We investigated the seasonal population dynamics of the bruchid beetle, Bruchidius dorsalis Fahraeus, in relation to the phenology of its host plant Gleditsia japonica Miq. (Fabaceae: Caesalpinoidea) in two geographically distinct areas of Japan. We compared life cycles to clarify influences of seasonal variation in temperature and host plant availability on insect life cycles. Results from a 4-yr study indicated that in Sagamihara (warmer climate): (1) B. dorsalis has a trivoltine life cycle, (2) it cannot reproduce during a period of more than 1 mo until new seeds become available in early or mid-August, and (3) it overwinters in several different developmental stages (young and old instars, and adults). However, studies over 3 yr in Tatsuno (cooler climate) indicated the following: (1) B. dorsalis has a bivoltine life cycle, (2) G. japonica phenology lags several weeks behind that in Sagamihara, and (3) B. dorsalis overwinters as diapausing larvae (late fourth instar) or as adults. Thus, the life cycles of B. dorsalis are more constrained in cooler areas, not only because of the direct effects of lower temperature on insect development, but also because of the phenological delay in host plant development.