Bees are entirely dependent on the resources that flowers provide in order to complete their life cycles. Nectar is the primary carbohydrate source and is used by most bees throughout their life cycles, but pollen provides almost all of the protein in a bee's diet and is especially critical for larval development. The availability of these resources likely affects survival, reproduction, and overall colony success in social bees. Our study addressed the following questions: 1) What is the availability of primary pollen resources and how do they vary diurnally? and 2) Does pollen collection by wild bumble bees (Bombus spp.) reflect diurnal changes in pollen availability? We focused on four species of wildflowers in a 20 ha early successional field in the Shenandoah Valley of Virginia. Previous work at this study site had demonstrated that these four species accounted for over 90% of all of the pollen collected by bumble bees. We collected pollen from flowers in both the morning and afternoon in June 2017. We simultaneously collected pollen loads from 199 bumble bees that were foraging at the site in order to determine the relative abundance of pollen species within the load.
Results/Conclusions
Bombus griseocolis (54%) and B. impatiens (44%) were the most abundant of the five species of pollen-foraging bumble bees that we collected, and we restricted our analyses to them. We collected 90% of pollen-foraging bees from the most abundant host at the study site, Carduus acanthoides, the only focal species that produced nectar. Securigera varia (46%), C. acanthoides (34%), Solanum carolinense (17%), and Verbascum thapsus (<1%) accounted for 97% of all the pollen collected by bumble bees. Overall, pollen availability was lowest for C. acanthoides and greatest for S. carolinense and S. varia. Both C. acanthoides and V. thapsus were almost devoid of pollen in the afternoon, but both S. varia and the buzz-pollinated S. carolinense retained high amounts of pollen throughout the day. Morning bumble bee pollen loads were dominated by C. acanthoides, but afternoon loads for B. griseocolis shifted to more readily available pollen from S. varia. Loads from B. impatiens shifted to S. varia and S. carolinense in the afternoon. Our study provides evidence of substantial diurnal pollen resource depletion and that in the afternoon, bumble bees become more dependent on pollen resources provided by flowers that are less accessible to competing pollinators.