Photoperiod-dependent adult diapause within a geographical cline in the multivoltine bruchid Bruchidius dorsalis

The bruchid beetle Bruchidius dorsalis Fahraeus ( Coleoptera: Bruchidae) has been known to undergo larval diapause during the final instar under short photoperiods ( Kurota & Shimada, 2001). This species has a multivoltine life cycle and the overwintering stages show a geographical variation across Japan ( Kurota & Shimada, 2002). In cooler areas, overwintering occurs during the final instar, whereas in warmer climates overwintering can occur during several developmental stages: non-diapausing young instars, diapausing instars, and adults. In this study, we investigated the adult reproductive diapause in three populations from different geographical regions to clarify the role of geographical variation on overwintering strategies. We found that: ( 1) B. dorsalis entered reproductive diapause in addition to larval diapause under short photoperiods, ( 2) diapause propensity was higher and the critical photoperiod was longer in populations from cooler regions, and ( 3) the sensitive photoperiod range was the first 5 days after emergence. Predictions of the overwintering stage, derived from critical photoperiods, were consistent with actual overwintering stages observed in each population. The geographical variation in diapause induction is likely to reflect the adaptive overwintering strategy in each local environment.