Effect of Rapid Cold Hardening on the Cold Tolerance of the Larvae of the Rice Stem Borer, Chilo suppressalis (Walker)

The effects of rapid cold hardening (RCH) on the cold tolerance of the last instar larvae of Chilo suppressalis (Walker) were evaluated for the first time. The discriminating temperature, induction, detection, duration and extent of RCH of the larvae in the laboratory were tested, and the supercooling points (SCPs) and the contents of water and lipid of the larvae after RCH treatment were determined, respectively. The results showed that the discriminating temperature of the larvae was about -21°C. Mean survival rates of the larvae which exposed to either 0 or 5°C for 2 and 4 h before exposure to the discriminating temperature for 2 h were significantly higher than those of the control groups (P<0.05). Moreover, the highest survival rate appeared in the larvae after 0°C for 4 h treatment. The protection against low temperature gained by RCH at 0°C for 4 h was rapidly lost on return to 28°C. Mean survival rates of RCH larvae were significantly higher than those of non-acclimated (NACC) larvae and acclimation (ACC) larvae when they were exposed to the discriminating temperature for 2 or 4 h (P <0.05). Moreover, the rates of NACC, ACC, RCH and ACC + RCH larvae from 2 to 6 h to the discriminating temperature resulted in a significant decline. The values of SCPs and the contents of lipid of the larvae which exposed to either 0 or 5°C for 2 and 4 h showed no significant difference at 0.05 level compared to those of the control groups. But the contents of water in the larvae were obviously decreased. Therefore, it could be concluded that RCH could enhance cold tolerance and affect partly physiological and biochemical components of the larvae of C. suppressalis, but the underlying mechanisms needs to be further explored.