Foraging preferences of bumble bee castes are weakly related to plant species cover on two arable agri-environment habitat types

Arable field margins provide important floral resources for insect foragers. This study assessed the significance of cultivated margins and floristically enhanced margins, both English agri-environment scheme (AES) options, to foraging bumble bees (Bombus species). We examined plant foraging preferences in each habitat according to species and caste. Additionally, detailed botanical surveys were carried out to determine vascular plant densities on the study margins. Overall, our results emphasised the importance of spontaneous (Asteraceae) species emerging from the seed bank in the provision of forage across Bombus species and castes, and highlighted that Bombus foraging preferences appeared to be only weakly related to floral species densities. Although found only occasionally in high densities, the popularity of these dicots was likely due to high nectar sugar mass. Bombus queens were recorded relatively infrequently, implying that these habitats are failing to provide the preferred floral resources of all Bombus spp. queens. Queens that were observed were found to favour earlier-flowering species (e.g. Anchusa arvensis) and species with longer corollas (e.g. Vicia sativa). Worker bees across Bombus spp. showed high overlap in plant preferences (e.g. Cirsium arvense, Ononis spinosa). However, some variability in preferences between castes within a species were noted, for example, only B. terrestris/lucorum drones were found to forage on Crepis vesicaria in cultivated margins. Additionally, bumble bee abundance was only found to increase as dicot cover increased. Overall, our findings highlight the importance of continuing to utilise multiple AES types in order to fully support Bombus and other pollinating insect populations on farmland.