Flexible Life Cycle of a Cockroach Periplaneta japonica with Nymphal Diapause

Abstract

A cockroach, Periplaneta japonica, is known to have a semivoltine life cycle in northern snowy areas. In the present study, nymphal development, nymphal diapause, and reproduction of a southern population were studied under laboratory and outdoor conditions to understand the life cycle strategy of this species. Nymphal development proceeded faster at a longer photoperiod, but diapause was induced at any photoperiod. The longer the photoperiod the later the nymphal instar at which diapause occurred. A sample collected early in the spring suggested that the overwintering population comprised two distinct size groups, corresponding to early and late nymphal instars. This favored the conclusion that this species requires two years to complete the life cycle, as reported for northern populations. However, by rearing cockroaches under outdoor conditions, it was found that some of the nymphs hatching early in the summer overwintered as late nymphal instars and emerged as adults in the following year. This pattern of development was observed not only in a year with an unusually hot summer but also in a year with an unusually cool summer. This species has a mixture of univoltine and semivoltine life cycles, and such a mixed voltinism appears to be adaptive for insects with a long reproductive period. The time of first oviposition was earlier at a higher temperature, but mean ovipositing interval was the shortest at around 25C and tended to be prolonged at a higher temperature. These results indicate that P. japonica is adapted to northern climates and has a highly flexible life cycle.