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By Michael Hanley

Urban gardens are increasingly recognised for their potential to maintain or even enhance biodiversity. In particular the presence of large densities and varieties of flowering plants is thought to support a number of pollinating insects whose range and abundance has declined as a consequence of agricultural intensification and habitat loss. However, many of our garden plants are not native to Britain or even Europe, and the value of non-native flowers to local pollinators is widely disputed.

We tested the hypothesis that bumblebees foraging in urban gardens preferentially visited plants species with which they share a common biogeography (i.e. the plants evolved in the same regions as the bees that visit them). We did this by conducting summer-long surveys of bumblebee visitation to flowers seen in front gardens along a typical Plymouth street, dividing plants into species that naturally co-occur with British bees (a range extending across Europe, north Africa, and northern Asia – collectively called the Palaearctic by biologists), those that co-occur with bumblebees in other regions such as southern Asia, and North and South America (Sympatric), and plants from regions (Southern Africa and Australasia) where bumblebees are not naturally found (Allopatric).

Rather than discriminating between Palaearctic-native and non-native garden plants when taken together, bees simply visited in proportion to flower availability. Indeed, of the six most commonly visited garden plants, only one Foxglove (Digitalis purpurea – 6% of all bee visits) was a British native and only three garden plants were of Palaearctic origin (including the most frequently visited species Campanula poscharskyana (20.6% of visits) which comes from the Balkans). The remaining ‘most visited’ garden plants were from North America (Ceanothus 11% of visits) and Asia (Deutzia Spp 7% of visits), while the second most visited plant, Hebe × francisciana (18% of visits) is a hybrid variety with parents from New Zealand (H. speciosa) and South America (H. elliptica).

However a slightly different pattern emerges when we consider the behaviour of individual bumblebee species. This is important because we know from work done in natural grassland ecosystems that different bumblebees vary greatly in their preference for native plant species. Some bumblebees visit almost any flower, while others seem to have strict preferences for certain plants. The latter group (‘dietary specialists’) include bees with long tongues that allow them to access the deep flowers of plants belonging to the pea and mint families that short-tongued bees cannot. One of these dietary specialists, the aptly named ‘garden bumblebee’ (Bombus hortorum), showed a strong preference for Palaearctic-origin garden plant species (78% of flower visits by this species); although we also saw this species feeding on the New Zealand-native, Cordyline australis. Even more interesting was the fact that our most common species the ‘buff-tailed bumblebee’ (B. terrestris) appeared to favour non-Palaearctic garden plants (70% of all visits) over garden plants with which it shares a common evolutionary heritage (i.e. Palaearctic plants). So it seems that any preference for plants from ‘home turf’ varies between different bumblebees; just like in natural grasslands, some bees are fussy about where they forage, and others not.

So what should gardeners do to encourage pollinators? Our results suggest that it is not simply a question of growing native species even if this is desirable for other reasons, but that any ‘showily-flowered’ plant is likely to offer some forage reward. There are caveats, however. Garden plants that have been subject to modification to produce ‘double’ flowers that replace or obscure the anthers and carpels that yield pollen and nectar (e.g. Petunias, Begonias, and Hybrid Tea roses) are known to offer little or no pollinator reward. A spring to autumn supply of flowers of different corolla lengths is important to provide both long- and short-tongued bumblebees with nectar. A reliable pollen supply is particularly important during nest founding through to the release of queen and male bees at the end of the nest cycle. Roses and poppies are obvious choices, but early season willows also offer pollen for nest-founding queens. Potentially most crucial of all however, are the pea family as they offer higher quality pollen vital for the success of the short-nest cycle, specialist bumblebees such as B. hortorum. It is also important that access to what gardeners refer to as ‘weeds’ is available. Where possible gardeners can set aside a small area to allow native brambles, vetches, dead nettles, and clovers to grow, but as long as some native weed species are available in nearby allotments, parks, or other green spaces, we suggest that a combination of commonly-grown garden plants will help support our urban bumblebees for future generations.

Dr Michael Hanley is Lecturer in Terrestrial Ecology at the University of Plymouth. He is co-author of the article ‘Going native? Flower use by bumblebees in English urban gardens’, which is published in the Annals of Botany.

Annals of Botany is an international plant science journal that publishes novel and substantial research papers in all areas of plant science, along with reviews and shorter Botanical Briefings about topical issues. Each issue also features a round-up of plant-based items from the world’s media – ‘Plant Cuttings’.

