Why are our bumblebees in trouble?

Bumblebees are my number one reason for gardening. I love hearing them, seeing them and knowing that I can help them, even though I live in the middle of a capital city.

I decided to look up the root cause of their decline over the recent past, to check if our efforts are worth it. I have tried to summarise the work of a leading expert in bumblebee ecology and behaviour, after reading one of his publications about bumblebees.* According to Dave Goulson, the main culprits implicated in bumblebee declines are…

1) Declines in floral diversity

Bees feed almost exclusively on pollen and nectar, and so they are completely reliant on flowers. For bumblebees to thrive, they require a continuous succession of flowers. Unlike honeybees, bumblebees do not store large quantities of honey or pollen to get through tough weather. The queen bumblebees will normally hibernate during autumn and winter, and the rest of the colony will have died. When the queen emerges from hibernation in Spring, she singlehanded gathers sufficient forage to feed her first batch of offspring, as well as keeping them and herself warm enough. She uses enormous amounts of energy at this time, and she needs to be efficient at collecting the forage. This is why Spring-flowering plants are particularly vital.

If there is a gap in the succession of flowering plants in their vicinity, bumblebee colonies will starve and die.

But there are fields everywhere around Cardiff! Surely they prefer the countryside?

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Agricultural crops will provide an abundance of food for bees during their brief flowering period, but then the land is completely devoid of flowers. Before artificial fertiliser, leguminous plants such as clover would provide a very favourable food source, particularly for long-tongued bee species. This practice has almost been completely abandoned, and is likely to be the main reason for the decline in long-tongued bee species. As few crops flower early, unless farms contain areas of wild flowers to provide the stable food supply, farms will not support bumblebees. If bees decline, the plants they pollinate will set less seed, and there will, in turn, be less food for future bees, and ourselves.

Most bumblebees are not fussy about habitat type, however to provide for larger numbers requires a high abundance and diversity of flowers. Traditionally, habitats with enough diversity include salt marshes, sand dunes, unimproved meadows, heathland, wet meadows and fens. All these habitats have declined with increased agricultural efficiency. With the introduction of cheap artificial fertilisers, ancient grasslands were replaced by faster growing species that can outcompete and exclude the ancient grassland species. 90% of unimproved lowland grassland was lost in the UK after WW2. Development grants were given to grub up hedgerows and to drain marshes. With all this increase in productivity, we lost biodiversity.

In general, it seems that bumblebees favour perennial and biennial plants over annuals. Longer-tongued bumblebees in particular strongly favour perennials. The bumblebees that have declined the most in the UK are the medium and long-tongued bumblebees.

2) Loss of nest and hibernation sites

Bumblebees need suitable nesting sites, and the requirements vary with species. Carder bees tend to nest in grassy tussocks, others nest in underground cavities. Many use abandoned rodent nests. Loss of hedgerows and unimproved grassland is likely to have reduced nest sites for above and below ground nesting bumblebees.  (There are less voles and mice on intensively managed land.) Bumblebees also need hibernation sites where young queens can remain undisturbed through the autumn and winter. Not much data are available, but for common species, they hibernate on north-west facing slopes or in the shade if trees.

3) Pesticide use

Pesticide risk assessments were routinely completed for honey bees, but not for bumblebees. Spraying crops early in the morning avoids the main honeybee foraging time, but this is the time when bumblebees are most active. Systemic pesticides can reach high concentrations in the nectar, which is eaten by the bumblebees. When colonies are large, it is likely that they can support some loss of workers. However, in the spring when the queens are foraging, and subsequently the nests are small and contain few workers, mortality may have a significant effect. Thus spring applications of pesticides are of particular concern.

4) Effects of habitat fragmentation

The six most common Bombus species can be found throughout the UK, but the rare UK species now persist in isolated areas, notably in south-west England, in south and west Wales, and in remote areas of Scotland. Some of the strongest remaining bumblebee communities are in military training areas such as Salisbury Plain and Castlemartin Range in Pembrokeshire, or other areas where agriculture is not economically viable, like marshes.

The population structure of bumblebees is rather different to that of most insects. A healthy population of butterflies can persist on quite a small area of land for many decades. A single bumblebee colony, in contrast, will forage over an area if 3km^2, sometimes much more. Most nature reserves are very small, and probably only provide sufficient forage for perhaps one or two colonies. Yet a colony is, essentially, just one breeding pair (since queens of most species are monogamous). So a species that may appear to be abundant, in terms of workers, may have a very small effective population size. To maintain a viable population in the long term, it is presumably necessary to provide enough habitat for dozens of colonies. A healthy bumblebee population requires at least 10km^2 of suitable habitat. In densely populated parts of the country like Cardiff, most reserves are tiny fragments of the original area, often just a few hectares. Very few nature reserves are large enough to support a viable population of bumblebees, thus only a few generalist species, those able to eke out a living in an impoverished agricultural landscape, have survived in most regions.

So what do we do about it and have the projects to help bees actually helped? Get in touch with your comments, and I’ll look to summarise this in the near future…

 

* Dave Goulson’s book was wonderful reading, and has increased my love for these complex and hard-working creatures. But any mistakes in the facts are from my own misinterpretation. Sorry!

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One comment

  1. Yes – bumble bees are really good to see. Have they worked out how they fly yet? I saw quite a few in Totnes this summer, all round one specific bush (don’t ask me what sort)

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