The aim of this study is to evaluate the physiological costs and the consequences on fitness of cold exposure on the parasitoid Aphidius ervi (Hymenoptera: Aphidiinae) under non-lethal conditions. We exposed 1-day-old mummies to different treatments: control at 20°C, 7°C constant, 7°C fluctuating (7°C for 22 h and 20°C for 2 h). Two performances of fitness were particularly affected after two weeks at 7°C constant: female longevity and the sex ratio of the progeny was male biased. In contrast, egg load at emergence, lifetime fecundity, frequency of asymmetric individuals in both sexes, and mating success were not significantly affected under all treatments. The effects of these treatments on trends such as utilisation of fat reserves and fertility of males and females are discussed.
In stressful environments, two main hypotheses have been proposed to explain the consequences of body size: (1) the absolute energy demand hypothesis (AED), which predicts that larger individuals are at a disadvantage under stressful conditions; (2) the relative efficiency hypothesis (RE), which predicts the reverse. We compared the effects of cold stress on different fitness traits of large and small individuals of the parasitoid wasp Aphidius ervi (Hymenoptera: Aphidiinae). For that, we exposed nymphs of this wasp to 5 treatment conditions as follows (control at 20°C; 7C1 and 7C2: constant cold temperature of 7°C for 1 and 2 weeks respectively; 4C1 and 4C2: constant cold temperature of 4°C for 1 and 2 weeks respectively).
After cold stress, only the large females that emerged in the 7C2 and 4C2 treatments displayed a reduction in the fitness traits studied (longevity, egg load at emergence, life‐time fecundity). The decrease in lipid content in large adults may have been responsible for their lower fitness. Our results thereby supported the AED hypothesis. Furthermore, the small females in these treatments produced more eggs at emergence than the control females. This highlights the fact that in stressful environments, small females switch their reproductive strategy from a synovigenic strategy (in which females mature new eggs after emergence) to a more pro‐ovigenic one (in which females emerge with more mature eggs).
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