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: Using unharvested cereal headlands combined with cultivated margins on heavy land

Author: Martin Lines
Farm: Papley Grove Farm, Cambridgeshire

Aims:

Using options that need spring establishment can be difficult on heavy land. We wanted to find a way of providing conservation management without needing a great deal of work in the spring and without causing undue problems to the crop following it.

I grow winter wheat, winter beans and oil seed rape. Because of my heavy soils we thought this combination would be ideal for using ground for two full cropping years, allowing me to follow it with another winter established crop and avoiding the need for trying to get a crop going in the spring. I worked with Niki Williamson of the RSPB, who gives free nature-friendly farming advice in the Fens area, to design the combination.

Management:

I plant a winter wheat crop as normal, leaving a headland as a conservation headland. This receives limited fertiliser, no insecticides after 15 March, and I apply limited fungicide (only when absolutely necessary to stop crop failure). This provides insect-rich habitat all summer, helping birds like grey partridges and corn buntings. The headland is left unharvested the following autumn, so provides winter seed food as well.

From 21 March, I cultivate the headland by ploughing or discing to form a cultivated margin. Problems with working the heavy soil at this time are not so important as I am trying to create a reasonably fine fallow, but not required to create a seedbed to establish a mix or a crop.

If there is blackgrass residue once I’ve cultivated I use a herbicide to control it. That needs doing as soon as cultivation is completed, to give as long a window as possible to allow the intended weeds to grow on the plot, forming cover and food sources for wildlife.

On my land there is no rare arable flora! But the plants that do grow will flower and set seed in time for turtle doves arriving back on the farm in late April, providing a much needed food source. It also provides a fallow, weedy area for other insects and birds to use during the summer months.

From 31 July I spray off the area to clean it up and cultivate it in time to follow it with autumn sowing.

In any one year there are two areas like this on the farm, one in each option. They are rotated around the farm to keep the weed burden in check.

Achievements:

This year we followed the cultivated margin with oil seed rape and I found that this was the best bit of rape in the field. It’s cleaner because of the seedbed preparation and it’s established better because there are less slugs where there are no stubbles.

On my soils blackgrass is an issue, and would normally make things like conservation headlands and summer fallows a no-no. However this management has not increased my weed burden any worse than the rest of the farmed field. There are two opportunities to cultivate the option and bury the blackgrass, one in March and one in July. There is also nearly 2 months across August and September that can be used to germinate weeds and spray them off to create stale seed beds before drilling, giving you plenty of opportunities to address blackgrass problems.

The seed from the unharvested headlands was being depleted by pigeons and crows really early, so I had to put out deterrents to make sure it lasted further into the winter for the smaller birds we were trying to target. In the end there were still big flocks of birds in it right up until March, finding something to eat. I also have other wild bird seed mixes on the farm, which are designed to retain seed through the winter months to make sure there’s a winter-long food source.

We noted that in shaded areas, wheat tended to stay in the ear and germinate there, making it inaccessible to birds. Although it’s clearly beneficial to site it near a hedge for birds to hop in and out, I think you need to avoid the north side of tall hedges or woodland s to avoid loss of seed in this way.

The plots are rotated around the farm, so this year it will be sited in a different place. The unharvested headland will be near woodland rather than hedge. Having observed it this winter, I now wonder if it will be as popular without the hedge for the birds to use as a base to feed from. Otherwise I am very happy with the option as it stands. It fits in well with our system and delivers what we wanted for wildlife.

I found the options very easy to manage. It needs an extra operation in March to create the cultivated margin but nothing onerous.

I do what it takes to make the margin work in terms of balancing wildlife benefits with weed control. Some years it takes more, sometimes less. Although I haven’t done the detailed costings, I’m confident it balances out overall and does not leave me out of pocket.

The hedge alongside the option was teeming with yellowhammers, corn buntings, reed buntings and sparrows for as long as there was seed left in the headland. There were still birds using it when I came to cultivate it in March. There are breeding turtle doves on the farm, which I hope would benefit from the early seed source the cultivated margin provides. I saw loads of starlings and blackbirds rummaging around in spring and summer, which must show how many insects there are in there.

You can also see areas where partridge have used the fallow as a sunbathing/dusting area.

It adds another feature to the farm that wouldn’t usually be there. It’s not common to find fallow areas on heavy land, so this adds to the diversity of our system.

It’s a simple straightforward combination option that has worked well for me in my farming system.

Case Study: Cultivated margins

 

Author: Nicholas Watts
Farm: Vine House Farm, Lincolnshire

Aims:

The aim was to create an insect-rich foraging habitat for farmland birds. I farm on fertile peat soils, so effective control of pernicious weeds is essential.

Management:

I started using this option 5 years ago. For the first four years, I cultivated annually in the spring. Blackgrass is hit by spring cultivations and the seed is short-lived, so successive spring cultivations along with lack of fertiliser have virtually eliminated it.

This year, I applied a herbicide in March to control grass weeds instead of cultivating. This has the advantages of providing seed food for birds through the ‘hungry gap’ (January – April inclusive), providing pollen and nectar throughout the spring, and I think that the plant diversity has increased as a result. It has taken me 4 or 5 years to get the margins to produce food in the lean time of year, because of the need for annual spring cultivations to get on top of the blackgrass. If someone has no blackgrass then an early spring food source can be produced in year two or three. In future, I may only cultivate in alternate years and use herbicide in the 2nd year.

Creeping thistle is a major concern with potatoes and sugar beet in the rotation, so I spot spray thistles with a knapsack sprayer. This has reduced the thistle numbers over time. I had to spot-spray some areas to remove couch and field bindweed with a boom sprayer at harvest time, when the annuals were dormant.

Achievements:

The floristic diversity of these margins is unparalleled by any other habitat on the farm. Plant species richness per margin varies from 55 to 70 species, almost an order of magnitude greater than my wild flower margins, and even more impressive compared with my grass margins, ditch banks and hedgerows.

In years when the spring cultivation is not required, it delivers pollen and nectar throughout the year as well as any sown mixture. Red dead-nettle feeds bumblebees in March and a progression of a wide range of flowers take over from then onwards. When spring cultivation is undertaken, these margins provide pollen and nectar from June onwards.

The full range of farmland birds forage for insects in these margins: grey partridges, linnets, reed buntings, corn buntings and even quail. Turtle doves are scarce on the farm now: common sense suggests that they should use them as a source of seed food too, but I have not been able to witness this. I now have ten times the density of linnets feeding on this farm compared with neighbouring farms. I put this down to the availability of seeds in these margins. I think that the accessibility to the ground in combination with the abundance of insects (and seeds in the case of linnets) provides the ideal conditions for foraging birds.

The combination of spring cultivations, use of selective herbicide and spot-spraying have effectively controlled all of the noxious weed problems. Uncropped cultivated margins are more work than grass margins, but less than some of the other arable options, such as wild bird seed mixtures.

http://www.vinehousefarm.co.uk