COMPARISON OF THE COLD HARDINESS OF 2 LARVAL LEPIDOPTERA (NOCTUIDAE)

Diapause larvae of the European corn borer (Ostrinia nubilalis (Hubn.)) and the related Mediterranean noctuid Sesamia cretica Led. possess sufficient supercooling ability to avoid freezing over their normal environmental temperature ranges. In progressive chilling experiments (10 days acclimation at each 5-degrees step in the temperature range from 15 to -5-degrees-C), mean supercooling points (measured at a cooling rate of 0.1-degrees-C min-1) were lowered from -20.4-degrees-C at 15-degrees-C to -24.0-degrees-C at 5-degrees-C (lower lethal temperatures: c.-28-degrees-C) in O.nubilalis, compared with -15.0 to -17.2-degrees-C (lower lethal temperatures: -15 to -17-degrees-C respectively) in S.cretica. Concentrations of glycerol and trehalose determined by gas chromatography of whole body extracts were consistently higher in the former than in the latter species at both 15 and 5-degrees-C, and may be responsible for the deeper supercooling in O.nubilalis larvae. Acclimation to 5-degrees-C increased glycerol levels in O.nubilalis extracts compared with 15-degrees-C, and this was enhanced in larvae exposed for a further 10 days at each of 0 and -5-degrees-C (glycerol being 438-mu-mol ml-1 body water). Haemolymph glycerol concentrations showed a similar pattern to whole body extracts in this species. Fat body glycogen was reduced during low temperature acclimation in both species. Body water contents did not change with acclimation in O.nubilalis, whilst S.cretica, containing significantly more water, lost c.7% during acclimation from 15 to 5-degrees-C. Haemolymph osmolalities increased during acclimation, especially in Ostrinia larvae, probably as a result of the accumulation of cryoprotectants. The majority of O.nubilalis larvae survived freezing,under the conditions of the cooling experiments, whilst larvae of S.cretica did not, thereby confirming an element of freezing tolerance in the former.