Abstract
During the last six years serious outbreaks of the oriental armyworm, Mythimna separata WALKER, occurred in the early summer of 1971 and again in 1972. The outbreak areas were located mainly in the western parts of the district. This pattern of distribution suggested that the outbreak might be caused by migrating moths carried by the strong southwesterly winds which blew through the district. Some of the local outbreaks could be explained by the topographic effect on the migration. The dates of the moth invasions were inferred from the mean air temperature and the direction and strength of the wind. Meteorological data showed that the inferred dates of moth invasion in the latter half of May coincided with the rapid movement of a depression from estern China to the northwestern coast of the Tohoku district.
