Pine caterpillar, Dendrolimus tabulaeformis Tsai et Liu, is a major pine pest in North China. The larvae enter diapause in the third or fourth instar before winter. Supercooling points (SCP) and cold hardiness of the diapausing larvae were investigated and compared in non-acclimated, acclimated and de-acclimated larvae. A bimodal frequency distribution was observed with a break point of -14°C in the SCP. Larvae in the low group (LG, SCP <= -14°C) were more cold tolerant with lower lethal temperatures than those in the high group (HG, SCP > -14°C). This bimodality occurred in three patterns, LG (> 60% of individuals in LG), LG-HG (< 60% of individuals in LG and HG) and HG (> 60% of individuals in HG), in response to cold acclimation and de-acclimation. The cold hardiness was ranked as: LG > LG-HG > HG pattern. Cold hardiness was enhanced by an increase in concentrations of trehalose, galactose, glucose and mannose in the haemolymph as well as by decrease in metabolism after cold acclimation, but was lost after de-acclimation. Loss of cold hardiness was correlated with decrease in sugars and increase in metabolic rate. In conclusion, the species is a chill tolerant insect, adopting the strategy of depressing SCP through accumulation of low molecular weight sugars in the haemolymph, concomitant with metabolic depression.
Overwintering insects must avoid injury and death from the freezing of tissues and from metabolic disruptions associated with exposure to low, non-freezing temperatures. The winter climates of the world are classified in relation to insect overwintering on the basis of their minimum temperatures and the duration of the winter (when temperatures are below the thermal range for activity and development). Outside the Tropical Wet zone, the severity of exposure to cold (temperature, snowfall, duration of exposure, predictability, variability) can vary from a few days at 0°C to months below -20°C with extremes as low as -60°C. The severity of the temperature exposure may be ameliorated by the selection by insects of overwintering sites (exposed, partly-exposed, protected). The relationships among overwintering habitats, the minimum winter temperature in climatic zones, and the supercooling points (SCP) of over 350 terrestrial insects from published reports were examined. Variability in the SCP among insects within each climatic zone and habitat was wide. Among the freeze-susceptible species that overwintered in exposed or partly-protected habitats the SCP and the cold severity of climate were correlated. This was not the case for insects that overwintered in protected habitats. The SCP's of freeze-tolerant insects were generally higher than the freeze-susceptible insects, and the SCP's were not tightly linked with the cold severity of climatic zone. Insects, both freeze-susceptible and freeze-tolerant, overwintering in exposed habitats had lower SCP's than insects from habitats that offered some protection from ambient temperatures. Thirty-eight species had reports of SCP's for different geographical locations. Although there were occasionally differences in the SCP's, there was no consistent pattern of insects having lower SCP's when overwintering in colder habitats. The incidence of freeze-tolerance was higher in boreal and polar climatic zones than in climatic zones with warmer winters. Holometabola insects had a higher incidence of freeze-tolerance than hemimetabola insects. Suggestions for future research directions are outlined.