Not only climate change but human-driven change of forest composition also can be a trigger for insect outbreaks in forests. Emperor moth (Saturnia japonica) is a common species in Japan. The outbreak of this species had been rarely reported in the past.

In Hokkaido, however, it has been reported that the outbreak of emperor moth has continued more than 7 years in deciduous broad-leaf tree forests, especially in pure and mature monarch birch stands established after artificial forest fire occurred about 100 years ago. On the other hand, there is no report of that monarch birch are suffering emperor moth’s herbivory and the outbreak of emperor moth hardly occurred in Tohoku area nevertheless emperor moth and monarch birch distribute in Tohoku also. Our experiment demonstrated significant higher performance of emperor moth larvae feeding mature monarch birch leaves than those feeding other species and immature monarch birch leaves. Our questionnaire survey clarified that the habitat of emperor moth is expanding recently in Hokkaido. These results may indicate that the expanding of emperor moth’ habitat and pure and mature monarch birch stands established after artificial forest fire have caused the outbreak of emperor moth in Hokkaido.