A multi-year dormancy strategy in a cabbage beetle population in southeastern China

The seasonal life cycle of the cabbage beetle Colaphellus bowringi in southeastern China is very complicated because there are four options for adult development: summer diapause, winter diapause, prolonged diapsuse, and nondiapause. However, detailed information on the multi-year emergence patterns of diapausing individuals in this beetle has not been documented. In this study, we monitor the adult emergence patterns of diapausing individuals and estimate the influence of the diapause-inducing temperature and photoperiod on the incidence of prolonged diapause under seminatural conditions for several years. The duration of diapause for adults collected from the vegetable fields in different years varied from several months to 5 years. Approximately 25.9-29.2% of individuals showed prolonged diapause (emergence more than 1 year after entering diapause) over the five years of observation. Furthermore, regardless of insect age, emergence always occurred in spring, between mid-February and March, and in autumn, between late August and mid-October, when the host plants were available. The influence of diapause-inducing temperatures (22, 25, and 28 °C) combined with different photoperiods (L:D 12:12 and L:D 14:10) on diapause duration was tested under seminatural conditions. Pairwise comparisons of diapause duration performed by the log-rank test revealed that the low temperature of 22 °C combined with the long photoperiod of L:D 14:10 induced the longest diapause duration, whereas the low temperature of 22 °C combined with the short photoperiod of L:D 12:12 induced the highest proportion of prolonged diapause. This study suggests that C. bowringi adopts a multi-year dormancy strategy to adapt to local environmental conditions and unpredictable risks in order to avoid local extinction.