Abstract

Existing markets in the US south and southern Japan, as well as new markets in China and Taiwan, require wood products to be resistant to the highly destructive Formosan subterranean termite (Coptotermes formosanus Shiraki). The ability of a proprietary buffered amine oxide carrier (Ross 2007, 2010, 2011;Ward and Scott 2011a, 2011b) to through-treat post–mountain pine beetle (MPB) lumber would minimize the competitive disadvantage of low-permeability Canadian species in comparison with US southern pine and New Zealand radiata pine (Pinus radiata Don) in markets where termite and decay resistance are important. Borates are one of the components used as a buffer and carrier, but the low levels are not designed to protect against Formosan subterranean termites without inclusion of carbon-based insecticides in the formulation. Borate-treated wood is susceptible to cosmetic damage when attacked by termites because the borate is not repellent nor immediately lethal to termites (Grace and Yamamoto 1994, Campora and Grace 2007). Such cosmetic damage can be an issue in appearance-based applications such as window and door frames. One of the principal advantages of including carbon-based insecticides is that they may be either repellent or more rapidly toxic to termites and reduce or eliminate the possibility of superficial damage from exposure of borate-treated wood to aggressive termites.

British Columbia is faced with harvesting large volumes of mountain pine beetle (MPB)–affected trees and funnelling products made from these trees into existing or new markets. This project capitalizes on documented properties of post-MPB wood, such as low moisture content and increased permeability to preservatives (Byrne 2003, 2004; McFarling and Byrne 2003). Earlier work demonstrated the feasibility of through-treating selected high-sapwood post-MPB lumber via pressure treatment or via the new buffered amine oxide carrier (Morris and Minchin 2006).

In the present study, post-MPB lumber, blue stained on three sides, was either dip or pressure treated with several preservative formulations using the buffered amine oxide carrier and then installed in field tests in Hawaii and Japan at sites with confirmed populations of Formosan subterranean termites. The termite resistance of this material was compared with that of a standard high-loading disodium octaborate tetrahydrate (DOT) pressure treatment and a reference preservative, alkaline copper quaternary type D carbonate (ACQ-D). This article describes the test methods and results after 2 and 5 years.

Materials and Methods

Sampling for analysis

We cut a 50-mm slice from one end of each 450-mm-long treated board, leaving a 400-mm test sample for the exposure test. The 50-mm subsamples were oven-dried at 50°C, and two 5-mm slices were removed from the innermost end, one for simulated boring retention and one for whole cross-section retention (with the exception of the ACQ treatment). The remaining subsample was retained for penetration measurement.

Simulated cores, 10 mm wide and 16 mm deep, were removed from one edge of one 5-mm slice per board and were analyzed individually for boron or ACQ retention. Another entire slice was analyzed for DOT by mannitol titration (Winters 1965). ACQ retentions were calculated from copper concentrations determined by energy dispersive X-ray spectrometric analysis (American Wood Protection Association [AWPA] 2002). The freshly cut face of the remaining subsample was sprayed with boron or copper indicator, and the preservative penetration was measured in three ways for each sample: from one edge, from the heartwood face, and the percent total cross section. Cross sections of all samples were sent to Kop-Coat for analysis of termiticides. Four samples from each group were analyzed for permethrin (Standards Australia 2006) by high performance liquid chromatography (HPLC) and for imidacloprid by neutron activation analysis for chlorine (AWPA 2012a). Where both insecticides were used together, the chlorine content of the permethrin calculated from the HPLC results was subtracted to determine the imidacloprid retention.

The borate retentions achieved (Table 1) were lower than the targets. Borate retentions are expressed as percent m/m BAE. For reference, the loading specified in the AWPA standards for Formosan termite zones (AWPA 2012d) is 4.5 kg/m³ B₂O₃, which is 2.0 percent m/m BAE in a wood species with a density of 400 kg/m³.

