Constrained patterns of pollen use in Nearctic Andrena (Hymenoptera: Andrenidae) compared with their Palaearctic counterparts

Andrena is a large genus of bees primarily distributed across the Holarctic. Despite their abundance in temperate regions, the pollen diets of many Nearctic Andrena remain incompletely resolved. The pollen diets of 50 species of Andrena found in Michigan were characterized using light microscopy. Twenty-four species (48%) were classified as pollen specialists, collecting pollen from one botanical family. The remaining 26 species (52%) were classified as pollen generalists, collecting from many botanical families. Andrena species fell into three broad foraging groups: (1) spring-flying species foraging predominantly from woody plants; (2) spring-flying species specializing on herbaceous ephemerals; and (3) summer-flying species specializing on herbaceous prairie plant species. Species of Nearctic Andrena specialized on pollens from Asteraceae, Geraniaceae, Hydrophyllaceae and Montiaceae or avoided them almost entirely. Botanical families that hosted specialized bees showed a higher variation in utilization by the Andrena community than botanical families without specialists. In contrast, Palaearctic species showed little temporal partitioning and low variation in the utilization of different botanical families. This pattern of pollen use supports previous findings that pollens from certain botanical families cannot be used as a food source without physiological adaptations, but that this phenomenon is more pronounced in the Nearctic Andrena fauna