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Image credit: Bumblebee on apple tree. By Victorllee [CC-BY-SA-3.0], via Wikimedia Commons

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Further to inventions that imitate nature, here is one of a series of bumblebee-themed images I was asked to design some years ago. A "friendly bee" was asked for, one that was busy about a garden. It was a fun assignment and I eventually came up with several versions of BumbleBee, a family of them and their garden, including hollyhocks. I have been reviewing the images recently, going through the files, and this morning as I walked by the lake I came upon this:

an end-of-summer drowsy bee nestled in the hollyhocks that have naturalized along the unfinished harbour wall, among the small convolvulus and michaelmas daisies.

Dave Goulson is Professor of Biological Sciences at the University of Stirling, and is the author of Bumblebees: Behaviour and Ecology. Research into bumblebees has accelerated in recent years and many new discoveries have been made with regard to their ecology and social behaviour. However, bumblebees are now suffering widespread decline - something all of us should be concerned about. In the post below, Dave Goulson explains why their numbers are declining, why we should care, and what we can do about it.

With their large size, furry, colourful bodies and slow, buzzing, slightly clumsy flight, bumblebees are among the most endearing and welcome of insect visitors to the garden. In my mind they are forever associated with the endless sunny summer days of childhood. So it is particularly alarming that these lovable insects are slowly but surely disappearing. Three of the 27 UK species have become extinct, and six more are critically endangered. Over most of the UK only a handful of species remain. Similar patterns can be seen in Western Europe, North America and even as far away as Sechuan. What has happened? Where have they gone?

The loss of our bumblebees is the result of profound changes to the countryside, particularly in the last 60 years. Pesticides were introduced widely during the second world war and their use increased steadily until the 1990s. Farmers were given subsidies to rip out hedgerows, and to plough up ancient flower-rich meadows. Well over 95% of our unimproved grasslands (haymeadows and chalk downland) were lost to cereal or silage production. Ponds and marshes were drained, heathlands ploughed, and woodlands cleared, all paid for by taxpayers money in the name of increased productivity. Faced with this onslaught, much UK wildlife suffered. Numbers of many farmland birds such as skylark and lapwing plummeted and many butterflies and wildflowers disappeared. Bumblebees depend on flowers for food, and they nest in hedgerows, woodland and old meadows. As these resources disappeared, so did the bees.

Photograph: Author’s Own

Perhaps more so than any other creatures, we should be concerned by the loss of our bumblebees. They are not just beautiful and endearing. They are a vital part of the ecosystem. They pollinate numerous wildflowers such as foxgloves, bluebells and cowslips. They also pollinate many important crops. Without them, some of the remaining wildflowers would fail to set seed, and could vanish. Crops such as oilseed rape, raspberries, currants, beans and peas would be less productive. Alarmingly, driven by concerns that wild bumblebee populations were becoming inadequate to pollinate their crops, growers of raspberries, strawberries and apples are now buying in colonies of non-native bumblebees from factories in Europe. This practice provides further threats to native bees, for they may be displaced by the incomers, or they may be exposed to novel diseases accidentally imported with the foreign bees. It would clearly be far better if we had sufficient populations of our native bumblebees to pollinate our crops.

The growing awareness of the importance of bumblebees is illustrated by the success of the Bumblebee Conservation Trust (BBCT), which was formed in 2006 and has already grown to a membership of over 5,000. The trust promotes bumblebee-friendly farming and gardening. Gardens cover over 1 million hectares, a larger area than all the UK’s nature reserves combined, so making gardens more wildlife-friendly could have a huge positive impact. Churchyards, parks and road verges can be great for wildlife including bumblebees, but are often mown too frequently, often without good reason. BBCT are trying to influence local authorities to consider bees when planning how they manage these areas.

If you would like to do your bit for the humble bumblebee, there are lots of things you can do. Look at the BBCT website for information on how to make your garden a haven for bumblebees, both by providing flowers and nest sites for them.

If we look after our bumblebees we will benefit much else besides. The flowers that they love also support a host of other creatures such as butterflies and hoverflies. By attracting and feeding the bees we can ensure that our flowers, vegetables and fruit trees are well pollinated, so that we directly benefit from better crops. With dedication and a little luck perhaps we can conserve bumblebees for future generations to enjoy.