Bruchidius dorsalis Fahraeus (Coleoptera: Bruchidae) enters winter diapause in either the fourth (final) instar or during the adult stage (Kurota and Shimada 2001, 2003b). In the middle part of Japan, it overwinters in several developmental stages: nondiapausing young instars, diapausing instars, and adults (Kurota and Shimada 2002). To understand the relationship between the complex overwintering stages and the adaptive diapause strategy of B. dorsalis, I investigated (1) overwintering survivorship in developmental stages observed in the field and (2) cold hardiness at all stages including nonoverwintering ones such as eggs and pupae after enduring a controlled chilling experiment at 5°C for 100 d. The following results were obtained: (1) survivorship of nondiapausing larvae under natural conditions was almost as high as that of diapausing larvae and adults, and (2) cold hardiness in eggs and pupae was much lower than the other developmental stages in terms of survivorship and reproductive ability after the chilling treatment. From these results, I concluded that larval and adult diapause of this species would be induced at the most appropriate time to avoid overwintering in developmental stages with low cold hardiness. The complex overwintering stages in species that inhabit the middle part of Japan are likely to result from the adaptive timing of diapause induction at the two stages.
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