Floral resource use of native bees in mature deciduous forests

The native bee fauna of the northeastern United States prior to widespread anthropogenic habitat conversion were likely forest-associated species that are active in the springtime. Yet research and conservation practices have primarily focused on bees associated with open meadow habitat in the summertime. Consequently, we know little about the floral resource requirements of bees that rely on deciduous forests. In particular, differences among forest-associated bee genera in their use of pollen resources, and the relative importance of canopy resources in mature forests with healthy spring ephemeral understories, are not well understood. We used floral visitation records for 3706 bee specimens belonging to 110 bee species, and analysis of the pollen loads of 677 bee specimens, to characterize bees’ use of floral resources at 13 deciduous forest sites spanning an ~550 km region of northern Indiana and Michigan.

Bees associated with 60 species of spring ephemerals, shrubs, and canopy trees. However, only eight spring ephemeral species accounted for 82% of bees’ visits to plants, and only ten plant taxa accounted for 79% of bees’ collection of pollen. Dominant bee genera (Andrena, Bombus and Lasioglossum) consistently differed in the plants that they visited the most and from which they collected the most pollen.Canopy trees and shrubs provided nearly one third of bees’ pollen collection, and were especially important resources for bees in the genera Andrena and Colletes, confirming the importance of canopy tree species and shrubs for bees in forest habitats, even in forests with healthy understory flowering plant communities.