Timber: fungi and insect pests
Note
TRADA Wood Information Sheet Section 2/3 Sheet 32. Revised November 2004
This Sheet outlines the more important types of fungi and insects which can affect timber. With its relatively cold temperate climate, the UK is not seriously afflicted by insect pests. Termites, a major scourge of tropical and sub-tropical countries, are absent altogether (occasional accidental introductions have been swiftly eradicated) and another widely prevalent pest overseas, the house longhorn beetle, has a very restricted distribution in the UK.
Much unwarranted alarm can be caused by insects in buildings although many do not attack timber. Some common types are shown in the WI Sheet 4- 17 Pests in houses. Attack by bacteria, soft rot fungi and marine borers is not dealt with here. Preservative treatment is covered in WI Sheets 2/3-33 Wood preservation: Chemicals and processes and 2/3- 16 Preservative treatment for timber - A guide to specification.
Timber is composed primarily of hollow fibres of cellulose, cemented together with lignin, a complex organic material constituting some 20% by weight of dry wood. The ratio of wood to air, the shape, variety and arrangement of the constituent fibres and the small proportion of loosely bound ‘extractives’ vary to give timbers which differ widely (eg balsa to lignum vitae), in their physical and mechanical properties. The chemical constitution varies little except for the ‘extractives’, which confer the specific properties of colour, odour and resistance to bio-deterioration by fungi and insects. Sugars and starch form part of the vital system of the living tree; they can persist in the log or converted timber for several years under the right conditions, and can influence the susceptibility of timber to degrade by fungi and insects.
Fungi can cause staining, decay and weakening, whilst insects disfigure the timber or render it unserviceable by boring holes or consuming it. Fungi require the timber to have a moisture content of at least 20%, if they are to develop and cause damage. In many situations out of contact with the ground the moisture content of timber may be kept below the danger threshold for fungal attack by correct design and detailing. This minimises wetting and ensures continued weathertightness and adequate ventilation. Such measures do not, however, necessarily confer immunity to insect damage since some insects can attack ‘dry’ wood.
Some timber species are resistant to fungal and/or insect attack. Typically, only the inner, or heartwood, zone contains the extractives, which give resistant properties to the wood. It follows that the outer sapwood, is essentially unprotected from fungi and insects, if the conditions for attack are satisfactory. This is important to bear in mind when the ‘natural durability’ of a timber is quoted. This is assessed on the performance of test stakes in ground contact or on standard laboratory tests, and relates only to the heartwood. The sapwood of most species is in the most susceptible two groups of the 5-category natural durability classification in BS EN 350-1 ie:
Class 1 Very durable
Class 2 Durable
Class 3 Moderately durable
Class 4 Slightly durable
Class 5 Not durable.
