Abstract
The greatest degree of cold-hardiness and middle instar larvae were tested to clarify some necessary factors for larval overwintering. The important factors were periods of effective temperature for larval development and the presence of food plants during overwintering. The effective temperature of more than 0.9 day-degrees per day was required for larval survival. More effective temperature was required in accordance with the length of exposure time below 0°C per day. Larval death caused by chilling was correlated with intensity and duration of low temperature below 0°C. When the larvae were exposed to 0°C and -2°C for 5hr per day, there was no significant difference in the survival period between both temperature levels. Longer exposure (e.g. 10hr per day) clearly decreased the surviving rete even at -2°C. The low temperature of -5°C was a low critical point because the larvae were not able to survive more than 45 days.
