Seasonal progression and differences in major floral resource use by bees and hoverflies in a diverse horticultural and agricultural landscape revealed by DNA metabarcoding

In their new study, Abigail Lowe and colleagues discuss why we need to know which pollinators use which plants in which seasons throughout the year, so that we can support them effectively.

In the last few years, we have seen an immense increase in public support for pollinators with many choosing to buy pollinator-friendly plants for their garden. However, even with these good intentions, it’s not always easy trying to find the right plants.

Many recommendation lists are available, but they are usually inconsistent, poorly supported by scientific research and often don’t specify which pollinator group (bees, hoverflies, moths, butterflies, etc.) they are targeting. To make sure we are supporting a range of pollinator groups appropriately through the year, we need a deeper understanding of which plants are used across the season, and by which pollinators. Our new study, published in the Journal of Applied Ecology, helps to answer some of those questions.

At the National Botanic Garden of Wales, UK, the Science team has spent the last few years using DNA barcoding expertise to study honeybee foraging. Through this we have developed an excellent understanding of which plants are used by honeybees throughout the season, not just in the Botanic Garden, but across the UK.

However, in the UK, honeybees are just one species; we have around another 280 species of bee. This includes bumblebees (24 species) and the hugely diverse solitary bees, with some of the latter potentially nesting in your bee hotels.

We wanted to use our DNA barcoding skills to find out more about what plants other bee species visit. Our previous research on hoverflies in species-rich grasslands has shown them to be important and overlooked pollinators, so we were keen to include this fascinating group.

Bombus terrestris (Buff-tailed bumblebee) foraging on Cirsium. Bees were found to utilise Cirsium/Centaurea/Hypochaeris spp. more than hoverflies. Photo: Natasha de Vere.

From March to October, over two years, I walked along transects in the Garden and surrounding Waun Las National Nature Reserve, collecting any bees (bumblebees, honeybees, and solitary bees) and hoverflies that I found. As the pollen found on insects contains plant DNA, we can use this as a source of information of which plants have been visited by individuals. Once I had collected the insects, the pollen was washed from their bodies. Then, the plant DNA was extracted from the pollen and our DNA regions of interest were amplified using PCR, before being sent for sequencing. After this, we compared the DNA sequences returned to our custom reference libraries, made possible through the Barcode Wales and Barcode UK projects, to find out which plants the insects had visited.

Across the 41 species of hoverfly, six species of bumblebee, nine species of solitary bee, and one species of honeybee that I collected, pollen from 191 different plants was found on their bodies throughout the year. Bees and hoverflies did use a lot of the same plants, but when it came to looking at the largest contributors to their diet, there were significant differences.

Dandelion, buttercups, and lesser celandine were the favourites in the spring. In the summer, bramble was popular with both bees and hoverflies but bees particularly liked knapweeds, thistles, and cat’s ear, whilst hoverflies favoured angelica and hogweed. In the autumn, Rudbeckia, Helenium, Bidens and Coreopsis came out on top.

Rubus fruticosus_ndv
Bramble, Rubus fruticosus was the most frequently visited plant across all insects sampled. Photo: Natasha de Vere

Even with over 5000 plant taxa from around the world to choose from in this landscape, the majority of plants visited by bees and hoverflies were native and near-native. However, horticultural plants still have a role to play during the later months when native plants have finished flowering, by extending the season and providing pollinators with a late food source.

We conclude that plant recommendation lists for pollinators should distinguish between bees and hoverflies and provide evidence-based floral recommendations throughout the year, that include native as well as non-native plants for use in the United Kingdom and Northern Europe. Specific management recommendations include reducing mowing to encourage plants such as dandelion Taraxacum officinale and buttercups Ranunculus spp. and reducing scrub management to encourage bramble Rubus fruticosus.

This work has already been used to inform the National Botanic Garden of Wales’ Saving Pollinators Assurance Scheme, a unique label scheme that ensures that plants are proven to be good for pollinators, contain no synthetic insecticides and have been grown in peat-free compost. The Saving Pollinators Assurance Scheme, funded through the Growing the Future project**, gives gardeners the confidence to know which plants are both good for pollinators and the environment.

* Abigail received funding from the Knowledge Economy Skills Scholarships (KESS 2), which is a pan-Wales higher level skills initiative led by Bangor University on behalf of the HE sector in Wales. It is part funded by the Welsh Government’s European Social Fund (ESF) convergence programme for West Wales and the Valleys.

** The Growing the Future project has received funding through the Welsh Government Rural Communities – Rural Development Programme 2014-2020, which is funded by the European Agricultural Fund for Rural Development and the Welsh Government.

Read the full paper Seasonal progression and differences in major floral resource use by bees and hoverflies in a diverse horticultural and agricultural landscape revealed by DNA metabarcoding in Journal of Applied Ecology

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