New research: What limits bumblebee populations on farmland?

Authors: Dr Tom Timberlake and Prof Jane Memmott 

A new study by Tom Timberlake and colleagues at the University of Bristol shows how important late summer flowers and rural gardens can be for supporting bumblebees on UK farmland.

Does this offer an opportunity to devise more targeted agri-environment schemes for pollinators?

Pollen and nectar are the main source of food for bees, including the charismatic and agriculturally important bumblebee. It is no surprise then that declining flower densities – particularly on farmland – are considered one of the most important drivers of pollinator decline. Agri-environment schemes which incentivise the planting of wildflower strips and the expansion of semi-natural flowering habitats such as hedgerows and field margins are an important tool for reversing pollinator declines, but are they really the most efficient way of supporting pollinators?

Image 1: For bumblebees like this one (Bombus terrestris), it’s not just about how much food is available, it’s also when that food is available through the year. A dandelion flowering in early spring for example, would be more valuable to a bumblebee than an equivalent flower in mid-summer.
Image: T. Timberlake

Whilst agri-environment schemes have successfully increased the overall numbers of flowers on farmland, they tend to overlook the timing of when these flowers are available to pollinators. Different plants flower at different times and most of the plants in agri-environment schemes flower in late spring and early summer which often isn’t the period of greatest need for pollinators. Pollinators need a continuous supply of food throughout their flight season, and for species with long flight seasons such as bumblebees, this means from late February, right through until October. ‘Hunger gaps’ of even one week could limit the number of pollinators surviving through the year.

Hunger gaps

To support pollinators in the most effective and cost-efficient way, it makes sense to find out when these hunger gaps occur and then devise targeted management or planting schemes to plug these gaps. A previous study by our team did just this and showed that nectar supplies on farmland were most limited in early spring (March) and late summer.

To check what effect these ‘hunger gaps’ were having on bumblebee populations, we carried out a study on 12 farms around the west of England. We captured, recorded and released hundreds of bumblebees and then measured all sorts of features of the farms to find out which aspects of the farm were most important in determining bumblebee density.

To our surprise, the supply of nectar in late summer (September) was by far the most important factor driving bumblebee density on these farms – more so even than the amount of natural habitat. Late summer is a very important stage in the lifecycle of bumblebees – it is when new queens are produced and must pile on the pounds before their winter hibernation. A rich supply of flowers is therefore crucial, but with fewer and fewer hay meadows, cover crops and weedy areas to provide this late summer nectar on farmland, bumblebees are struggling.

Image 2: Low nectar supplies in late summer coincide with an important stage in the colony lifecycle, limiting colony density the following year. How might we change the shape of this curve to reduce the September bottleneck?

How to plug the gap

So what can we do to plug this late-summer hunger gap on farmland and support bumblebees? On our farms at least, ivy was the most important plant for providing nectar during this sparse time, so managing your hedgerows and woodland edges to promote this amazing plant is a good first step. Leaving some rough weedy corners for late flowering species like thistles, knapweed, scabious and dandelions can also contribute to plugging the gap.

We found that Environmental Stewardship Scheme pollen and nectar mixes were really effective at increasing overall nectar supplies but were far more effective when mown early, or in multiple phases, to extend flowering into the late summer.

Finally, if you want to give those hungry queen bumblebees a real treat in the run-up to autumn, a tasty cover crop of late-flowering red clover would do just the trick. A single hectare of this crop could provide around 1 kilogram of raw nectar sugar each day and completely close the late summer gap.

There was one final surprise in our results… Small patches of garden were having a significant influence on the density of bumblebees. Farms with more gardens had more bumblebee colonies.

Gardens have a far denser and more diverse supply of flowers than farmland and are often managed to keep things flowering throughout the whole year. These little floral oases seem to be throwing bumblebees a lifeline during periods when farmland offers very little.

Image 3: Species which flower in September such as dandelions, red clover, ivy and thistles (clockwise from top left) are likely to be disproportionately important to bumblebees and other pollinators. Including these and other late-flowering species in conservation schemes will help fill the late summer hunger gap.
Images: T. Timberlake & Wikimedia Commons

How much and when

The take home message is that it’s not just about how much food we provide for pollinators, it’s also about when that food is available through the year.

Providing more flowers in the early spring and late summer when bumblebees are at their hungriest is a great way to support these important creatures.

Image 4: Some examples of low quality (left) and high quality (right) farmland habitats for pollinators. The more diverse and flower-rich high quality habitats tend to provide a more consistent supply of nectar through the year, helping to fill those important hunger gaps.
Image: Nick Tew

This article is the summary of a University of Bristol-led research paper published by Thomas Timberlake, Jane Memmott, Ian Vaughan and Mathilde Baude in the Journal of Applied Ecology. The work was funded by the Natural Environment Research Council (NERC) with additional support from the NERC Biomolecular Analysis Facility.

Case Study: Pasture for Pollinators

Authors: Owain Rowlands ( Menter a Busnes ) & Anna Hobbs ( Bumblebee Conservation Trust )

 

Welsh dairy farmers and bumblebees don’t normally crop up in the same context but a group of organic dairy farmers in Wales, who market their products under the Calon Wen brand, are hoping to change things.

Six of the Calon Wen farmers have been growing multi-species Herbal leys, which include flowering herbs, legumes and grasses, for a three-year European Innovation Partnership (EIP) Wales project.

The project is aiming to show that some simple changes to grassland management can go hand in hand with boosting pollen and nectar resources for bumblebees and other pollinators.

