Flower preferences and pollen transport networks for cavity‐nesting solitary bees: Implications for the design of agri‐environment schemes

Abstract

Floral foraging resources are valuable for pollinator conservation on farmland, and their provision is encouraged by agri‐environment schemes in many countries. Across Europe, wildflower seed mixtures are widely sown on farmland to encourage pollinators, but the extent to which key pollinator groups such as solitary bees exploit and benefit from these resources is unclear. We used high‐throughput sequencing of 164 pollen samples extracted from the brood cells of six common cavity‐nesting solitary bee species (Osmia bicornis, Osmia caerulescens, Megachile versicolor, Megachile ligniseca, Megachile centuncularis and Hylaeus confusus) which are widely distributed across the UK and Europe. We documented their pollen use across 19 farms in southern England, UK, revealing their forage plants and examining the structure of their pollen transport networks. Of the 32 plant species included currently in sown wildflower mixes, 15 were recorded as present within close foraging range of the bees on the study farms, but only Ranunculus acris L. was identified within the pollen samples. Rosa canina L. was the most commonly found of the 23 plant species identified in the pollen samples, suggesting that, in addition to providing a nesting resource for Megachile leafcutter bees, it may be an important forage plant for these species. Higher levels of connectance and nestedness were characteristic of pollen transport networks on farms with abundant floral resources, which may increase resilience to species loss. Our data suggest that plant species promoted currently by agri‐environment schemes are not optimal for solitary bee foraging. If a diverse community of pollinators is to be supported on UK and European farmland, additional species such as R. canina should be encouraged to meet the foraging requirements of solitary bees.

Does a strong reduction of colony workforce affect the foraging strategy of a social pollinator?

“…In the period after removing the workers ("after" phase, 20 th -23 rd July), pollen pellets of 18 workers for each colony were collected in the same way as the "before" period (17 workers for one of the nests). The number of samples collected was similar to other studies on DNA metabarcoding of pollen 44,45…”

Mitigating the Effects of Habitat Loss on Solitary Bees in Agricultural Ecosystems

“…In Europe, several farms have adopted floral provisioning schemes and planted wildflowers alongside their fields. Such schemes can provide a greater amount and variety of floral resources for the bees, though there are several considerations to keep in mind when selecting flowers, including seed costs, pollinator floral preferences, regional growing conditions, and bloom period [97][98][99]. The cost of buying wildflower seeds is one of the major factors determining whether a floral provisioning scheme will be implemented at all.…”

The best wildflowers for wild bees

“…Seventy three percent of foraging visits by solitary bees were to Asteraceae and Apiaceae found naturally on farmland and which were not part of sown mixes (Wood et al 2015a). Additional research shows no visits were made by cavity-nesting bees in the UK to flowers sown as part of an AES seed mix (Gresty et al 2018). This demonstrates that a large proportion of UK bee species are not benefitting directly from 'pollinator' seed mixes, and that there is scope to develop new mixes that cater for the needs of a greater diversity of wild bee species.…”