The relative importance of number, duration and intensity of cold stress events in determining survival and energetics of an overwintering insect

The relationship between abiotic stress and fitness in an individual is usually described by the intensity and duration of stress. Yet in natural systems, variability in abiotic stress is common. Since individuals have physiological and fitness responses to single bouts of stress, frequency of stress may also determine the lifetime success of an organism. However, the majority of laboratory studies have focused only on the effects of single stress events. We investigated the relative importance of stress parameters including duration, intensity and number of cold events on the short-term physiology and long-term fitness in the freeze-avoiding eastern spruce budworm Choristoneura fumiferana (Lepidoptera: Tortricidae, Clemens). We exposed overwintering 2nd-instar larvae of C.fumiferana to -5 degrees C, -10 degrees C, -15 degrees C or -20 degrees C, for either a single exposure of 120h or repeated 12h exposures (3, 6 or 10 exposures). Changes in short-term physiology were quantified by cryoprotectant content, energetic stores and supercooling point. Long-term fitness effects were measured by rearing individuals after overwintering and recording successful eclosion as adults, development time from 2nd instar to adult, and adult size. We found that long-term survival of C.fumiferana was most strongly affected by the number of low-temperature stress events rather than intensity or duration, despite increased investment into cryoprotection at the expense of glycogen reserves. Sublethal measures such as adult size were unaffected by low-temperature stress. Thus, we show that frequency of stress is an important, yet frequently neglected, parameter in the study of the effects of abiotic stress. The responses we documented here suggest that frequency of stress may be an additional important parameter for modelling the effects of abiotic stress on populations.