Abstract
The black cutworm, Agrotis ipsilon HUFNAGEL, is an immigrant pest harmful to newly sown grasses and other crop plants in Tohoku district. Numbers of the trapped adults fluctuated rather irregularly, presumably because of their migratory habit. The outbreak records accumulated since 1966 however suggested that there might be some regularity in the seasonal oviposition trend, although the outbreaks due to the mass invasion were always local. The oviposition period was estimated to extend from April to August with three peaks in late April to early May, around early June and in mid to late July, respectively. The remarkable larval occurrence was found in spring in the southwestern plain of Tohoku district, being spread over the whole district thereafter. The outbreaks in tame grasslands mainly established in higher lands were referred to the oviposition in June to August, and were more frequent in Aomori, Iwate and Miyagi prefectures, where the grasses were sown continuously during summer, than those in Akita, Yamagata and Fukushima prefectures, where sown in late August or later usually. The delayed sowing was thus recommended to escape the heavy cutworm attack on young grasses. The oviposition prevalence was discussed in conjunction with the synoptic trend of the seasonal adult migration.
