Simulation of Remedial Borate Treatments Intended to Reduce Attack on Douglas-Fir Lumber by the Formosan Subterranean Termite (Isoptera: Rhinotermitidae)

Abstract

Formosan subterranean termites, Coptotermes formosanus Shiraki, cost residents of Hawaii over U.S. $100 million each year. The aggressiveness of this termite, the unique climate and soil conditions in Hawaii, and the use of Douglas-fir wood, which resists impregnation with chemical preservatives, make the adoption of termite-control methods developed in other regions problematic. Both laboratory and field experiments were designed to simulate spray applications of disodium octaborate tetrahydrate for preventive and remedial termite control. For laboratory assays, sodium borate solutions in water (10%) or in ethylene glycol and water (23.5%) were applied to one face of Douglas-fir heartwood lumber and allowed to diffuse for 1 or 10 wk. Test blocks were cut from the front (treated face) and rear (untreated face) of each board and assayed for boron content and penetration and for susceptibility to attack by C. formosanus. Termites died rapidly upon contacting the board face treated with sodium borate in ethylene glycol. After a 10-wk diffusion period, mass losses from termites feeding on boards treated with this solution were significantly less (7-9%) than on those treated with aqueous borate solutions (34-35%) or with solvent controls (37%). However, borate diffusion below the wood surface was < 3 mm after 10 wk with all treatments. When Douglas-fir boxes treated either with two or four applications of sodium borate in water or with two applications of sodium borate in ethylene glycol solution were exposed to a field colony of 1.3 million termites for 10 wk, severe damage from termite feeding occurred in all treatments (30-54% mass losses), and there were no significant differences among treated boxes and untreated controls. Although multiple applications of sodium borate solutions are of value in protecting the treated wood surface from insect penetration and also provide some protection from decay fungus growth, such surface applications do not diffuse readily into Douglas-fir heartwood nor wick down termite galleries in the wood rapidly enough to protect the interior of Douglas-fir lumber from termite feeding.