Water relations, fat reserves, survival, and longevity of a cold-exposed parasitic wasp Aphidius colemani (Hymenoptera: Aphidiinae)

Parasitoids exposed to low temperatures may suffer from extreme physiological conditions inducing direct or indirect chill injuries. Under long cold exposure (which also includes starvation), individuals face a great challenge in maintaining both water balance and energy reserves. Key parameters associated with individual fitness and physiological parameters related to cold-hardiness were analyzed. One-day-old mummies of the aphid parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiinae) were exposed to cold (2 and 4 degrees C) for various periods (1-3 wk) under a high relative humidity (75 +/- 5% RH) and darkness. High mortality and shortened adult longevity occurred with increasing duration of exposure at low temperatures. Individual parasitoid mass loss increased with cold storage duration and was associated to a marked decrease in dry mass caused by lipid reserve depletion. Water content (water mass/dry mass) slightly increased with cold exposure duration because of starvation. Similar patterns were observed for both temperatures tested. This study emphasizes (1) how energetic reserves may be critical to survive at low temperatures and (2) that the survival and longevity are related, at least in part, to the depletion of energy reserves during starvation.