Over-wintering stage polymorphism of a bruchine beetle: geographical variation in optimal diapause strategy

Abstract.  1. Most temperate insects undergo diapause at a specific developmental stage to overcome severe winters. The bruchine beetle Bruchidius dorsalis in a warmer region in Japan, however, has diverse over-wintering stages – non-diapausing larvae, diapausing larvae, and diapausing adults, whereas in a cooler region, it over-winters only as the final instar larva or adult in diapause.
2. The geographical pattern of over-wintering stages in 12 populations was investigated over a wide range along the mainland of Japan. It revealed that a distinct geographical pattern of over-wintering stages exists in relation to temperature conditions. In regions with warmer climates, the proportion of non-diapausing larvae increased and B. dorsalis had a more complex over-wintering stage structure.
3. Life cycles were also compared between two areas of Japan by field experiments. In the cooler area, the first generation over-wintered in the diapausing larval or adult stage. Conversely, in the warmer area, diapause was induced later and some of the first-generation adults produced second-generation offspring before over-wintering.
4. Based on the geographical cline of climates and the differences in cold hardiness among stages, we can demonstrate that the over-wintering stage variation among and within populations results from an adaptive timing of diapause induction in each region, because the late larval or adult diapauses protect pupae or eggs – which unlike other stages are not cold hardy – from being produced late in the season.