Abstract
Bumblebees (Bombus spp.) rely on an abundant and diverse selection of floral resources to meet their nutritional requirements. In farmed landscapes, mass‐flowering crops can provide an important forage resource for bumblebees, with increased visitation from bumblebees into mass‐flowering crops having an additional benefit to growers who require pollination services. This study explores the mutualistic relationship between Bombus terrestris L. (buff‐tailed bumblebee), a common species in European farmland, and the mass‐flowering crop courgette (Cucurbita pepo L.) to see how effective B. terrestris is at pollinating courgette and in return how courgette may affect B. terrestris colony dynamics. By combining empirical data on nectar and pollen availability with model simulations using the novel bumblebee model Bumble‐BEEHAVE, we were able to quantify and simulate for the first time, the importance of courgette as a mass‐flowering forage resource for bumblebees. Courgette provides vast quantities of nectar to ensure a high visitation rate, which combined with abundant pollen grains, enables B. terrestris to have a high pollination potential. While B. terrestris showed a strong fidelity to courgette flowers for nectar, courgette pollen was not found in any pollen loads from returning foragers. Nonetheless, model simulations showed that early season courgette (nectar) increased the number of hibernating queens, colonies, and adult workers in the modeled landscapes. Synthesis and applications. Courgette has the potential to improve bumblebee population dynamics; however, the lack of evidence of the bees collecting courgette pollen in this study suggests that bees can only benefit from this transient nectar source if alternative floral resources, particularly pollen, are also available to fulfill bees’ nutritional requirements in space and time. Therefore, providing additional forage resources could simultaneously improve pollination services and bumblebee populations.
Highlights
Loss of floral resources due to changes in land management is generally thought to be the primary driver of reported declines in pollinator populations Brown and Paxton (2009)
This study clearly demonstrates a mutualistic relationship between courgette flowers and B. terrestris that is beneficial to both, im‐ proving pollination success and colony dynamics (Bailes, Ollerton, Pattrick, & Glover, 2015; Holzschuh et al, 2016)
Results showed that over 24 hr pistillate flowers produce signifi‐ cantly more sugar than staminate flowers. This overall higher sugar content combined with nectaries which are harder to access than staminate flowers is thought to be why bee species show a prefer‐ ence for, and spend longer at pistillate flowers (Artz & Nault, 2011; after anthesis and by the end of the morning, stigmas had received an adequate number of pollen grains (4,749 ± 441) for optimum fruit set as ~1,200 are thought to be required for maximal fruit set in pumpkin (Vidal, Jong, Wien, Morse, & a., 2010)
Summary
Loss of floral resources due to changes in land management is generally thought to be the primary driver of reported declines in pollinator populations Brown and Paxton (2009). All bee species visiting courgette were recorded during pollinator surveys, B. terrestris was the focus of this study because of their natural abundance at study sites and availability as commercial colonies (Biobest Biological Systems, Belgium) which were required to quantify the proportion of courgette pollen in B. terrestris’ diet (Figure 1).
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