What are the possible benefits of small size for energy‐constrained ectotherms in cold stress conditions?

Abstract

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).

Comparing thermal tolerance across contrasting landscapes: first steps towards understanding how landscape management could modify ectotherm thermal tolerance

“…For instance, Ismail et al . () showed that smaller individuals of the parasitoid A. ervi had better resistance to cold stress than larger wasps. Since parasitoid size is intimately linked to fitness‐related traits (e.g.…”

“…Our results are, however, consistent with the AED hypothesis which states that a winter should impose a selective pressure that favours smaller parasitoids. For instance, Ismail et al (2012) showed that smaller individuals of the parasitoid A. ervi had better resistance to cold stress than larger wasps. Since parasitoid size is intimately linked to fitness-related traits (e.g.…”

“…Many studies have been devoted to these species (e.g. Thies et al, 2005;Le Lann et al, 2008 including investigations of their thermal tolerance (Giri et al, 1982;Legrand et al, 2004;Le Lann et al, 2011a,b;Ismail et al, 2012). Aphid parasitoids are interesting biological models for landscape microclimate studies because they closely track the distribution of their hosts, especially during the larval stage when they are immobilised inside an aphid mummy and have to withstand the climatic conditions imposed upon them.…”

Body size affects host defensive behavior and progeny fitness in a parasitoid wasp, Lysiphlebus fabarum

“…Surprisingly, daughters of small mothers emerged with more mature eggs in their ovaries than did daughters of large mothers (Table 4). Working with Aphidius ervi Haliday, Ismail et al (2012) showed that, under stressful conditions (exposure to cold treatments), small females had more eggs at emergence relative to larger ones and inferred this to represent a shift toward a more pro-ovigenic strategy under stress. Thus, large body size may entail an initial fecundity disadvantage in L. fabarum, in addition to the host handling disadvantages detailed above.…”

“…Notably, maternal egg size was positively correlated with progeny egg load among small females ( Figure 2B), suggesting that the negative effect on egg load may only occur above some threshold egg size. Working with Aphidius ervi Haliday, Ismail et al (2012) showed that, under stressful conditions (exposure to cold treatments), small females had more eggs at emergence relative to larger ones and inferred this to represent a shift toward a more pro-ovigenic strategy under stress. Because of the low L. fabarum survival in fourth instar hosts, our results did not permit analysis of effects of host instar on progeny egg load.…”