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Abstract

Low temperature tolerance or cold hardiness refers to the capacity of an organism to survive exposure to low temperatures. The tolerance is commonly divided into two classes, freeze susceptible and freeze tolerant. Since freeze susceptible insects cannot survive ice formation in their bodies, they enhance their supercooling capacity in winter to decrease the probability of freezing. In contrast, freeze tolerant insects can survive the extracellular freezing, but not intracellular freezing. In this paper the mechanisms of low temperature tolerance and the role of ice nuclei in larvae of the rice stem borer, Chilo suppressalis Walker are reviewed. Both non-diapausing and diapausing larvae were freeze tolerant even though the freeze tolerance of the former was much lower than that of the latter. High levels of glycerol were produced in the haemolymph from glycogen in the fat body as a cryoprotectant in overwintering larvae during pre-diapause to diapause stages in the field. An increase in freeze tolerance in the diapausing larvae coincided with an increase in glycerol content in the haemolymph. A number of factors involved in the regulation of crystallization temperature were identified. Ice nucleating active fungus. Fusarium moniliforme var. subglutinans, was present in the gut of non-diapausing larvae as an exogenous ice nucleus and induced freezing at relatively high subzero-temperatures. As food particles including the fungus in the gut of overwintering larvae were excreted in autumn, the crystallization temperature of the gut decreased. Instead of the exogenous ice nuclei, endogenous potent ice nucleating active proteins were produced in the muscle and epidermis, thereby, allowing overwintering larvae to avoid the potentially lethal effects of intracellular freezing. The nuclei were present in the cell membranes of these tissues and induced freezing of the haemolymph to prevent intracellular freezing. Endogenous ice nuclei and high glycerol content enhance overwintering survival in rice stem borer larvae.