Steamed wood of Japanese larch (Larix leptolepis (Sieb. et Zucc.) Gord.) was preferably attacked by termites. Steam treatments have often been applied to Japanese larch wood for improving its penetrability or color. For preventing termite attack on steamed timber, we have to elucidate why termite likes it. Choice tests with Coptotermes formosanus Shiraki were conducted using larch boards steamed for 30-240 min at 160-170°C. Weight losses due to termite attack were very small in non-steamed boards while they were very large in steamed boards. Steamed samples extracted with hot water were not attacked by termite. This means that increased palatability to steamed samples was affected by some components of hot water extracts which were possibly produced by steam treatments. Forced tests were also carried out by exposing the steamed wood samples to Coptotermes formosanus in an acrylic cylinder with plaster bottom. The overall results suggested that steam treatments degraded and/or removed some larch wood constituents which suppress termite attack.

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