Seasonal and geographical variation in diapause and cold hardiness of the Asian corn borer, Ostrinia furnacalis.

Asian corn borer, Ostrinia furnacalis (Guenée), is a key corn pest in the Asian-Western Pacific countries. It overwinters as full-grown larvae in plant stalks or in a spun-silk covering located in the plant debris in the temperate regions of China. Supercooling point (SCP) and survival rate after low sub-zero temperature treatment were assessed for field-collected populations in the laboratory using a cool bath with a 1°C/min cooling rate until -40°C. Mean SCPs were varied among geographical populations, with a significant decline from -22.7°C of Haikou, the multivoltine tropical population in the south, to -28.5°C of Gongzhuling, the univoltine temperate population in the northeast of China. In addition, there was more than 1°C difference in SCP between Gongzhuling univoltine and bivoltine populations that were from the same geographic origin. Mean SCPs of the Guangzhou population fluctuated over the year, with significantly lower SCPs in winter than in other seasons, which correlated with a significantly higher proportion of diapausing larvae in winter than in other seasons. Over 41% of overwintering larvae from the northeast population could withstand to be supercooled for a few minutes to the low sub-zero temperature of -40°C, but only 6.7% of their southern counterparts did so. The findings from this study suggest that O. furnacalis mostly takes advantage of freeze avoidance as diapausing larvae for overwintering in the southern region, whereas it exhibits freeze tolerance in diapause in the northeastern region.