Diapausing Cavity-Nesting Bees (Osmia, Megachile) Resist Winter Desiccation Stress

All solitary and primitively eusocial bees of the temperate zones survive winter in a dormant diapausing state. Weight loss of <i>Osmia</i> and <i>Megachile</i> bees has been solely attributed to metabolism of stored triglycerides, as evidenced by loss of fat body. I hypothesized that uptake or loss of water vapor may represent another contributor to wintering weight change. Using a range of atmospheric humidities (0-88%), it is shown that wintering cocooned <i>O. californica</i> (adults) and <i>M. rotundata</i> (prepupae) gained weight in humid atmospheres and lost weight in drier air, the inflection point being between 35 and 47% humidity. Most of this weight change occurred in early winter. No humidity treatment enhanced mortality. Winter storage humidity is an available management variable for these bees when kept for commercial pollination.