Can’t See the Wood for the Trees

Termite expert and director of ATP Research Scott Kleinschmidt explores the issue of timber susceptibility to termites, explaining why some types of  timber are more prone to attack than others. 

In order to understand what makes timber more or less susceptible to termite attack, it’s important to understand a bit about wood and how it is made.

From a chemical point of view, wood is a natural polymer consisting primarily of cellulose, hemicellulose and lignin. Together they provide the structural support to the tree. Cellulose and hemicellose are carbohydrates found in the primary cell walls. Lignin is an aromatic alcohol found in the secondary cell walls, where it binds the cellulose fibres and serves many biological functions including water transport, mechanical support and resistance to decay and insect attack.

Tree trunks and branches are made up of a series of layers. Wood is the collective name given to all these layers (except the bark). The heartwood is actually the dead material in the centre of trunks and branches. The next layer is the sapwood, which is a living layer. Another name for the sapwood is the xylem and it moves water and nutrients up the plant from the roots. Outside the xylem is the phloem, which moves sugars, produced inside the leaves via a process known as photosynthesis, down the plant. In between the xylem and phloem layers is a thin but remarkable layer called the cambium. This is the growth layer and will lay down new cells to the outside which become the phloem and new cells to the inside to add to the xylem. The old layers of phloem become the bark and as the old layers of the xylem die, they turn into heartwood.

During this ageing process, when the xylem cells die on the inside of the sapwood layer, creating the outer layers of the heartwood layer, the cells become storage vessels for the various by-products (extractives) from the biological processes in the plant. The extractives can be divided into three major sub-groups: aromatic phenolic compounds, aliphatic compounds (fats and waxes), and terpenes and terpenoids. The composition and quantity of these extractives will determine the level of resistance of the heartwood to fungal and timber pest attack.

Timber susceptibility to termite attack

Timber susceptibility to termite attack is largely driven by the type and quantity of the extractives in the various parts of the wood. Timber density can also have an impact on the level of resistance to termite attack.

Termite resistance increases as you move out radially from the centre of the tree – termite resistance of the pith is poor, with extractives and therefore resistance increasing as you move to the outer heartwood. However, as soon as you hit the sapwood, the level of termite resistance drops significantly. In fact, the sapwood of any tree species is not resistant to termite attack. Resistance also varies depending on the height within the tree, with decreasing termite resistance the higher up the tree the timber is sourced from. As trees age, more heartwood is created and more extractives are stored, increasing the resistance to fungal decay and timber pest attack. Wood from juvenile trees is both less dense and contains less extractives, which make it highly susceptible to termite attack.

Although heartwood may generally be more resistant to termite attack, this is not the case in these juvenile trees and the centre of young trees is often seen eaten out. The tree trunks used for the creation of didgeridoos highlights this phenomenon. As such, it is not possible to make sweeping statements about how resistant a particular type of wood may be, as it will depend on the age of the tree and which part of the tree the timber was cut from.

Some tree species are naturally more resistant than others. The list of termite resistant timbers listed in the various Australian Standards, including AS.5604 and AS.3660, was actually generated in the 1950s. The data used to compile these standards relied on testing the mature outer heartwood only, purposely avoiding the inner juvenile wood closer to the pith. This made sense at the time, as Australia was blessed with an abundant supply of high-quality timber sourced from native forests and the small section of juvenile wood was often discarded when milled.

However, changing public policies have resulted in restricted access to many native forests. Plantations are expected to replace these resources, but there are questions concerning whether timber from these trees will have the same resistance as the native forest material since they are likely to be grown on much shorter rotations with more aggressive management and contain a higher proportion of juvenile wood. The standards make no distinction between the termite resistance of timber from plantations, regrowth or from natural stands. Given its age, this list probably needs some revisiting. For example, in a recent trial, Mr Kleinschmidt presented data which demonstrated Jarrah (probably plantation grown) suffering a 73% weight loss with Coptotermes and a 97% weight loss with Mastotermes. According to the list in AS.5604 and AS.3660, Jarrah is supposed to be termite resistant!

It is important to understand that the susceptibility of the different timbers to termite attack will also vary depending on the termite species. Presenting work taken from a baiting trial that included a softwood (radiata pine) and a hardwood (Tassie oak, Eucalyptus regnans), Mr Kleinschmidt showed the very different responses of Coptotermes and Nasutitermes. Both woods are generally accepted to be highly susceptible to termite attack, yet although Coptotermes readily attacked both species, Nasutitermes only attacked the hardwood (Tassie oak), leaving the softwood (radiata pine) untouched, despite it lying directly next to the Tassie oak timber (Figure 1).

Wood preservation

Whilst natural resistance to decay and timber pest attack is an important consideration when choosing structural timber, the use of preservative-treated timber, particularly treated pine, is common in the construction industry.

H2 treated timbers are so classified as suitable for use internally. These timbers are treated with a LOSP (light organic solvent preservative) through either dipping or spraying, typically using bifenthrin or permethrin to provide the timber pest protection. Timbers treated as such can be identified by their typical blue colour. As these are only surface treatments, there is always the concern that if the timbers are cut, it would expose untreated timber and therefore allow potential entry of fungi or timber pests. Although it might be the case if the cut end remains exposed, a large, replicated trial demonstrated that if the cut end is placed on a treated piece of timber, the structure is still protected from decay and timber pests.

CCA treated timber is under increasing environmental pressure, but it’s still important to understand what it is and how it works. CCA stands for copper (fungicide), chromium (fixative) and arsenic (insecticide). It is a water-based preservative and consequently swells the wood, which makes the timber unsuitable for some uses. The current environmental, health and safety regulations have restricted its use further. It is applied to timber through a vacuum pressure process to get it throughout the sapwood layer; it cannot penetrate the heartwood layer. Softwoods have a large sapwood layer relative to the heartwood, whereas hardwoods have a small sapwood layer relative to the heartwood. The level of CCA penetration reflects this.

Generally speaking, preservative-treated timber provides excellent protection from termites. However, the level of protection is only as good as the quality of the preservative treatment; if the preservative has not been applied properly and parts of the timber remain unprotected, then they are vulnerable to termite attack.

In terms of understanding termite foraging patterns in buildings – trying to understand why termites are attacking some timbers and not others, and which timbers may be susceptible to future termite attack – using the list of termite resistant timbers published in AS.3660 is good starting point (for now). But as this article has highlighted, the level of resistance to termite attack will vary between tree species, within tree species (by age and location of wood within tree) and between the various termite species.