Which field margins are good for wild bees?

Summary

  • Modern agricultural methods have led to simpler landscapes, less diversity, and biodiversity loss.
  • Pesticides remove both insects themselves, as well as weeds. These weeds supported other insect species, which are then lost as well. Fewer hedgerows, woodlands and meadows mean more open, uniform areas of cropping.
  • This study looked at two kinds of field margin, which are designed to support pollinators to see which plants were most beneficial for wild bees.
  • More solitary bees were found in cultivated margins, which are cultivated and then left unsown for plants to colonise.
  • More bumblebees were found in floristically enhanced grass margins, which are sown (or naturally regenerated) with a mixture of grasses and wildflowers.
  • A broader range of flower species in enhanced margins would benefit more wild bee species.
  • Providing both kinds of margin in a landscape will benefit wild pollinators the most.

Background

Wild flower field marginIn the UK, a third of pollinating insects declined between 1980 and 2013 because of intensified agriculture and climatic changes. Pollinators are critically important to plants worldwide, with around 75% of global food crop types dependent on pollinators. Almost half of the leading crops are pollinated by at least one wild bee species. Wild bees consist of both bumblebees and solitary bees.

To try to help provide for pollinators in the UK landscape, some agri-environment scheme (AES) options focus on providing resources such as food and shelter for pollinators. Looking after the pollinators by increasing the amount of flowers and diversity in the landscape can benefit not only the insects themselves, but the whole agricultural system as well. Floristically enhanced grass margins are one example of an AES option designed to support pollinators. These are permanent strips at the edge of fields that are sown (or regenerated) with a mix of grasses and perennial wildflowers. However, these predominantly contain plants that are known to be preferred by bumblebees and few that provide for solitary bees, which visit a different selection of food plants.

Another form of AES option, which are designed to benefit rare arable plants, are cultivated margins. These are strips at the edge of fields that are cultivated (usually by shallow plough or minimum till) to around 15cm, then left unsown. Rare arable plants can then regrow from seeds in the soil. These conditions are ideal for some of those rare farmland plants that may otherwise be lost, as well as more common annual plants, both of which can also help support pollinators. The bare ground on cultivated margins is good for some ground-nesting bee species.

The potential benefits of cultivated margins to pollinators had not been examined before this study, but this paper looked at both types of margin to explore:

  • How solitary bee species use the two types of margin
  • Which beneficial weeds are found and how they are used by pollinators
  • How the establishment and management techniques affect their use by pollinators

What they did

The scientists looked at farms with both types of margin, surveying 15 farms per year across the south, midlands and east of England in both 2018 and 2019. They collected information about the establishment and management of each margin; for example, was it sown or naturally regenerated, how was it cultivated, is it cut or not? Also, information about its age, size, position relative to other features such as hedges, and the fertility of the soil.

They surveyed the number of bees and hoverflies using each margin for food or nesting each month from April to September, and also recorded the type of vegetation across each margin, with height, cover, plant density and plant species.

What they found

Cultivated margins

Over 1,000 bees were recorded (and almost 1,500 hoverflies) in cultivated margins, of which only 10% were honeybees and the rest were wild bees. Of these, a third were bumblebees and two thirds were solitary bees. A wider range of solitary bees used cultivated margins compared to floristically enhanced margins. There were fewer bumblebees present as the age of the margin increased, but fewer solitary bees if the margin had been ploughed when it was established. Spear thistle, beaked hawk’s beard and bird’s-foot trefoil were the most visited species by bumblebees. The most popular species for solitary bees were dandelions, beaked hawk’s-beard, cow parsley and ox-eye daisy. Most popular for hoverflies were dandelions, the daisy family and corn marigold.

Margins that remain in the same place each year had more cover of broad-leaved species and taller vegetation compared to those whose location is rotated. Those that were ploughed to establish have more bare ground, less grass cover and shorter, less dense vegetation.

Floristically enhanced margins

879 bees were observed in floristically enhanced margins, 21% of which were honeybees. The rest were wild bees, with around three quarters being bumblebees and a quarter solitary bees. There were over 1,500 hoverflies. More bees and hoverflies were observed on margins that were sown compared to those that were regrown.

Wild bees and bumblebees were less abundant on margins that were next to hedgerows compared to those next to other habitats. 27 plant species that were identified as forage plants were also present in cultivated margins.

The ox-eye daisy, spear thistle and bird’s-foot trefoil were most used by bumblebees; hoverflies preferred cow parsley, spear-thistle and beaked hawk’s-beard. Solitary bees preferred cow parsley.

What does this mean?

Overall, this paper makes the following recommendations for this type of field margin to be most attractive to wild bees and hoverflies:

Cultivated margins

Cultivated margins can be important not only for the rare arable plants and weeds that they are designed for, but can also support wild pollinators. If pollinator support is the aim, cultivated margins should not be ploughed, should be carefully sited and then kept in the same place rather than rotated, unless they become dominated by invasive or less beneficial weeds and grasses. Some weeds that are attractive to pollinators should be tolerated in the margin – such as spear thistle and cow parsley. 

Floristically enhanced margins

Margins sown with a seed mix are more beneficial to most wild bee species compared with those that regenerate naturally. Species such as spear thistle, bird’s-foot trefoil, ox-eye daisy, cow parsley and beaked hawk’s-beard will support many common bumblebee species as well as hoverflies. For the greatest benefit to a range of rarer bumblebee species, they should be sown with a broad mix of plant species, and again some weed species such as those mentioned above should be tolerated. As the age of the margin increases, grass species can take over and become dominant, with fewer flowers persisting.

The seed mixes that are available now for floristically enhanced margins contain common species of vetches, clovers and knapweed, which support a relatively small group of bumblebee and solitary bee species. To increase the number of species that benefit from these margins, we suggest the addition of:

  • Smooth hawk’s-beard
  • Bearded hawk’s-beard
  • Greater knapweed
  • Wild carrot
  • Spiny restharrow
  • Cow parsley

As cultivated margins are favoured by solitary bees and floristically enhanced margins are favoured by bumblebees, for the maximum benefit to wild pollinators, both habitats should be provided where possible.

Read the original abstract

McHugh, N & Bown, B & McVeigh, A & Powell, R & Swan, E & Szczur, J & Wilson, P & Holland, J. (2022). The value of two agri-environment scheme habitats for pollinators: Annually cultivated margins for arable plants and floristically enhanced grass margins. Agriculture Ecosystems & Environment. 326. 107773.

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