Possible Role of Glycerol in the Winter-Hardiness of Insects

FURTHER OBSERVATIONS ON GLYCEROL AND COLD-HARDINESS IN INSECTS

Changes in glycerol content and supercooling points were studied in three overwintering species. In larvae of Hyponomeuta evonymellus L. and eggs of Acrolita naevana (Hb.), glycerol was accumulated and supercooling points were lowered during the fall. After diapause was broken, the glycerol content decreased, accompanied by an increase in supercooling points. Similar changes were observed in all larvae of Laspeyresia strobilella (L.), although some of them pupate after one winter, while others remain in diapause for at least 1 more year. The larvae that remain in diapause are able to accumulate glycerol during exposure to low temperatures, and in this way increase their cold-hardiness, Glycerol was lost at 20 °C in the fall when all larvae of L. strobilella were in diapause, whereas diapausing eggs of A. naevana retained their glycerol content at this temperature. Eggs of A. naevana from a location in eastern Norway accumulated more glycerol than did eggs from a location with a milder climate in western Norway, thus suggesting differences in adaptation to low temperatures, Larvae of H. evonymellus and eggs of A. naevana were killed by freezing, while larvae of L. strobilella were freezing-tolerant in the middle of the winter.

EFFECTS OF GLYCEROL ON COLD-HARDINESS IN INSECTS

The effects of glycerol on cold-hardiness in insects and seasonal changes in glycerol concentrations were investigated. The presence of this polyhydric alcohol was demonstrated in overwintering stages of 10 species. Larvae of one species also contained sorbitol, and eggs of two species another polyhydric alcohol, probably mannitol.Evidence gathered from various diapausing species showed that glycerol accumulated during the fall. This increase in concentration was observed in eggs of one species at temperatures ranging from −5° to 20 °C. No decrease in glycerol content was observed in any species as long as it was in diapause. After diapause was broken glycerol was lost in all species, in some even at temperatures down to −5 °C. Increase in concentration was never found in postdiapause insects. In two species, which do not have a diapause, the glycerol content increased below a certain temperature and decreased at higher temperatures.Three species were freezing-tolerant, although one contained less than 3% and another no glycerol, whereas eight species, most of which contained more than 15%, were killed by freezing. Thus glycerol alone cannot protect against freezing injuries.In several species the cold-hardiness was increased by the presence of glycerol because supercooling points were depressed. These depressions were more than those of the corresponding melting points. The regression of amount of supercooling on concentration of glycerol was linear in five species.