GLYCEROL SYNTHESIS BY DIAPAUSING LARVAE IN RESPONSE TO THE TIMING OF LOW-TEMPERATURE EXPOSURE, AND IMPLICATIONS FOR OVERWINTERING SURVIVAL OF THE SPRUCE BUDWORM, CHORISTONEURA-FUMIFERANA

Spruce budworm larvae, Choristoneura fumiferana, were exposed at different times during diapause to a lower temperature regime (from 18 to 2 degrees C with or without 1-week at -5 degrees C in between), A lower temperature clearly elicited a stronger response in glycerol synthesis by the larvae, However, the magnitude and the pattern of increases in glycerol Level were associated with the timing of the cold exposures, indicating the existence of a physiological window, These laboratory results were related to those from two experiments conducted outside during the winters of 1992-1993 and 1993-1994. By delaying the starting date of outside exposure in the 1993-1994 winter, larvae failed to produce a significant amount of glycerol and most died, This was most likely due to the effective change of the physiological window together with an ''earlier'' arrival of extreme cold temperatures, Based on this evidence, we argue that overwintering success of an insect depends not only on temporal changes in external parameters (e.g. temperature) but on internal developmental processes and their interactions. Timing of such events plays a very important role in insect overwintering survival and deserves particular attention in future investigations of insect winter biology.