Seasonal occurrence of the non-native nymphalid butterfly Hestina assimilis in the Kanto region, Japan

Abstract

The seasonal occurrence of the non-native butterfly Hestina assimilis assimilis (Linnaeus) in Honshu, central Japan, was investigated by field observations of larvae on Celtis sinensis as the host plant and adults. The study sites were parks and green spaces in urban and suburban areas of Chiba, Saitama, and Aichi prefectures. Surveys of the sites in the Kanto region from 2019 to 2023 revealed four periods of 5th instar larval occurrence per year, which were in April, June, August, and September to October. Adults emerged 16-38 days after the molting to 5th instar larvae. Most of the yearly final larvae could be divided morphologically into two forms: 4th instar of the diapausing morphotype and 5th instar of the normal morphotype via 4th instar at a population ratio of about 5 : 1. Adults of the fourth generation laid eggs, a few of which hatched and developed into diapausing 4th instar larvae, indicating that H. a. assimilis exhibits three generation cycles per year with a much lower frequency of the fourth generation producing overwintering larvae. Field observations also revealed that H. a. assimilis larvae preferred to live in young host trees, in contrast to the allied native species Hestina japonica, suggesting that interspecific competition of these species for food plants at the larval stage is avoided through spatial segregation. These results expand our knowledge of the ecology of H. a. assimilis in central Japan and provide further insights into its ecological relationship to allied native species with which it shares C. sinensis as the host plant.