Pollinators in Winter: Go native or not?

Buff-tailed bumblebee Bombus terrestris on Gorse
Image: Kevin Thomas, Falmouth Nature

One of the big questions raised in the analysis of New Year Plant Hunt (NYPH) results by Kevin Walker, BSBI Head of Science, is around what the impacts of changes in flowering times of wild and naturalised plants might have on pollinators and other insects. 

As Kevin says, "NYPH has shown that in milder winters, more plants flower because of warmer temperatures and fewer frosts. We don't yet know what the implications of this are for plants and associated insects - but what we do know is that weather patterns are changing and that plants are responding".

Bombus terrestris on a range of plants
Photographed on 30th January 2019
Image: Charlotte Rankin

Of course, before we can get to grips with the possible impacts of changing weather patterns on our plants and insects, we need to know what is "usual" at this time of year. So, to help those of us (like me!) whose botany is much stronger than our entomology, I asked conservationist and insect ecologist Charlotte Rankin to talk us through which pollinators are usually on the wing at this time of year and which plants they are most likely to visit - native species? Naturalised garden plants? Or perhaps the winter-flowering shrubs we see in parks and gardens which rarely naturalise? 

Charlotte has a first class degree in Conservation Biology and Ecology, and a strong track record in community engagement and public outreach. She is also a botanist and took part in this year's North-East New Year Plant Hunt-Off in Northumbria, so she is ideally placed to bridge the gap between flowers and pollinators! 

Bombus terrestris heads for a Rosemary plant
Image: Charlotte Rankin

Over to Charlotte:

    

"For most flowering plants and their visiting insects, Winter is a time for rest and preparation for the warmer seasons ahead in the UK. However, there are some quiet buzzes in the urban air and, as shown by this year’s New Year Plant Hunt results, over 600 species of wild or naturalised flowering plants can be found in bloom. Honeybees and various species of fly can be active on particularly mild winter days and most notably, brave and bold Bombus terrestris (Buff-tailed Bumblebee) queens can establish winter-active colonies.

"What forage can urban areas offer for such pollinators in Winter? At a glance, results from the New Year Plant Hunt provides insight into what wild or naturalised plant species may well be available for urban pollinators out in Winter. The top 20 plants found in flower during this year’s Hunt shows that there are species particularly attractive to pollinators, such as the trusty Taraxacum spp. (Dandelion), Achillea millefolium (Yarrow), Sonchus oleraceus (Smooth Sow-thistle), Hedera helix (Ivy) and Jacobaea vulgaris (Common Ragwort), that can hang onto Winter. 

Bombus terrestris on Winter Heliotrope
Image: Charlotte Rankin

"Urban areas also tend to have more naturalised non-natives that could be exploited, including Centranthus ruber (Red Valerian) and Petasites pyrenaicus (Winter Heliotrope). However, private gardens, parks and other amenity areas also introduce a variety of exotic winter-flowering plants, that by flowering at their peak during these months, can offer a rich resource of nectar and pollen throughout the Winter.

"Winter-active nests of Bombus terrestris are associated with Southern, urban areas of England and were first noted in the 1990’s. Rather than hibernating, some queens produced in late Summer/Autumn may establish a nest and maintain it throughout the Winter months. 

"In order to do this, nests need a continuity of nectar and pollen and it seems that colonies deal with the depths of Winter by utilising exotic winter-flowering plants, particularly mass-flowering shrubs.

Bombus terrestris on Winter Honeysuckle,
Mahonia and Winter Heather
Image: Mike Robinson  

"There has been some published research on the winter foraging activity of Bombus terrestris: A study by Stelzer et al. (2010) showed that commercial Bombus terrestris colonies in London relied upon mass-flowering garden shrubs such as Mahonia and could achieve foraging rates like that in Summer. An ask on Twitter also showcased the exotic tastes of Bombus terrestris, with the large majority of sightings on a great range of garden plants including Mahonia spp., Lonicera fragrantissima (Winter Honeysuckle), Hebe spp., Camellia spp. such as ‘Cornish Snow’, Winter-flowering Erica spp., Jasminum nudiflorum (Winter Jasmine) and Helleborus spp. (Garden Hellebores).

"These plants all share something in common: they flower at their peak during Winter, providing these bumblebees with an abundance of nectar and pollen to develop their colonies. As a generalist species active during these tricky months, it makes sense for these bumblebees to focus their foraging on these mass-flowering plants. While wildflower species attractive to bumblebees could well be present, their flowers are few and far between compared to their prime months of flowering to be relied upon.

Episyrphus balteatus on Gorse
Image: Kevin Thomas, Falmouth Nature 

"Winter workers can be seen using the native Ulex europaeus (Gorse), appearing to collect its pollen. Some workers may even venture into allotments and make use of broccoli that has been left to flower and have been spotted using the invasive and naturalised Winter Heliotrope.

"On mild Winter days, there are three species of hoverfly likely to be seen: Meliscaeva auricollis, Eristalis tenax and Episyrphus balteatus. Winter hoverfly sightings from Twitter and the UK Hoverflies Facebook Group provide insight into what plants they visit, including visits to wildflowers such as Taraxacum agg. (Dandelion), Ulex europaeus (Gorse) and early-flowering Ficaria verna (Lesser Celandine). 

Meliscaeva auricollis on Viburnum
Image: Will George

"Again, exotic plants appear to be a valuable forage resource, with sightings on species such as Mahonia, Viburnum tinus (Viburnum), Lonicera fragrantissima (Winter Honeysuckle), Erysimum spp. (Perennial Wallflower) and Jasminum nudiflorum (Winter Jasmine).

"In the depths of winter, exotic mass-flowering plants can provide a reliable and rich resource of nectar and pollen. Winter-active nests of Bombus terrestris appear to have a strong association with these plants and currently, wildflowers found straggling on into Winter likely do not provide sufficient forage to be utilised alone. 

Eristalis tenax on Dandelion
Image courtesy of Trevor Kerridge 

"Towards the end of Winter and early Spring, a variety of early-flowering species such as Lesser Celandines and Blackthorn begin to flower, and their importance as magnets for early-emerging pollinators should certainly not be overlooked. More data is needed on the winter foraging activity of bumblebees and other pollinators that may venture out on mild winter days. Sightings of winter-active bumblebees can be submitted online to the Bees, Wasps and Ants Recording Society (BWARS)’s winter bumblebee project."

Many thanks to Charlotte for these helpful insights into a pollinator's eye view of our winter flowers, whether wild or in gardens. Thanks also for the superb images she sourced to illustrate this post: some are her own and some were sourced from her network of contacts. Many thanks to them too and we have shared links to their Twitter accounts so you can follow them. You can also follow Charlotte on Twitter for more of her observations on the natural world and our wonderful wildlife, both plants and animals.