Decomposition of Basamid in Douglas-fir heartwood: Laboratory studies of a potential wood fumigant

Basamid (3,5-dimethyl-l,3,5,2H-tetrahydrothiadiazine-2-thione), a crystalline powder first used as a soil sterilant, has shown promise as a wood fumigant. This chemical decomposes to methylisothiocyanate (MITC), the primary active ingredient, as well as other products; but the rate of decomposition in wood is too slow to be effective unless certain catalysts are used. This study explored conditions and additives that accelerated Basamid decomposition in Douglas-fir heartwood. MITC production increased with increasing temperature and moisture content. Decomposition was also enhanced by a buffer powder formulated to reach pH 12 when mixed with 100 ml of water and, more efficiently, by copper sulfate. Carbon disulfide, a less fungitoxic compound, was the only other decomposition product detected in this study. Additional tests using a purge-and-trap system indicated that MITC was produced at a steady, moderate rate over a 28-day period. Addition of copper as a catalyst in these tests increased MITC production only during the first 7 days. Chemical analyses of residues at the conclusion of the tests indicated that most of the Basamid remained unchanged, providing a reservoir of chemical that should provide long-term MITC release