Bumblebee Population Dynamics: Assessing Risks Associated with Resource Availability and Parasitism

Bumblebees (Bombus spp.) are a vital group of pollinators in wild and managed ecosystems, but we know relatively little about the ecological factors that regulate their populations. Although floral resource availability and natural enemy abundance are important components of habitat quality for bees, and have been implicated in bumblebee declines, the influence of these factors on bumblebee population size remains understudied. Through five studies that focus on bumblebee populations in northern Virginia (USA), this dissertation asks how floral resource availability and parasitism influence bumblebee populations, and how risks associated with these factors vary among species within a community. Chapters 1-3 reveal differences among bumblebee species in their sensitivity to changes in the flowering landscape and in their susceptibility to parasite infections. Chapters 1-2 show that two locally rare species have narrow diet breadths, tend to be absent from meadows with lower floral diversity, and are more frequently infected by the fungal parasite Nosema (sp.). This research points to changes in floral resources and parasitism as candidate factors involved in their declines. Chapter 3 demonstrates that one of three evaluated bumblebee species is resistant to a behavioral manipulation by an endoparasitic fly (Conopidae, Diptera). This study is the first to show that expression of this manipulation varies across bumblebee species. Chapters 4-5 evaluate how two seasonally variable factors, flower abundance and lethal conopid fly parasitism, affect bumblebee colony performance. Results show that steep reductions in flower abundance associated with midsummer drought conditions limit colony size and reproduction, and that larger colony size is more important when food is scarce. Through development of a simulation model, I demonstrate that the impact of conopids on colony reproduction is generally small, but increases when food is more limited during the colony cycle. These findings support current hypotheses that low food availability and heightened parasite pressure have resulted in the disproportionate representation of late-season bumblebees among declining species. Overall, this research reveals differences in the vulnerability of bumblebee species to changes in resource availability and natural enemies, and provides novel information on the influence that these factors, independently and in combination, have on bumblebee demographics.