International audience 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 effi ciency hypothesis (RE), which predicts the reverse. We compared the eff ects of cold stress on diff erent fi tness 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. Th is 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).