The ability of insecticidal soil drenches or spray-on insecticide/fungicide treatments to protect mass timber elements exposed in a high moisture environment but out of direct soil contact was assessed using a modified AWPA ground proximity test established in 2017. Three-ply Douglas-fir CLT pieces measuring 305 x 355 x 105 mm (L x H x W) were installed at the Harrison Experimental Forest (HEF) (Saucier, Mississippi) in 2017. Test samples were placed in sets of two on bricks approximately 150 mm apart, 75 mm above the soil and covered with ventilated waterproof covers. A total of 20 test sets (40 total samples) of four different treatments were installed. Treatments were a soil termiticide (fipronil), a spray-on borate (23% DOT) preventative treatment at test initiation, a remedial borate treatment after one year and an untreated control treatment. To mimic an in-service structural component, borate treated samples were sprayed on all sides except the bottom of the sample where it sat on the bricks near soil. This was to mimic a worst-case scenario where an in-service member could not be fully treated after building construction. Samples were monitored for moisture and biological attack annually for the next six years. Moisture contents of the samples were high throughout the duration of the test. Sample mass losses after six years were elevated in the control and untreated sister sample pairs. Samples treated with borates at test initiation and remedially after one year of exposure showed limited decay or termite attack and low mass loss compared to other treatments or untreated paired samples. Borate treated samples were also cut and indicator stained at the end of the test to examine boron diffusion. Soil termiticide treated plots showed no sign of termite attack, but some of the samples incurred heavy fungal decay resulting in substantial mass loss compared to non-soil termiticide treated plots. The results provide critical baseline data for field evaluation methods of mass timber in areas with high subterranean termite and decay pressure.

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