Hibernation and Migration of Diamondback Moth in Northern Japan

It was assumed that one of the most important factors preventing the hibernation of the diamondback moth, Plutella xylostella (L.), in northern Japan is the low temperatures, 0 ° C or below, and snow cover during the winter. To test this hypothesis, individuals of each developmental stage of diamondback moth were reared at 1 5 ° C in the laboratory and kept at 0°C for varying periods of time and returned to 1 5 ° C . The mortality of chilled insects increased with the duration of chilling. Al l larvae and pupae died after chilling for more than 60 days. Although 7.5 10% of adults survived after chilling, none of their eggs could hatch normally. These results indicate that hibernation of diamondback moth in fields is impossible in areas where continuous snow cover lasts longer than 60 days. This condition exists in Hokkaido and a large part of Tohoku and Hokuriku districts of Honshu. Since Morioka city is located in the northern part of Honshu Island (39°42'N), diamondback moth hibernation in this area is impossible. A large number of adults, however, were caught by pheromone traps from April to November in Morioka. The first oviposition was observed in the latter part of May and adults which grew from these eggs began to emerge in mid June. Therefore, diamondback moth adults trapped from April to May could not be considered to have hibernated and emerged in Morioka City. This strongly suggests that these moths were migrants from the southern part of Japan.