Overwintering strategies in the red mason solitary bee-physiological correlates of midgut metabolic activity and turnover of nutrient reserves in females of Osmia bicornis

The catabolic activity of midgut and the turnover of metabolic reserves in fat body and hemolymph was studied during 7 months of overwintering period (September-March) in the European solitary red mason bee Osmia bicornis L. (Hymenoptera: Megachilidae). Assays of lipids, protein, and carbohydrates in midgut, fat body, and hemolymph as well as carbohydrates in crop, were conducted during overwintering period, including the 3 months of diapause (November-January). In general, in midgut, lipids were used mainly during pre-wintering period (September-October); whereas in fat body and hemolymph, the stores of lipids as well as carbohydrates were mobilized. In midgut and crop, carbohydrate mobilization occurred during both beginning and end of the overwintering period. The activities of midgut enzymes (proteases and amylases) changed substantially during the overwintering period. While the midgut protease activity was relatively low at the beginning of the overwintering period (September), its activity increased significantly in the final stages of overwintering, during post-diapause quiescence (February-March). On the other hand, amylase activity showed high activity during pre-wintering and post-diapause quiescence, but its activity was low during the deep diapause period (December-January). During winter months, the fat body and hemolymph lipids and free hemolymph carbohydrate concentration slowly decreased. On the other hand, glycogen concentration dramatically dropped down after November and was at a very low level till February and then increased with the rise of ambient temperature. This reflects the dynamic physiological strategies adopted by the red mason bee to successfully overwinter.