Defects Caused to Timber by Insects and Marine Borers
Timber may in certain circumstances be attacked by various wood-boring insects, which differ in their choice of species and condition of the wood- from standing trees to woodwork which has been in service for many years. Infestation in the tree or log usually dies after felling but evidence of the attack remains permanently in the seasoned wood as wormholes. Some timbers are generally recognized as more liable to attack which may be immune from infestation by some wood boring insects.
In hardwoods, the following are the most common types of defect caused by insect pests:
Ambrosia (Pinhole) Beetles (Platypodidae and Scolytidae)
The defect caused by these insects may occur in heartwood as well as in sapwood. It takes the form of numerous circular holes or short tunnels of 1/50-1/8 in. diameter, according to the insects responsible, and is often associated with dark discoloration of the surrounding wood.
The full extent of the defect is not usually evident until infested logs are converted. Attack ceases when timber is seasoned. This defect is more frequent occurrence in timbers from tropical and sub tropical countries and may originate in the standing tree but much commonly in recently felled logs.
Langhorn Beetles (Cerambycidae)
Attack by these insects, of which there are many species in tropical and temperate countries, results in the presence of dust filled tunnels, varying in size, in newly felled logs and sometimes in standing trees. Infestation is usually confined to bark and sapwood but occasionally occurs in heartwood. Most of these insects attack green timber only and are relatively unimportant.
Powder-post Beetles (Bostrychidae and Lyctidae)
These wood borers affect only the sapwood of most hardwoods during and after seasoning. The Bostrychidae are most abundant in the tropics and the Lyctidae are cosmopolitan in temperate regions. Infested sapwood may be completely reduced to powder even after processing.
The susceptibility of timber to this type of insect attack is governed primarily by its starch content, which may be affected by methods of handling and storing logs or of drying after conversion
Furniture Beetles (Anobiidae)
These insects, which include the common furniture beetle and the death watch beetle, are widespread in most temperate countries and are troublesome particularly in old woodwork, including furniture, paneling and structural timbers. Some types of plywood and wickerwork are highly susceptible to attack. Sapwood is preferred but heartwood is not immune especially if slight fungal decay is present. Information is at present lacking on degree of susceptibility of most tropical hardwoods to this type of insect attack after utilization in temperate regions.
Termites or White Ants (Order Isoptera)
Information on the resistance of hardwoods to attack by termites in tropical and sub tropical countries is incomplete and data often conflicting. Some timbers have the reputation of being highly resistant if not immune from attack but it is noteworthy that the resistance of any particular timber may vary according to local conditions and the species of termite, e.g. of the subterranean type nesting in the ground, or dry wood species infesting timber direct and not maintaining any contact on the ground.
Resistance to Marine Borers
Timber used in sea of brackish waters is subject to attack by marine boring animals (Teredo spp., Limnoria, etc.). Marine borers are widely distributed but they are particularly destructive in tropical waters. Around the coast of Great Britain Limnoria appears to be generally active and although Teredo attack is spasmodic it is always liable to occur.
Most timbers have not sufficient resistance to marine borers to be of much use where they are liable to be attacked.
The timbers listed below are generally recognized as being resistant. None of them, however, is immune from attack.
- Andaman Padauk
- Southern blue gum
Following are believed to be best for marine work:
- Brush box
- Red louro
- Totara (softwood)
One of the most widely used material for this purpose is pressure creosoted timber and long experience has shown that when the timber is thoroughly impregnated it will last a very long time.