Repeated exposure of borate-treated Douglas-fir lumber to Formosan subterranean termites in an accelerated field test

Douglas-fir boards (ca. 74.5 g) pressure treated with disodium octaborate tetrahydrate (DOT) to retentions of 0 (controls), 0.88, 1.23, 1.60, or 2.10 percent (weight/weight) DOT were sequentially exposed to four active field colonies of Formosan subterranean termites, Coptotermes formosanus Shiraki (Isoptera: Rhinotemitidae), in an aboveground field test. Samples were placed in contact with each colony for 10 weeks, with ovendry weight losses determined between exposures, for a total termite exposure period of 40 weeks. Feeding activity differed among termite colonies. The two lower borate retentions (0.88% and 1.23% DOT) were virtually equal in efficacy, with mean wood weight losses during each individual 10-week exposure ranging from 1.2 to 4.6 percent. Feeding was negligible on wood treated to the two higher borate retentions. Mean wood weight losses from termite feeding during each 10-week period ranged from 0.7 to 1.3 percent with an initial retention of 1.60 percent DOT, and 0.3 to 0.9 percent with 2.10 percent DOT. Total cumulative wood weight losses over the 40-week exposure were: 10.2 percent (0.88% DOT), 8.7 percent (1.23% DOT), 3.6 percent (1.60% DOT), and 2.4 per- cent (2.10% DOT). Under conditions of high termite hazard, wood treatment to retentions greater than 1 percent DOT can be expected to provide protection from serious structural damage, although minor feeding may still occur. Treatment to higher retentions can be expected to progressively minimize the possibility of minor cosmetic damage.

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