Some Consideration on the Chronological Records of Outbreaks of the Armyworm (Leucania separata WALKER)

Abstract

The outbreak of the armyworm was studied based on the records from 1941 to 1968 throughout Japan as well as the records from 1957 to 1968 in Akita Prefecture. The place where the worms broke out appeared to be a comparatively limited area. This limitation in an outbreak of the worms was not always related to the distribution of suitable food plants for the worms. Although the places of the outbreak varied depending on years, generations in a year, and emergence period of the year, the outbreak of the worms tended to converge in a generation or an emergence period throughout Japan as well as in Akita Prefecture. When the whole country of Japan was divided into six districts of Hokkaidô-Tôhoku, Kantô, Chûbu, Kinki, Chûgoku-Shikoku, and Kyûshû, the outbreaks showed a tendency to occur simultaneously in the comparatively wide area of several districts in some years and in the restricted district in other years. The seasonal trends of the outbreak differed among each of districts, but generally speaking, there appeared some similarity between districts such as Hokkaidô-Tôhoku and Kantô, Chûbu and Kinki, and Chûgoku-Shikoku and Kyûshû.