Overwintering ecology of two social halictine bees,Lasioglossum duplex andL. problematicum

Overwintering of two social halictine bees, Lasioglossum duplex (Dalla Torre ) and L. problematicum (Blüthgen ), was studied. In L. duplex many females stay near the old nests, each female preparing a hibernaculum separated from the natal nest. In L. problematicum most females overwinter communally within the natal nest. This difference in overwintering habits relates to the social structure in the next spring. L. duplex is nearly always solitary in spring although later becoming eusocial, whereas many nests of L. problematicum are polygynous, beginning in early spring. L. duplex overwinters much deeper () in the soil than does L. problematicum (), but both species are safe from drops in soil temperature, which is above 0°C even in midwinter, and the bees' supercooling points are lower than −6°C. This cold resistance and the storage of sufficient food in the crop are reflected in the winter survival, which is much higher (79%) than the rate of successful nests in the spring active phase (25%).