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High latitude insect populations must cope with extreme conditions, particularly low temperatures. Insects use a variety of cold hardiness mechanisms to withstand this temperature stress, and these can drive geographic distributions through overwintering mortality. The degree of cold hardiness can be altered by two evolved responses: phenotypic plasticity and local adaptation. Phenotypic plasticity can occur within or between generations (transgenerational plasticity; TGP), and local adaptation can evolve through directional selection in response to regional climatic differences. We used the eastern spruce budworm, Choristoneura fumiferana (Lepidoptera: Tortricidae) as a model to explore the role that variable winter temperatures play in inducing two aspects of plasticity in cold hardiness: TGP and local adaptation in phenotypic plasticity. This species is one of the most destructive boreal forest pests in North America, therefore accurately predicting overwintering survival is essential for effective management. While we found no evidence of TGP in cold hardiness, there was a long term fitness cost to larvae that experienced repeated cold exposures. We also found evidence of local adaptation in both seasonal and short-term plasticity of cold hardiness, as our more northerly populations that would experience lower overwintering temperatures had more plastic responses to cold exposure. These findings provide evidence for the importance of phenotypic plasticity and local adaptation when modelling species distributions., (Crown Copyright © 2021. Published by Elsevier Inc. All rights reserved.)

Temperate, polar and alpine insects generally do not feed over winter and hence must manage their energy stores to fuel their metabolism over winter and to meet the energetic demands of development and reproduction in the spring. In this Review, we give an overview of the accumulation, use and conservation of fat reserves in overwintering insects and discuss the ways insects modify fats to facilitate their selective consumption or conservation. Many insects are in diapause and have depressed metabolic rates over winter; together with low temperatures, this means that lipid stores are likely to be consumed predominantly in the autumn and spring, when temperatures are higher but insects remain dormant. Although there is ample evidence for a shift towards less-saturated lipids in overwintering insects, switches between the use of carbohydrate and lipid stores during winter have not been well-explored. Insects usually accumulate cryoprotectants over winter, and the resulting increase in haemolymph viscosity is likely to reduce lipid transport. For freeze-tolerant insects (which withstand internal ice), we speculate that impaired oxygen delivery limits lipid oxidation when frozen. Acetylated triacylglycerols remain liquid at low temperatures and interact with water molecules, providing intriguing possibilities for a role in cryoprotection. Similarly, antifreeze glycolipids may play an important role in structuring water and ice during overwintering. We also touch on the uncertain role of non-esterified fatty acids in insect overwintering. In conclusion, lipids are an important component of insect overwintering energetics, but there remain many uncertainties ripe for detailed exploration., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

Thermal performance curves enable physiological constraints to be incorporated in predictions of biological responses to shifts in mean temperature. But do thermal performance curves adequately capture the biological impacts of thermal extremes? Organisms incur physiological damage during exposure to extremes, and also mount active compensatory responses leading to acclimatization, both of which alter thermal performance curves and determine the impact that current and future extremes have on organismal performance and fitness. Thus, these sub-lethal responses to extreme temperatures potentially shape evolution of thermal performance curves. We applied a quantitative genetic model and found that beneficial acclimatization and cumulative damage alter the extent to which thermal performance curves evolve in response to thermal extremes. The impacts of extremes on the evolution of thermal performance curves are reduced if extremes cause substantial mortality or otherwise reduce fitness differences among individuals. Further empirical research will be required to understand how responses to extremes aggregate through time and vary across life stages and processes. Such research will enable incorporating passive and active responses to sub-lethal stress when predicting the impacts of thermal extremes., (© The Author 2016. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

Cold tolerance is a key determinant of insect distribution and abundance, and thermal acclimation can strongly influence organismal stress tolerance phenotypes, particularly in small ectotherms like Drosophila. However, there is limited understanding of the molecular and biochemical mechanisms that confer such impressive plasticity. Here, we use high-throughput mRNA sequencing (RNA-seq) and liquid chromatography - mass spectrometry (LC-MS) to compare the transcriptomes and metabolomes of D. melanogaster acclimated as adults to warm (rearing) (21.5 °C) or cold conditions (6 °C). Cold acclimation improved cold tolerance and led to extensive biological reorganization: almost one third of the transcriptome and nearly half of the metabolome were differentially regulated. There was overlap in the metabolic pathways identified via transcriptomics and metabolomics, with proline and glutathione metabolism being the most strongly-supported metabolic pathways associated with increased cold tolerance. We discuss several new targets in the study of insect cold tolerance (e.g. dopamine signaling and Na(+)-driven transport), but many previously identified candidate genes and pathways (e.g. heat shock proteins, Ca(2+) signaling, and ROS detoxification) were also identified in the present study, and our results are thus consistent with and extend the current understanding of the mechanisms of insect chilling tolerance.

Insects experience repeated cold exposure (RCE) on multiple time scales in natural environments, yet the majority of studies of the effects of cold on insects involve only a single exposure. Three broad groups of experimental designs have been employed to examine the effects of RCE on insect physiology and fitness, defined by the control treatments: 'RCE vs cold', which compares RCE with constant cold conditions; 'RCE vs warm', which compares RCE with constant warm conditions; and 'RCE vs matched cold' which compares RCE with a prolonged period of cold matched by time to the RCE condition. RCE are generally beneficial to immediate survival, and increase cold hardiness relative to insects receiving a single prolonged cold exposure. However, the effects of RCE depend on the study design, and RCE vs warm studies cannot differentiate between the effects of cold exposure in general vs RCE in particular. Recent studies of gene transcription, immune function, feeding and reproductive output show that the responses of insects to RCE are distinct from the responses to single cold exposures. We suggest that future research should attempt to elucidate the mechanistic link between physiological responses and fitness parameters. We also recommend that future RCE experiments match the time spent at the stressful low temperature in all experimental groups, include age controls where appropriate, incorporate a pilot study to determine time and intensity of exposure, and measure sub-lethal impacts on fitness.

Insects in the field are exposed to multiple bouts of cold, and there is increasing evidence that the fitness consequences of repeated cold exposure differ from the impacts of a single cold exposure. We tested the hypothesis that different kinds of cold exposure (in this case, single short, prolonged and repeated cold exposure) would result in differential gene expression. We exposed 3 day old adult female wild-type Drosophila melanogaster (Diptera: Drosophilidae) to -0.5°C for a single 2 h exposure, a single 10 h exposure, or five 2 h exposures on consecutive days, and extracted RNA after 6 h of recovery. Global gene expression was quantified using an oligonucleotide microarray and validated with real-time PCR using different biological replicates. We identified 76 genes upregulated in response to multiple cold exposure, 69 in response to prolonged cold exposure and 20 genes upregulated in response to a single short cold exposure, with a small amount of overlap between treatments. Three genes--Turandot A, Hephaestus and CG11374--were upregulated in response to all three cold exposure treatments. Key functional groups upregulated include genes associated with muscle structure and function, the immune response, stress response, carbohydrate metabolism and egg production. We conclude that cold exposure has wide-ranging effects on gene expression in D. melanogaster and that increased duration or frequency of cold exposure has impacts different to those of a single short cold exposure. This has important implications for extrapolating laboratory studies of insect overwintering that are based on only a single cold exposure.