Abstract. 1. From the early 1960s to 2000 Nezara viridula (Heteroptera: Pentatomidae) expanded its range northwards in Japan and reached Osaka following climate warming recorded in the region.
2. The timing of diapause induction and its effect on life-history traits were studied under quasi-natural conditions in Osaka. Egg masses were placed outdoors in six series in July–November 1999. Developmental events were monitored until September 2000.
3. Adult diapause was induced in September–October in agreement with the photoperiodic response obtained under laboratory conditions. Induction of diapause in early October ensured the highest winter survival. Nymphs that hatched after late September died by December–April showing that the species cannot survive winter in the nymphal stage.
4. Life-history traits varied between the early (non-diapause reproduction) and late (post-diapause reproduction) series. Thus, non-diapause females produced significantly fewer egg masses than did females that reproduced only after diapause. The timing of diapause induction strongly affected overwintering success and post-diapause performance: females that became adults and entered diapause in October lived longer, had a longer period of oviposition, and produced more eggs in larger egg masses than females that attained adulthood and entered diapause in September.
5. Females from the early series reproduced until late November, although progeny from the late September eggs were destined to die during the winter. Pre-winter reproduction of adults that emerged in mid-September or later was a result of the imperfect timing of diapause induction. It is an ineffective allocation of resources and may be considered the ecological cost of range expansion.
6. To establish in the region, N. viridula will probably evolve a lengthening of the critical photoperiod of the diapause induction response. This will allow the species to enter diapause earlier and, thus, avoid maladaptive late-autumn reproduction but, perhaps, increase the cost of diapause because of a possible adverse impact of pre-winter high temperature conditions on overwintering.