Laboratory tests were done to measure the efficacy of addition of steamed larch (Larix leptolepis (Sieb. et Zucc.) Gord.) heartwood extracts for the uptake of boric acid against the termite, Coptotermes formosanus Shiraki. Paper discs were treated with the water-soluble fraction obtained from hot-water extracts of steamed larch wood (S-Water) with or without 0.01-2.0% boric acid, followed by exposure to this of termites, C. formosanus. The consumptions of the discs, termite mortalities, and boron contents in the termite bodies were measured. The S-Water significantly accelerated the consumptions of the discs at the retentions of boric acid below 0.1% (w/w) (p<0.01). However, regardless of the addition of the S-Water, the termite mortalities were almost the same at all the retentions of boric acid examined after 21 days of exposure. When the paper discs treated with the S-Water and 2.0% boric acid were given to workers and soldiers, boron contents in workers were lower than those of the same experiment without soldiers (p<0.01). This is because ingested boron was incorporated into soldier bodies from workers by trophallaxis. In the experiment using only workers, boron contents of termites fed with 2.0% boric acid plus the S-Water were larger than those of the termites fed without the S-Water. From the result, it was supposed that the S-Water inhibited the excretion of boron from the termite bodies or increased the accumulation of it in the termite bodies.

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