Plagiodera versicolora Laicharting (Coleoptera: Chrysomelidae) stops reproduction in mid-August at Ishikari, Hokkaido, Japan, despite the fact that the degree-days required for complete development predict an additional generation late in the season. We examined whether the life cycle of P. versicolora was constrained by the seasonal decline in quality of its host plant, Salix sachalinensis Fr. Schmidt (Salicaciae). P. versicolora was maintained for three successive generations throughout a season under constant laboratory conditions and fed host leaves obtained from the field site. Larval and adult performance declined in the generations that were produced later in the season. Developmental time in the last generation was significantly longer than in the first generation, although larval survivorship did not differ significantly between generations. Preoviposition periods were significantly longer and the number of eggs laid from the first oviposition to the 10th day of oviposition was significantly fewer in the last two generations compared with the first generation. In addition, reproductively inactive females that laid no eggs were significantly more frequent in the last two generations. This reproductive inactivity in female adults in late generations may be caused by diapause, which can prevent P. versicolora from producing an additional generation on poor-quality host plants. However, the physiological condition of the reproductively inactive females was different from that induced by short photoperiod in diapausing females because feeding was observed in the former females but not in the latter ones.
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