in color and softer in texture. White rot typically develops in very humid and dark areas. As with brown rot, the only solution is to replace the damaged wood with new, treated wood.

Soft Rot

This type of fungus develops inside the cell walls of the wood. It slowly and invisibly reduces the wood’s mechanical resistance. To check for soft rot, insert a screwdriver into the suspected area — if it penetrates easily or leaves a visible trace, the affected section should be replaced.

Mould and Bacteria

Mould is an imperfect fungus that does not produce fruiting bodies. It can develop on the wood surface, forming a visible whitish to grey mycelium. It causes colored stains on the wood, which can be green, grey, or black, depending on the mould species. Mould is found in humid conditions but does not affect the wood's mechanical properties. It can be removed by brushing or washing. Preventive measures include keeping the wood away from excessively humid environments.

Excessive moisture also promotes the growth of various bacteria and microbes that can damage the wood's cellular structure. Lignivorous bacteria, in particular, cause irreversible damage, softening and cracking the wood until it completely decays.

Blue Stain Fungi

This type of fungus develops in freshly cut wood. It causes a blue discoloration along the grain, which affects the wood's appearance but not its physical properties — unless the infestation is severe. In such cases, the wood’s mechanical properties can be compromised. Blue stain fungi require a wood moisture content of around 30% to develop. Once the wood is in use and dries out, the fungus becomes inactive. Even if the wood later becomes humid again, the fungus cannot develop due to a lack of nutrients. Some fungi, such as blue stain fungi, cause only cosmetic damage, while others can destroy the cellulose in wood.

Biological Attacks – Insects

Insects

Wood is also vulnerable to damage from insects that feed on it. Wood-destroying insects lay their eggs in cracks or crevices. Their larvae then feed on the wood, compromising both its appearance and structural integrity. These insects often live partly or entirely within the natural biological environment of the wood, posing a serious threat to its durability. They are divided into two main groups: larval insects and social insects.

Larval Insects

These insects go through complete metamorphosis, including egg, larva, nymph, and adult stages. The larval stage is the longest and most destructive, as the larvae feed on the wood to meet their nutritional needs. As they grow, the wood deteriorates. Larvae often make audible chewing noises while eating.

The two most damaging families of larval insects for construction wood are:

Anobiidae (Woodworms):
These insects usually attack only the sapwood of certain softwood and hardwood species. They can damage both interior and exterior structural and carpentry wood.

Lyctids (Powderpost Beetles):
These insects target non-coniferous wood species with high starch content, commonly affecting parquet flooring and construction platforms.

Social Insects

The most well-known example of social insects that damage wood is the termite. The most destructive species is Reticulitermes lucifugus, also known as the subterranean termite or white ant. Their main nest is always located underground, outside the building. Termites enter buildings through damp wall joints, wooden beams, or door frames. Damage is hard to detect because termites leave the outer 1–2 mm of the wood intact. Unlike larval insects, termites do not make any sound while feeding.

Termite colonies are highly organized and divided into three castes:

Reproducers, workers, and soldiers