Field-collected 229 colonies of a pemphigine social aphid Colophina arma, which has a soldier caste in the first instar, were examined for its population dynamics, reproductive schedule, and ecological factors related to soldier production. Colony size was small in July and became larger in August and on; this change probably reflects the flowering phenology of the host plant Clematis stans. Soldiers, which neither grow nor reproduce but are specialized for colony defense, were found throughout the seasons. Winged sexuparae, which migrate to Zelkova serrata and produce the sexual generation, appeared in late September. Midget first instar nymphs, which persist on C. stans for hibernation, were produced in October. Statistical analyses suggested that soldier proportion in colonies of C. arma might be related to the proportions of unwinged adults, colony size, and seasonal change. Sexuparae and midget nymphs were produced only in autumn, which reflects the overwintering roles of these morphs, and suggests that production of these morphs might be influenced by environmental factors related to the season, such as short photoperiod and low temperature.