Managing hedges for pollinators

Author: Steven Falk

It’s easy to be dismissive of insects, yet about one-third of all the food we consume has required a pollinator to put it there, and by pollinator, I don’t just mean honey bees. Nearly one-quarter of Britain’s 24,000 insect species visit flowers and wild bees, hoverflies and moths are especially important. Even the dungflies that sit on cowpats and the blowflies that develop in carrion pollinate flowers. In fact, some research has suggested that honey bees only do about one-third of Britain’s crop pollination.

Image (c) Steven Falk

Farmland provides a variety of broad habitats and more specialised microhabitats that support pollinators and help sustain pollinator abundance and diversity within the British countryside. Hedges and the many microhabitats that they support are especially important, so the way you manage them, or establish new ones, is crucial.

There are five broad ways:

  • As a source of blossoms and flowers for adult foraging
  • As a source of many larval habitats
  • As a windbreak that aids pollinator activity and movement
  • As a source of shade and humidity, especially during droughts and heatwaves
  • As a component of a larger, interacting, landscape-scale habitat mosaic

Hedge blossoms are crucially important in early and mid spring before other flowers have got going, and I’m always keen to promote the concept of a ‘good blossom sequence.’ A simple blossom sequence might just entail blackthorn (peaking mid April) and hawthorn (peaking mid May). But if further blossoming species can be added to a hedge network, this can provide a longer and more continuous source of pollen and nectar. This could include cherry plum, goat willow and common gorse (which peak before blackthorn), field maple and crab apple (which peak between blackthorn and hawthorn), and guelder rose, dogwood and elder (which peak after hawthorn). The choice can be shaped around location and soil type and can be arranged at a farm unit level – I’m not advocating all those species in one hedge! But bear in mind that an abundance of spring blossom will help ensure you see more bumblebees, hoverflies and butterflies in summer.

Image (c) Steven Falk

Blossoming hedge trees such as wild cherry, willows or outgrown field maples or crab apples can add to that blossom offer. Hedge trees of all sorts (including ash and oak) can also provide an important larval habitat for pollinators. The foliage can be a food source for herbivorous butterflies and moths. Heart rot and aerial rot holes are the breeding sites for various hoverflies, and any dead limbs or dead trunks in the sun can be a breeding site for a variety of solitary bees and wasps, including the red mason bee – a fabulous pollinator of fruit trees.

Further crucial hedge microhabitats for pollinators are hedge banks, hedge ditches and hedge margins. Hedge banks (which can be very ancient) will often support large nesting aggregations of mining bees. These can be very important pollinators of fruit trees and oilseed rape. Abandoned mouse and vole burrows in banks are important nesting sites for bumblebees. Water-filled hedge ditches can be a breeding site for a variety of hoverflies and also double up as very flowery features, often supporting an abundance of meadowsweet, great willowherb, yellow iris etc. Even where no ditches are present, the margins of hedges can provide a useful source of flowers such as brambles, cow parsley, hogweed, thistles, hedge woundwort and white dead-nettle. That becomes enhanced if you have a decent buffer strip between the hedge and any crop, or a fence that stops stock grazing right up to the hedge.

Image (c) Steven Falk

The final benefit of hedges, which is all-too-often overlooked is their value as windbreaks. Pollinators don’t like strong breezes. Hedges help create pockets of calmer, warmer air that helps pollinator movement and activity. On a cool, breezy spring day of perhaps 10 °C, a sheltered, sunny edge of a field with blackthorn blossom might be reaching 15 °C and supporting huge amounts for pollinator activity. Warm microclimates are also important for the development of herbivorous larvae such as caterpillars and the nesting activity of bees. Hedges play a crucial role in shaping microclimates and therefore pollinator activity.

There is so much – but if I had to recommend just three things they would be:

  1. Enhance your hedge blossom sequences – check what is currently there and consider what extra things could be added that enhance the blossom sequence, especially prior to the Blackthorn peak (given that warm weather increasingly starts in late winter).
  2. Cut your hedges on a 3-4 year rotation (i.e. one-third or one-quarter each year) because less frequently cut hedges produce more blossom, become structurally more diverse, and produce better microclimates (including valuable humid-shaded microclimates within them or on their shaded sides as well as the warm ones on their sunny sides).
  3. Allow flowery hedge margins to develop – encourage those lovely shows of Cow Parsley, Hogweed, Teasel etc. and embrace some limited Bramble, thistles and ragworts. Don’t cut these areas whilst they are still flowery, and don’t feel you need to sow an artificial pollen and nectar mix here if nature is already producing a nice range of flowers.

This is a summary of a very big subject. But I hope it is useful.

 

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.