Habitat loss is one of the important factors in the decline of native bumble bees. Habitat restorations such as planting wildflower and warm season grass meadows have been advocated as an approach to improving the resource landscape for bees. These meadows are expensive and labor-intensive undertakings, and their benefits relative to less expensive alternatives (e.g., allowing fields to go fallow with minimal management) have not been well documented. A drawback of fallow fields is they tend to be dominated by non-native plants. In this study we examined four warm season fields and five fallow fields in northern Virginia. We 1) documented the abundance of floral species in both fallow and restored warm season grass field, 2) documented diurnal variation in pollen resource availability, 3) characterized pollen loads from field-collected bumble bees, and 4) evaluated the effects of floral resource availability on the foraging efficiency.
Results/Conclusions
We found there was no significant difference of total floral density between the warm season and fallow fields, but native species were at significantly higher densities in warm season grass fields. Most species experienced significant pollen depletion between the morning and afternoon. The rate of pollen return and the proportion of bees with pollen returning to colonies did not differ significantly between warm season and fallow fields. Higher abundance of Penstemon digitalis and Securigera varia significantly increased the rate at which bees came into the hive with pollen and significantly increased the proportion of bees returning with pollen. Conversely, bees returned to the hive with pollen at a significantly slower rate with increasing abundance of Carduus. The rate at which pollen entered the colony had no effect on bee body size or total brood production. Our results ultimately revealed no consistent differences between warm season and fallow fields, but a small number of species play an outsized role as pollen resources for bumble bees.