Woodboring Insects

What are they?

Woodboring insects are the larvae of woodboring beetles that eat wood. Many attack living or recently fallen trees but a small number attack seasoned timbers in structural timbers in buildings and in wooden objects. The commonest woodborer found inside buildings is the Furniture Beetle (Anobium punctatum) or ‘woodworm’. In the UK, Death-watch Beetle (Xestobium rufovillosum) can be a major cause of damage in damp oak structural timbers, and the Powder Post Beetle (Lyctus brunneus) can be an occasional, but serious, pest in newly seasoned hardwoods such as freshly laid maple flooring.

What do they look like?

Furniture Beetle adults are small (4mm) brown beetles that can fly into buildings during the summer months, or are brought in from infested wood (particularly partially seasoned firewood and old furniture). They lay eggs in cracks, splits and fresh exit woodworm holes but not on exposed wood. The eggs hatch in the autumn and the emerging larvae eat the surrounding wood to grow. The larvae bore tunnels (about 1-1.5mm in diameter) in the wood, usually from 3 to 5 years. When fully grown, the larvae pupate just below the wood surface and emerge as fully grown adults in the Spring by biting through the wood surface making the distinctive ‘woodworm holes’. The adults live for only a few weeks to mate and for the females to lay eggs to start the next generation.

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Furniture Beetle

Death-watch Beetle

The ‘woodworm’ eggs are laid directly on their potential foodstuff. The emerging larvae will happily eat most UK hardwoods such as beech, ash, oak, walnut, and are particularly fond of the more nutritious outer sapwoods. Heartwood oak and tropical hardwoods such as teak, mahogany and rosewood are not normally attacked.
Softwoods – such as pine etc., are all attacked.
Particle boards, ply boards etc., are not normally vulnerable, but pre 1950’s boards that were glued with animal glued and some foreign boards glued with resin are particularly susceptible.
Paper, cardboard etc., can be attacked particularly if there is animal size or animal glue used with them – such as in the spines of books.
Cellulose fibres such as cotton and linen are occasionally attacked, again especially if animal sized or contaminated with animal glues etc.

Warm temperatures above 10°C are necessary for the larvae to develop. Temperatures above 20°C encourage the adult beetles to fly and spread an infestation.
High relative humidities above about 65% induce high moisture contents in wood (above about 15%) necessary for Anobium development. Deathwatch beetle development need even higher moisture contents.
Poor environmental conditions can cause splits, cracks and opening joints etc., that give good egg laying sites.
Poor housekeeping and lack of cleanliness can encourage Anobium development.

Fresh exit holes in wood, from the emerging adult beetle. They are small (~2mm diameter) clean cut holes, pale interior of the hole, sometimes with a slight raised ‘crown’ round the edge.
Frass : the pale coloured oval shaped pellets of digested wood often fall out of the exit holes. The frass has a gritty feel and the shape and size are obvious under a hand lens.
Dead adult beetles can often be found below the exit holes.

Fumigation with toxic gases such as phosphine and carbon dioxide. Fumigation is effective as the gas penetrates deeply into wood etc., killing all stages of the insect. It is useful when large quantities of material need to be treated quickly. However, fumigants are toxic and may react chemically with the objects being treated. They also do not provide any residual protection against the objects being reinfested.
Anoxic fumigation provides very low oxygen environments, either with inert gases such as nitrogen or with oxygen absorbers such as Ageless^TM. When suitably carried out, it can be a safe and effective method of killing insects by asphyxiation and desiccation. It does, however, need long treatment times of several weeks and is not very effective against woodborers.
Temperature manipulation using high and low temperatures will kill insects. Temperatures above 52^oC will kill all stages – eggs, larvae, pupae and adults in a few hours. This is the basis of the Thermolignum system. Temperatures of -30^oC will kill all stages in 2-3 days, however at -18^oC, the temperature of a domestic deep freezer, up to 2 weeks is necessary. Although generally safe on most materials, low temperature treatments can damage by embrittlement and shrinkage of, for instance, thick paint films.
Insecticides consisting of an active ingredient in a solvent system are effective and give residual protection. Care must be taken in the choice of insecticide. In some instances an organic solution base may be preferable to a water solvent base.
Water-based insecticide are very effective against woodborers and other insect pests as they penetrate deeply into the wood. Colourless and odourless there are many recommended for domestic use.