Location of Thesis Examination

Room 2084 B&GS

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. Brent J. Sinclair

Abstract

While insect cold tolerance has been well studied, the vast majority of work has focused on the effects of a single cold exposure. However, many abiotic environmental stresses, including temperature, fluctuate within an organism's lifespan. In this thesis I address two major questions. First, does frequency of cold exposure impose additional stress on insects? Second, how does this stress translate to performance and fitness? I first summarize the literature on the effects of repeated cold exposure in insects, critically examining experimental designs. I then address my questions experimentally using four insect species with contrasting life histories and responses to cold exposure. I examine the fitness costs of repeated cold exposure in the chill-susceptible fly Drosophila melanogaster by recording survival, generation time, and number of offspring in flies that had received five 2 h, one 2 h, or one 10 h exposure to -0.5 ºC. I found that D. melanogaster trades off immediate survival and reproductive output in response to repeated cold exposure. In the freeze-tolerant caterpillar Pyrrharctia isabella, repeated freezing did not deplete energy reserves, but did damage hemocytes and Malpighian tubules, and led to decreased survival in repeatedly-frozen caterpillars. Similarly, I found in the freeze-avoiding caterpillar Choristoneura fumiferana that frequency of exposure, independent of period, intensity, or duration of cold exposures significantly decreased mortality, likely due to increased investment in cryoprotection at the expense of glycogen reserves. Finally, in the freeze-tolerant fly Eurosta solidaginis, the frequency of exposure, again independent of period, intensity, or duration of cold exposures led to a significant decline in egg production. Repeated cold exposure therefore imposes additional costs to insects, even when intensity and duration of cold are controlled for. This cost may either be a result of accrued damage, or may be energetic as individuals trade off investment in survival mechanisms for reproductive output. Given that many environments are not static, these effects indicate that investigating the effects of repeated stress exposure is important for understanding and predicting physiological responses in the wild.