Cold Hardiness of Diapausing Rice Stem Borer, Chilo suppressalis Walker (Lepidoptera: Pyralidae)

This study was carried out to investigate the supercooling point (SCP), cold hardiness, and content of glycerol in the rice stem borer, Chilo suppressalis, larvae field-collected at different dates to understand the relationship between diapause and cold hardiness. Field-collected C. suppressalis larvae became cold-hardy from late autumn and higher level of survival was maintained from October to April. Lab-reared C. suppressalis larvae pre-treated at 0°C for 1 h or 2 h enhanced their survival to low temperature (-10°C). The SCP of C. suppressalis larvae changed seasonally, with dramatic decrease toward February and then drastic increase again from March onwards. The osmolality and protein content of field-collected C. suppressalis larvae showed their peak in January, and then dramatically declined afterwards. Overwintering C. suppressalis larvae have the ability to accumulate glycerol. Higher osmolality may be closely linked to lower SCP. This lower SCP probably develops a high level of cold hardiness during diapause development. The thermal hysteresis activity of C. suppressalis larva field-collected increased gradually to 12.7± 1.6°C in January, and then gradually declined afterwards. It can be recognized that field-collected C. suppressalis larvae have the thermal hysteresis proteins. Diapause- induced C. suppressalis larvae (kept at 15°C under a photo-period of 10L: 14D h) enhanced a significantly higher survival and osmolality. In diapause-destined C. suppressalis, therefore, increased cold tolerance can be linked to the diapausing state itself.