Could balsa join the ranks of timbers approved for use in Australian construction jobs? Researchers from the University of Tasmania test the timber against the destructive termite Coptotermes acinaciformis.

In February, a team of researchers from the University of Tasmania published a paper in International Biodeterioration & Biodegradation that evaluated the termite susceptibility of a range of timbers used in the Australian construction industry. Papua New Guinean balsa (Ochroma pyramidale), radiata pine (Pinus radiata) sapwood and southern blue gum (Eucalyptus globulus) heartwood – were tested against Australia’s most economically important wood-destroying termite species, Coptotermes acinaciformis.

As well as determining the termites’ appetite for the different timbers, the researchers also tested the extent to which an application of bifenthrin prevented termite attack on balsa wood. The aim was to test whether balsa timber should be considered as an alternative timber for use in the Australian construction industry, particularly for interior lining applications.

Bifenthrin is commonly used as a treatment to repel and kill termites in wood products in Australia. In the experiment, three samples of balsa were dip treated in bifenthrin emulsion concentrate that penetrated the wood to 2mm below the surface. The samples were treated at three different strengths – 0.02, 0.04 and 0.08%m/m. The trial compared the susceptibility of these treated timbers with untreated balsa, untreated radiata pine and untreated blue gum.

The timbers were exposed to seven different colonies of C. acinaciformis at a field site in the Northern Territory. Holes were bored into the trunks of seven eucalyptus trees to access the foraging galleries of termites, then plastic piping was inserted into the holes and linked to a container of wood. This technique was used to ensure each exposure sample of wood was linked to termites from a different termite colony.

At the end of 20 weeks of exposure, the results showed average mass losses for radiata pine and southern blue gum were 90.3% and 98.9% respectively, meaning the termites ate nearly the entire wood samples. For the non-treated balsa, the average mass loss of timber was 30.9% – this being based on a range of 3.8-99.9% mass loss. Although susceptible to termite attack, the level of feeding on non-treated balsa appeared very variable. As one would expect, the samples of bifenthrin-treated balsa suffered very little termite attack.

Australian Standard 1604.1 specifies a minimum application of 0.02% m/m bifenthrin in the outer 2mm of structural softwood construction materials. However, this speci cation assumes that treated timber has a density of 450-500kg/m3 – much higher than the density of balsa – which in the case of this experiment, had an average density of 105kg/m3. The researchers therefore decided to test bifenthrin retentions of 0.02, 0.04 and 0.08%m/m, with the 0.08%m/m retention being broadly equivalent (on a mass/volume basis) to the minimum requirement for standard softwood framing specified in AS 1604.1.

What is most interesting is that the results show that balsa can be successfully protected from termite attack with the use of bifenthrin, even at mass/volume retentions as much as 75% lower than those typically specified for structural softwood framing in Australian standards. With balsa appearing to be a much less preferred food source for C. acinaciformis compared to radiata pine sapwood and southern blue gum heartwood, these findings suggest balsa has the potential to be used as a construction timber, subject to further research and field testing.

Further reading: Kotlarewski et al. (2019). Bifenthrin treatment for balsa: Susceptibility of Papua New Guinea-grown Ochroma pyramidale to attack by Coptotermes acinaciformis (Blattodea: Rhinotermitidae) in an Australian context. International Biodeterioration & Biodegradation. Volume 137, pp153-157.

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