Table 1. Analytical results for samples installed the field tests.^a

View Fullscreen

Field test

Based on the results of the preservative penetration and retention assessments, the 20 samples within each set of 30 closest to the target retention were selected for the field test. The freshly cut ends of the field exposure samples were coated with the field-cut preservative Boracol 20 BD, containing DDAC (Sansin) for the six DOT and buffered amine oxide treatments and copper naphthenate containing 2 percent Cu (Recochem) for the ACQ-D–treated set. Ten boards from each of the eight treatment sets (a total of 80 samples) were shipped to Hawaii and to Japan for installation in termite field trials, which follow the AWPA Standard E21 test method (AWPA 2012b).

The Hawaiian test plot is on the island of Oahu, located at the Waimanalo Experiment Station of the College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa. The Japanese field test site, maintained by the Research Institute for Sustainable Humanosphere, Kyoto University, is located in Kagoshima Prefecture on the island of Kyushu, Japan. Both test sites support active populations of the Formosan subterranean termite, C. formosanus, and the Kagoshima test site also has the native subterranean termite, Reticulitermes speratus (Kolbe). Results from previous tests have indicated that termite attack on untreated controls is about 10 times faster at the Hawaii site (Tsunoda et al. 2002, Morris et al. 2011). The more tropical environment in Hawaii facilitates year-round foraging and growth of the colonies, in contrast to the seasonality of attack found in Kagoshima (Grace et al. 2004).

The exposure test method (AWPA 2012b) involves laying wood samples on top of hollow concrete blocks standing on the soil surface and then covering the structures with a box to protect them from rain and to maintain high humidity. There is no direct contact between the wood samples and the soil, other than that brought by the termites to construct shelter tubes. In Hawaii, the white-painted plywood covers were untreated and were replaced as necessary due to termite damage and weathering. The covering boxes in Japan were made of white plastic.

Through the two perforations in each block, 25 by 25 by 300-mm pine or Douglas-fir (Pseudotsuga menziesii) feeder stakes were driven into the ground so that the top of the stake was within 2 to 5 mm of the top of the concrete block. The test samples were situated one per block such that they covered the holes in the block but were not in direct contact with the feeder stake. This was designed to prevent direct tunnelling by termites from the untreated wood stakes into the test samples.

Test material was nondestructively rated using a visual examination for signs of termite attack. Each sample was carefully removed, examined, and assigned a rating on the pre-2011 AWPA rating scale (Table 2) in Japan and on the new AWPA scale (Table 3) in Hawaii. The mean ratings of each group were compared with the untreated controls by t test, and analysis of variance (ANOVA) was performed with the dip plus kiln treatments.

Table 2. Termite attack visual rating system in the American Wood Protection Association (AWPA) E21 field test prior to 2011.

View Fullscreen

Table 3. Termite attack visual rating system in the American Wood Protection Association (AWPA) E21 field test after 2011.

View Fullscreen

Results and Discussion

DOT pressure treatment resulted in good penetration of sapwood but poor penetration of heartwood. Use of the buffered amine oxide carrier resulted in more even penetration (shell treatment) around the perimeter of the test specimens. After 2 years of exposure in Hawaii, the imidacloprid and permethrin plus imidacloprid samples dip treated with the buffered amine oxide carrier were in perfect condition, with average ratings of 10 (Table 4). All but one of the samples treated with permethrin in the buffered amine oxide carrier were completely sound, but one had failed (rated 0). The samples treated with the buffered amine oxide carrier with no added insecticides had all failed with an average rating of 0, showing that the ca. 0.5 percent m/m BAE loading of buffer (lower than the 0.75% m/m BAE targeted) provides no protection against C. formosanus at this aggressive test site. The DOT pressure-treated samples with 1.3 percent m/m BAE had an average rating of 8, and the ACQ pressure-treated samples had an average rating of 9.2, with one sample rated as low as 4. At this point, 8 of the 10 untreated controls were rated 0, 1 was rated 8, and 1 was rated 10.

