Climate change is modifying winter conditions rapidly and predicting species’ reactions to global warming has been the “the holy grail” of climate sciences, especially for managed systems, like agro-ecosystems. Intuitively, increased winter temperatures should release insects from coldinduced mortality, but warmer winters can lead to fitness costs via increased metabolic rate and a drain on energy reserves.The overall objective of this thesis was to investigate differences in species responses in mild(+5°C) and cold (−5°C) winters. This was done by assessing organismal physiology, overwintering fitness and post-winter performance in two species: 1) a natural enemy Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), the Harlequin Ladybird, overwintering as an adult and 2) the agricultural pest Delia radicum (L.) (Diptera: Anthomyiidae), the cabbage root fly, overwintering as pupae.Physiological traits measured during winter were as follows: Standard Metabolic Rate (SMR), Q10, energy reserves: dry mass, lipids, glycogen and proteins. The overwintering fitness: longevity, mortality and body mass (loss) were assessed.Post-winter fitness was tested, in H. axyridis: the status of survivors, feeding capacity, mating and the length of the pre-oviposition period. Post winter fitness in D. radicum: the time of emergence of flies.Energy metabolismSMR was higher in mild compared to cold winters (H. axyridis) especially at the end of wintertreatment (in both H. axyridis and D. radicum), indicating earlier termination of diapause. Ingeneral, SMR increased with time and temperature measured in respiration chambers, indicatingloss of endogenous control of metabolism.In cold winter H. axyridis relied on dry mass depletion and lipid catabolism for energy and in mild winters they lost water, probably due to higher respiration evidenced at the end of winter. Glycogen was equally catabolized by H. axyridis in both winter conditions, again supporting a lack of endogenous control of metabolism to specific winter conditions.
Winter and post-winter fitnessMild winters increased mortality of H. axyridis for two subsequent years, compared to cold winters. Conversely, D. radicum had increased mortality during cold winters. The mortality risk factor in H. axyridis seemed to depend on high SMR and concurrent water depletion, in short the failure to adapt to the mild winters. H. axyridis males suffered increased mortality with decreasing body mass.Post-winter performance of H. axyridis was positively affected by mild winter conditions such that higher feeding capacity and shorter pre-oviposition period was recorded, but may be affected negatively by cold winters, which caused chill injury related mortality (recorded after winter) and could affect post-winter reproduction. In the case of D. radicum, mild winter caused earlier emergence of flies.Overall, this study showed that mild winters increased mortality of H. axyridis, but earlier onset of population growth may compensate for this, and in D. radicum mild winters decreased mortality and induced earli er emergence of flies. Thus, for some species, like H. axyridis, mild winters may not benefit nor conclusively stress the species, but for others like D. radicum, mild winters may benefit the population and be of concern to farmers.Agricultural insect fauna is a prime example of a rapid, contemporary evolution, optimal formeasuring species response under constant selection pressure, including organismal physiology.Thus, the methodological approach applied in this study, could prove a valuable tool for improving predictability of field population dynamics during climate change.