Introduction

The Oriental Armyworm Mythimna separata is an irregular pest of pasture grasses, maize, rice and other grain crops in the Republic of Korea (Table 5.1). Outbreaks occur every few years, most recently in 1981, 1987 and 1990. As pasture acreages are currently being expanded, M. separata is expected to become an increasingly serious pest of Korean agriculture. Research on the species has so far been quite limited, especially in comparison with the intensive long-term studies that have taken place in China and Japan.

It is almost certain that M. separata does not overwinter in Korea. The eggs, larvae and pupae do not survive the winter in Kyung-gi province (Fig. 5.1) (Choi & Cho, 1976), and even the southernmost extremities of the country (at 34° N) only just reach the boundary zone (32-34° N) for winter survival on the Chinese mainland (Yan 1991). As M. separata adults are able to migrate long distances (~1000 km) on the wind (Li et al., 1965; Yan, 1991), it seems likely that at least the early outbreaks each year are initiated by moths migrating across the Yellow Sea from lower-latitude regions of China. Similar northward migrations in spring lead to annual outbreaks in northern China (Lin & Chang, 1964; Li et al., 1965; Chapter 4, this volume). It has been hypothesised (Choi & Cho, 1976) that the flights into Korea take place in the warm sectors of eastward-moving depressions, as has been established for M. separata migration into northeastern Japan (Hirai, 1988a).