Table 4. Ratings of treated post–mountain pine beetle ponderosa pine lumber samples installed in termite field tests in Hawaii and Japan after 2- and 5-year exposures.^a

View Fullscreen

After 5 years in Hawaii, the samples that were imidacloprid and permethrin plus imidacloprid dip treated with the buffered amine oxide carrier were still in perfect condition with average ratings of 10 (Table 4). There was no further progress in deterioration of the amine oxide permethrin-treated dip-treated samples, with 9 of 10 completely sound (rated 10). The one sample that had failed at 2 years was removed from the test after the 5-year exposure, cross cut, and half was sent to Kop-Coat for chemical analysis (see below). The DOT pressure-treated samples with 1.3 percent m/m BAE had an average rating of 7.2 with no sample rated lower than 6. The attack was primarily in the heartwood, which had received poor penetration compared with the dip plus kiln-conditioning treatments. Better heartwood penetration can be achieved using hot treating solutions (Baker et al. 2000). The DOT pressure-treated samples with 0.9 percent m/m BAE plus permethrin had an average rating of 9.5 with no sample rated lower than 8. The ACQ pressure-treated samples had an average rating of 7.7 with two samples failing (rated 0). These failures may have been associated with poor penetration in the heartwood (Table 2) because this material was not incised. Incising is required in Canadian (Canadian Standards Association 2008) and American (AWPA 2012c) standards for lodgepole pine to be used as sill plates, irrespective of sapwood content. At the 5-year inspection, all the untreated controls were rated 0.

As with a previous 10-year field test (Morris et al. 2011), termites at the Japan test site were much less aggressive against the untreated controls than was the case at the Hawaii test site, although there was more surface damage to the treated samples. After 2 years of exposure in Japan, all samples were sound except one from the control group and one from the group treated with the buffered amine oxide carrier with no added insecticides (Table 4). After 5 years of exposure in Japan, the samples that were permethrin, imidacloprid, and permethrin plus imidacloprid treated with the buffered amine oxide carrier had, respectively, average ratings of 9.4, 9.5, and 9.5 with six, five, and two samples rated 9 (however, the permethrin plus imidacloprid formulation also had one sample rated 7). The samples treated with the buffered amine oxide carrier with no added insecticides had an average rating of 7.1, which was the same as the untreated controls. The DOT pressure-treated samples with 1.3 percent m/m BAE had an average rating of 8.4 (two rated 7), and the ACQ pressure-treated samples had an average rating of 8.9 (one rated 7 and one rated 8). All the controls had failed after 5 years of exposure. After 5 years, all of the preservative-containing treatments were statistically significantly different from controls using a t test, with the exception of DOT and ACQ-D pressure treatments at the Japan test site. The three buffered amine oxide carrier dip treatments with insecticides were statistically significantly different from dip treatment with no preservative using ANOVA.

Analysis of the sample treated with permethrin in the buffered amine oxide carrier that had failed in Hawaii revealed 106 ppm of permethrin and 0.13 percent m/m BAE in the intact treated shell, with no permethrin (below detection limit) and 0.36 percent m/m BAE in scrapings from the hollow interior. It appeared that termites had found a weak point in the treated shell, possibly where the preservative penetration was not as deep.

Conclusions

• Dip plus kiln-conditioning treatment with imidacloprid, permethrin plus imidacloprid, and permethrin in a buffered amine oxide carrier has provided good protection to post-MPB lumber over 5 years in Hawaii and Japan.

• Dip plus kiln-conditioning treatment with the buffered amine oxide alone provided no protection because of the low loading of borate used as the buffer.

• Samples that were DOT pressure treated to below the retention required for Formosan termites in AWPA standards did not perform as well as dip plus kiln-conditioning treatment with imidacloprid, permethrin plus imidacloprid, or permethrin. This may have been due to poor heartwood penetration. Better heartwood penetration can be achieved using hot treating solutions or by incising. Addition of permethrin to DOT pressure treatment improved performance.

• ACQ-D pressure-treated samples performed similarly to DOT pressure treatment but had two failures, likely associated with poor penetration in the heartwood because this material was not incised. Incising is required in Canadian and US standards for lodgepole pine sill plates, irrespective of sapwood content.