We report the fourth year of field study results from a protected above-ground field test in Hawaii simulating the sill plate (dodai) used in conventional Japanese housing construction. Field tests were established in both Hawaii and Japan to examine the efficacy of disodium octaborate tetrahydrate (DOT, 2% and 3% shell and through) wood treatments. In Hawaii, chromated copper arsenate (CCA, 4 kg/m3) and ammoniacal copper zinc arsenate (ACZA, 4 kg/m3) were included in the test, along with untreated western hemlock and Pacific silver fir controls. Both field sites support active Formosan subterranean termites, Coptotermes formosanus Shiraki, although termite pressure is greater in Hawaii due to the uniformly favorable environmental conditions. This report updates the three-year results previously reported to IRG (Grace et al. 2000). After four years, minor damage (visual rating of 7, where 10 = sound and 0 = failure) has been noted to five individual treated dodai (out of a total of 10 boards from 7 treatments) as follows: 2% BAE shell treatment (2 boards), 2% BAE + DDAC through treatment (2 boards), and CCA treatment (1 board). Between the third and fourth years of the test, only four treated boards progressed to lower visual ratings: one 2% BAE shell treatment (from 10 to 7), one 3% BAE shell treatment (from 10 to 9), and two 3% BAE through treatments (from 10 to 9). In contrast, all untreated control boards were virtually destroyed during the past 11 months of exposure. Mean ratings for all wood treatments ranged from 9.2 to 10. These four-year results suggest that each of the DOT, CCA and ACZA treatments would provide long-term protection of dodai from termite attack.

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