At Midagahara, Mt. Gassan (about 1, 400 m above sea level), Yamagata prefecture, a light trap survey of moths had been made from 23rd to 27th July, 1971. On 24th July, northwest wind of about 5 to 6m/sec. blew over this plateau, and at this night
Agrotis ypsilon HUFNAGEL,
Euxoa sibirica BOISDUVAL and a few other moths were attracted to the light.
A. ypsilon found at this night was very active and not alighted on a white screen for collecting. Moreover, many moths passed over the collecting site on the air current. About twenty specimens among them were caught by an insect net with elongate stick, and most of these were A. ypsilon. At other nights, particularly on 25th July,
A. ypsilon had not been collected, despite the fact that many other moths were attracted to the light. From these facts, it seems highly probable that
A. ypsilon migrated in a mass at night on 24th July across Midagahara. Another interesting fact is that E. sibirica with seasonal migratory behaviour was also found only on 24th July.
From light trap records and sampling in a soy-bean field severely infested by
A. ypsilon, it was confirmed that the adults occurred abundantly in mid July, 1971, on the plain in ShOnai district west to Mt. Gassan. Although the occurrence of adults on the plain nearly agrees with the migration observed of high land in time, the original source of migrating adults is open to question. In 1972, ovaries of the female adults caught by a light trap on the plain were examined. A large part of females had developed ovaries from May to July, while those caught after August had undeveloped ones. This result well agrees with a previous records obtained in Iwate prefecture (Chiba, 1972).
In relation to these findings, some problems in migration and mass outbreaks of
A. ypsilon were discussed.
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