Maintaining diverse bee communities on farmland: the importance of floristic diversity

In this post Thomas Wood discusses his recent paper ‘Providing foraging resources for solitary bees on farmland: current schemes for pollinators benefit a limited suite of species

Farmland biodiversity was negatively affected across most European nations throughout the 20th century, predominantly due to a period of rapid agricultural intensification following the Second World War. Flowery hay meadows were ploughed up and herbicides, fertilisers and other farming techniques reduced the abundance and diversity of arable plants. In common with many other insect groups, farmland bees have been badly affected by this reduction in floral diversity. Bees feed their young on pollen and nectar and so the loss of flowery habitat on farms has reduced the abundance and range of foraging resources available to them.

Since the 1990s, numerous agri-environment schemes have been designed and implemented to halt and reverse the declines. Farmers receive money from the government to reduce the intensity of their farming and hopefully benefit farmland species. In the United Kingdom, pollinator-focused schemes have been developed to help farmland bees by creating new patches of flower-rich habitat. These are usually grassy strips that run alongside arable fields and typically contain flowers such as Ox-eye Daisy (Leucanthemum vulgare), Wild Carrot (Daucus carota), Bird’s-foot Trefoil (Lotus corniculatus) and many more (Figure 1).

Example florally enhanced margin
Figure 1. Typical floristically enhanced grass margin alongside a field of barley in early July.

Previous work has shown that bumblebees (Bombus spp.) respond positively to the creation of this new foraging habitat, but much less is known about how other bees use these schemes. Bumblebees only represent 25 of the roughly 250 bee species found in the UK, so understanding how the other bees response is important if we want to conserve a diverse bee community on farmland. This project set out to compare solitary bee pollen use on farms with and without patches of sown flowers in order to quantify the relative contribution that these schemes made towards their overall pollen diet.

Whilst these schemes significantly increased floral abundance and represented the majority of observed flowers, they were used relatively infrequently by solitary bees. Only 27.0% of observed solitary bee pollen foraging visits were to sown flowers, and a microscopic analysis of pollen removed from solitary bees showed that only 23.3% of this pollen was from sown plants. Moreover, even though solitary bee species richness was fairly high with 72 species seen, only 25 of these (34.7%) used sown plants for pollen to a meaningful extent.

Why did so few solitary bees use the sown flowers? Because of concerns over declining bumblebees, many of which collect a lot of pollen from members of the pea family Fabaceae, pollinator-focused schemes contain a large Fabaceae component. Whilst there are several solitary bees that are entirely dependent on Fabaceae, most are not and only around 3% of observed pollen foraging visits in this study were to Fabaceae. Instead, most solitary bees are generalists and prefer simple, open flowers like hogweed (Heracleum sphondylium) and other umbellifers from the family Apiaceae, weedy Brassicaceae like charlock (Sisymbrium officinale) and members of the daisy family Asteraceae, like dandelions (Taraxacum officinale agg., Figure 2) and cat’s-ear (Hypochaeris radicata, Figure 3). None of these flowers are included in current agri-environment schemes.

Whilst providing lots of Fabaceae for threatened bumblebees is important, the rest of the bee community needs to be remembered as well because they make up the bulk of bee diversity, both in Britain and across the wider world. Agri-environment schemes should promote a wider variety of flowering plants in order to more effectively conserve diverse bee communities on farmland.

Andrena nigroaenea on Taraxacum officinale
Figure 2. The solitary mining bee Andrena nigroaenea grooming on a dandelion flower Taraxacum officinale agg. This bee is a pollen generalist but like many other spring flying bees it uses dandelions as they are one of the most abundant spring flowers.
Osmia spinulosa on Leucanthemum vulgare
Figure 3. The solitary bee Osmia spinulosa resting in the flower of cat’s-ear Hypochaeris radicata. This bee is a pollen specialist and only collects pollen from the daisy family Asteraceae, particularly from yellow ‘hawkish’ Asteraceae such as cat’s-ear.

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