Wood preservation Minimize

May 2007. An unusual question: a member from Niue asked about the preservation of timber. A local tree (Toi), which has lovely grain and hard heart wood has been cut down and the bark removed to prevent boring insects destroying the timber as it dries out. Are there any chemicals to prevent the damage to the sapwood?

The tree is Alphitonia zizyphoides (Spreng.) A. Gray, common throughout the Asia-Pacific region. A medium to large tree with hard, durable wood, good for canoe paddles, furniture, and firewood. It has an attractive grain, orange-red heartwood, fragrant bark and the leaves are used as a soap substitute. A pioneer species, typically in second growth, canopy gaps and on roadsides. Local names are doi (Fiji), toi (Samoa, Tahiti, Rarotonga, and Niue), huremi (Vanuatu). In Niue, it is a favoured furniture wood.

Borax was suggested. It is a low-toxicity mineral with insecticidal, fungicidal and herbicidal properties. It does not evaporate or volatilize into the air or pose the considerable health risks associated with synthetic pesticides. Boric acid (H3BO3) and boron containing salts (borate salts, such as sodium octoborate (Na2B8O13) are the refined forms of borax commonly used in structural pest control. Boric acid and boron containing salts are borax that has been refined with low grade acids and allowed to crystallize to form borates.

Use and Mode of Action: Registered in 1983 for control of cockroaches, ants, silverfish and several beetles, it has also been used as an herbicide along rights-of-way and as a fungicide for citrus, as a wood preservative/fire retardant, and as an insect repellent in insulation. As an insecticide, boric acid acts as a stomach poison affecting the insects metabolism and protoplasm; and the dry powder is abrasive to the insects exoskeleton. Various formulations are registered for use in Alaska to control cockroaches, silverfish and carpenter ants. As an herbicide, boric acid causes desiccation and interrupts photosynthesis in plants.

Boric acid may be used either in an insect bait formulation containing a feed attractant or as a dry powder. The powder may be injected into cracks and crevices, where it forms a fine layer of dust. Insects travel through the boric acid dust, which adheres to their legs. When the insects groom themselves, they then ingest the poison, which causes death 3 to 10 days' later of starvation and dehydration. As long as the material is not allowed to become wet, its continuous presence ensures that hatching insects, which insecticide sprays commonly spare, are exposed and die. Many boric acid formulations can be effective for more than a year.