Providing the ideal habitat for bumblebees

By John Holland

Bumblebees are declining across Europe and some species have become extinct in recent decades, whilst the range occupied by others is declining.

Agri-environment schemes have offered the opportunity to create the flower-rich habitats that they desperately need on farmland. There is plenty of evidence to show that areas planted with good bumblebee plants are utilised for the gathering of nectar and pollen, but until now there has been little evidence that this helps increase the size of bumblebee populations on farms providing such resources.

One way to measure a population increase is to measure nest density, and this can be achieved using a molecular technique because all the workers in a nest share the same genes and are distinct from other nests. A recent study, supported by the Trust, compared total bumblebee abundance and the nest density of four common species on arable farms with and without flower-rich habitats. The abundance of bumblebees along transect walks was higher on farms with flower-rich habitats compared to without.

The density of bumblebee nests was almost twice as high on farms with flower-rich habitats and three times greater for Bombus hortorum, also known as the garden bumblebee.

The provision of flower-rich habitats did not, however, increase the overall diversity of wild bees, of which 104 species were found in total, almost half those that occur in southern England, highlighting the importance of farmland for wild bees. However, it wasn’t all good news, as the rare species were seldom found. Looking at which flowering plants were visited revealed that bees visited 124 plant species. Just over half of bumblebee nectar visits were to black knapweed, with twenty percent of pollen visits to bird’s-foot trefoil.

The other popular forage plants for bumblebees were clovers, spear thistle, hedge woundwort, lesser burdock, white dead-nettle and wild teasel. For other wild bees, such as the solitary bees, black knapweed was also the most popular, but flowers with more open structures were also used such as hogweed, catsear, ox-eye daisy, scentless mayweed, smooth hawksbeard, red bartsia and fleabane.

Many of these species are not included in commercial wildflower mixes, which shows the importance of protecting the hedge and woodland edge, where they naturally occur, from fertiliser and herbicide drift, which damages the plant community.

To benefit the rarer bumblebees and other wild bees that typically only forage over short distances (less than 200m) it is important to provide flower-rich habitats evenly across the farm and, for mining bees, patches of bare ground for nesting.

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Increasing food and bees - the results

Since 2009 we have been working in partnership with Conservation Grade to develop multi-functional wildlife crops and produce them in a more integrated way, reducing the need for agrochemicals.

We tested a wildlife crop rotation of a grass and legume mix across different soil types, aimed to support bees, butterflies and insect chick-food; generate soil nitrogen; and inhibit noxious weeds.

This was followed by two years of a wild bird seed mix that would provide seed and insect food for birds and cover in winter. The grass legume mix was made up of two grasses (meadow fescue and timothy), four clovers (red, alsike, crimson and white), bird’s-foot trefoil, sainfoin, early English vetch, yellow trefoil and lucerne.

Main results

• On the clay soils, little extra soil nitrogen was detected after the two-year legume ley, however on the sandy soils there was on average an extra 65kg of nitrogen per hectare, with a maximum of 188kg of nitrogen per hectare. In one year heavy rainfall destroyed the ley, so in future years we will sow legumes within the wild bird seed mixture, as many legumes leak nitrogen while they grow.
• The diversity and abundance of wild bees was higher in the legume ley compared with nearby wild flower habitats. There was much less variation between plots, so legume leys provided a more reliable source of flowers.
• Long-tongued bees and solitary bees preferred the legume ley compared with wild flower habitats.
• Over 90% of wild bee visits were to just 10 plant species, of which six were in the legume ley and four common wild flowers.
• Hoverflies, whose larvae are voracious aphid predators, preferred to forage on smaller flowers such as black medic and other flowers found in the wild flower habitats.
• The legume ley supported over four times more grey partridge chick-food insects and almost twice as much general farmland bird food and pest natural enemies compared with wildflower or grass-only habitats.

Pollinators in Westminster

We presented this work at the Game & Wildlife Conservation All-Party Parliamentary Group (APPG) meeting in Westminster this spring.

Parliamentarians heard that we need to up our game in terms of creating the right farmland environment to protect vital bees and other important pollinating insects, while at the same time increasing food production.

A vibrant discussion followed with a number of questions from MPs including; Caroline Spellman MP, Angela Smith MP, Richard Benyon MP and Baroness Byford.

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