Seasonal occurrence of and adult diapause in the tortoise leaf beetle Aspidimorpha difformis (Motschulsky, 1860) (Coleoptera: Chrysomelidae)

Leaf beetles have diverse life cycles and temperate species exhibit several characteristics, such as a long life cycle and long diapause. Aspidimorpha is a group of tortoise beetles in the family Chrysomelidae that inhabit the Oriental and eastern Palearctic regions, which were originally derived from tropical and subtropical regions; however, voltinism and diapause in their life cycles remain unclear. Because the tortoise beetle Aspidimorpha difformis inhabits the northernmost region of the temperate zone among Aspidimorphini, a more detailed understanding of its life history will provide insights into the evolution of its life cycle. We herein performed quantitative field research on the seasonal occurrence of A. difformis in Kanagawa Prefecture, Japan, as well as rearing experiments. In the field, adults overwintered, oviposited, and first-generation larvae occurred between mid-May and mid-July. Most larvae and adults disappeared by early August, except for several individuals in autumn and early winter. Under a constant temperature (20 or 25°C) and photoperiod (LD 12:12 or 16:8), the larval period was short (16–29 days), whereas the pre-oviposition period was long (83–197 days). In conclusion, A. difformis is mainly univoltine and partially bivoltine, which is regulated by an obligatory adult diapause. A life cycle with a long diapause may be adopted to avoid high summer temperatures and poor conditions in food because larvae fail to develop in summer. In contrast to A. miliaris, which is multivoltine with a facultative summer diapause, environments in cooler regions may favor the evolution of univoltinism in A. difformis.