Herbal ley plants in flower. © Anna Hobbs

In 2018, each of the farmers sowed a Herbal ley from Cotswold Seeds which includes bird’s-foot trefoil, clovers (red, white, sweet and alsike), sainfoin, and other species which can help to enhance pollinator populations.

During the silage season, the farmers have been leaving a 100m by 4m uncut (or ungrazed) strip along a field margin.  During first or second silage cuts, they are leaving one headland uncut; then at the next silage cut the previously uncut area is mowed and a different area of headland is left uncut. Late grazing of permanent grassland has also been explored by some of the farmers.

These grass management techniques aim to help provide bees and other pollinators with the continuous supply of flowers they require to forage on. By alternating headlands, they can also keep the whole field in good agricultural condition.

Uncut margin in herbal ley silage field © Anna Hobbs

Monitoring by Bumblebee Conservation Trust staff in 2018 and 2019 has shown higher numbers of bumblebees and other pollinators, and also greater numbers of pollinator species, in the uncut/ ungrazed margins compared to cut/ grazed field margins.

With one year of monitoring to go, the results indicate that this is a practical management technique for livestock farmers to help support bumblebee and other pollinator populations, especially in terms of ‘bridging gaps’ in nectar and pollen forage resources throughout the season.

In one survey in 2019 at Cop House Farm, Chester where David Edge milks 300 proCROSS cows producing an average of 7500 litres per cow, pollinator numbers rose from 12 in the cut part of the field to 189 in the uncut margin.

Common carder bee on clover. © Anna Hobbs

Anna Hobbs, the Bumblebee Conservation Trust Officer responsible for surveying the farms remarked “the results that we have collected so far are really encouraging.”

David Edge, Calon Wen farmers says “We’ve run this project as farmers which I hope will show others how bumblebee conservation can be practical and beneficial to the farm. I believe that more often than not, wildlife conservation can go hand in hand with productive farming.”

Pollination is one of the most critical services that nature provides, underpinning food production and biodiversity. Around a third of the global food crop and three-quarters of British wild plants are dependent on pollination. Compared to horticultural and arable systems, much less attention has been given to how grass-based livestock farming, such as dairying, can benefit pollinator populations.

Watch the Video. for a visual summary of the project.

Case study: Herb-rich leys

Author: Ian Boyd
Farm: Whittington Lodge Farm, Gloucestershire

Aims:

Whittington Lodge Farm has predominately thin Cotswold Brash soils on 280ha, mostly over 800 feet in altitude. The cultivated half of the farm was in a continuous arable system, but average yields meant it was only marginally viable and blackgrass was creeping in.

In 2015, it was decided to undergo organic conversion on all the arable area to combine with the fully organic grassland. Herb-rich leys were introduced into the rotation to build fertility for the organic cereals. They are grazed by an existing and expanded suckler herd of native Hereford cows and calves.

Cattle grazing a herb-rich ley at Whittington Lodge Farm. Image credit: Ian Boyd

Management:

The herb-rich leys contain at least 5 species of grass, 5 legumes and 5 herbs which all add to the biodiversity with varying rooting depth in the shallow soils. They are established by under-sowing in spring barley crops, broadcast with an Opico seeder/harrow once the barley plants have reached the 3 leaf stage. This way the undersown plants are still small enough not to interfere with the barley combining. Grazing can begin in the autumn.

The herb-rich ley can last for 4 years and can be grazed or conserved as hay or silage. It can fit in well with other grasslands like permanent pasture with the management of grazing livestock.

Mob grazing can help achieve the best from the leys. The cattle are moved frequently, usually daily, onto fresh grazing and a back fence stops them eating any re-growth after about 3 days. The adage is that you should not see the cattle’s knees when they go in and you should not see their ankles when they come out. Some of the herbage is eaten, some trampled and some left so that the root structure underneath the plants is maintained and re-growth is rapid.

Then, most importantly, the ley is rested for a couple of months to allow plants to flower and set seed before the next grazing. This will provide habitat for invertebrates, including crop pollinators, and improve soil structure and water infiltration.

Moving the electric fence is a daily commitment but it makes it easier to check and count the stock as they run past.

The cost of seed is significant but no nitrogen is required, and it is possible to incorporate this option into a Countryside Stewardship scheme and claim payments for it.

As the soil organic matter level is improved, more grass will be grown and stocking rates will be able to be increased. This system suits both cattle and sheep on organic and non-organic farms.

Achievements:

There are multiple benefits to herb-rich leys. When terminated, they provide an ideal entry to cereal crops for several years, organic or not. The arable weed spectrum is dramatically reduced, especially blackgrass.
Soil health and organic matters levels will have been increased with large amounts of carbon sequestered into the soil, far more than the effects of methane produced by the cattle.

The nutrient density of the beef will be greater when multiple species and biodiversity are eaten with knock-on health benefits for the cattle and consumer.

The farmland wildlife is a big winner here, with many different plants flowering producing nectar for pollinating insects and producing seed (and insects) for many bird species to eat. Skylarks nest everywhere, hares and roe deer love to graze it. Kestrels and barn owls hunt for short-tailed field voles living in the long grass and snipe are increasingly being seen looking for worms in the winter time.

It really helps to be organised and to have planned the rotational grazing before the start of the season. Access to water, ease of moving electric fences and having the cattle in the right place at the right date all have to be factored in.

This system really suits British native breeds of cattle, which can readily be managed to Pasture-for-Life standards (essentially no grain fed). Pasture-for-Life beef allows the opportunity to brand and market it yourself with a great story for a higher price than the beef commodity market can pay.

www.cotswoldbeef.com