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Modelling bee pollination: enter the ‘flight arena’

Laboratory studies investigate feeding and flying behaviour

10 March 2011

  Modelling bee pollination: enter the ‘flight arena’

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BBSRC-funded researchers are engaged in a number of activities to investigate the decline in pollinating insects such as bees. Among them are laboratory-based experiments to find out what cues bees use when visiting flowers.

Insects are important pollinators of wildflowers and agricultural crops. Among the insects, bumblebees are essential components of both natural habitats and managed food production systems.

In ecosystems such as wild meadows, bumblebees pollinate a number of flowers that can only have their pollen spread by certain species of bumblebee due to the length of their tongues which have co-evolved with the sexual organs of the flower. It's a relationship that Charles Darwin noticed when he found a flower with a very long nectar tube and predicted that an insect would be found to match the plant, which was found many years later.

Bees are invaluable pollinators of crops and wildflowers. Image: Jane Memmott

Bees are invaluable pollinators of crops and wildflowers. Image: Jane Memmott

But lose the bee and you lose the flower. And many agricultural crops have their flowers almost entirely pollinated by bees, such as strawberries and raspberries and sunflowers. In fact, some crops are exclusively pollinated by bees – such as broad beans and tomatoes in glasshouses – losing bee pollinators could have a devastating effect on agricultural systems and have major impacts on rural economies, trade, and global food security.

Sadly, alongside the mysterious collapse and decline of honeybee hives and populations, respectively, the UK has seen declines in the populations of its bumblebees, with two of its 27 species extinct, and seven species threatened (ref 1).

Mystery decline

The reasons behind the declines are not well understood and are now subject to a significant research effort. BBSRC-funded PhD student Nicholas Charlton at Bristol University is looking into whether agricultural land management practices are having a detrimental effect on bumblebees; namely large swathes of land growing a single flowering species, such as oilseed rape, which flowers simultaneously providing a glut of sugary nectar for a short time but a drought in both amount and variety at other times.

"It is generally considered that changes to agricultural methods and land use have contributed to the decline in wild bees, mostly due to loss of habitat and a reduction in the number of flowers available," says Charlton. "All pollinators need floral resources to support them throughout the season."

But how do you tell whether land management practices are affecting bees, as opposed to other possible factors such as cumulative exposure to pesticides, pathogens or increased inbreeding which have all been implicated? One way is to monitor bee behaviour in relation to agricultural landscapes.

Bumblebees exhibit a crafty behaviour called 'nectar robbery' where they chew holes in flower petals to obtain nectar, rather than taking the traditional route of using their tongues, which, broadly speaking, are short or long depending on which flowers they have co-evolved with. Thus, nectar robbery can be seen as a cheeky shortcut to exploiting flowers that they would not otherwise be able to feed on (see 'The chief culprits')

Agricultural land management and habitat loss may play a part in bumblebee decline. Image: Claire Carvell

Agricultural land management and habitat loss may play a part in bumblebee decline.
Image: Claire Carvell

This is important because although the profusion of flowering plants such as oilseed rape has been shown to positively affect bumblebee colony growth and the densities of bumblebees, which enhances pollination services in agroecosystems (ref 2), if nectar robbery by one species deprives another species of its natural food sources, then population declines could result. In short, agricultural practices could be to the benefit of some bumblebees and to the detriment of others if their food is being stolen, distorting plant-pollinator interactions in the local ecosystem.

"In terms of how common is nectar robbery, it varies considerably with area," says Charlton. "One of the things I've been looking at is the high spatial variation in robbery levels."

A study in Germany published in 2010 indicated that agricultural landscapes do appear to affect robbery levels (ref 2). Diekotter and colleagues investigated robbery of red clover, Trifolium pratense, by Bombus terrestris which can only access nectar from clover by robbery. They found that increased amounts of oilseed rape resulted in increased robbery levels which coincided with numbers of long-tongued bumble bees (who cannot nectar rob) decreasing. The researchers concluded that this was probably because of increased competition for the red clover. The evidence implicates high levels of oilseed rape, via nectar robbery by species that benefit, in the decline of long-tongued species.

Flight of fancy

Charlton wants to see if oilseed rape landscapes in the UK are affecting bees' nectar robbing behaviour as appears the case in Germany. It's important work, because the EU-wide push for more biofuel plants such as oilseed rape means that such landscapes, oilseed rape already covers 600,000 hectares, or 6% of the UK's arable land, are likely to expand.

A bee has chewed through this petal, indicated at the yellow arrow, to steal the nectar inside. Image: Jane Memmott

A bee has chewed through this petal, indicated at the yellow arrow, to steal the nectar inside. Image: Jane Memmott

To complement his field studies, Charlton is experimenting with a laboratory bee 'flight arena' to see what cues bees use when choosing which flowers to feed from. Bees can be trained to feed from certain containers, and by varying the shape, colour and texture of different artificial flowers, he hopes to find out the cues related to feeding and nectar robbing.

Ultimately, using the advantages of working in the lab (see 'The great indoors') he hopes to induce nectar robbing in an artificial environment so experiments can be run that would test the different environmental thresholds needed to cause the behaviour, such as number and density of certain types of flower for example.

So far, he's had trouble persuading the bees to realistically make holes in the artificial paper flowers. "I'm hoping to use real flowers later this year," says Charlton. "I've found out that no man-made material can match flowers. All the designs and materials I tried did not replicate what the bees can do on real flowers."

Undaunted, he says his next big experiment is similar to the flight arena but on a different scale. "I'm planning to have captive bees with whole plants in large 3x4m poly-tunnels and look at the effect oilseed rape has on foraging behaviour."

The robbers

In the UK, several species of bumblebee have been recorded as nectar robbers including Bombus pratorum, B. jonellus, B. lapidarius, B. lucorum, B. rupestris and the most common nectar robber B. terrestris.

A species of carpenter bee, Xylocopa violocea, has started to become established here from Europe, and is also a common nectar robber.

Honey bees (Apis melifera) will use the holes made by other nectar robbers but do not make the holes themselves.

Other nectar robbing insects include ants and wasps, so there is something about the social insects (who all belong to the same order, the Hymenoptera) which appears to predispose them to using this behaviour.

Charlton has written an article, When bees exploit flowers: nectar robbery, for the British Entomological and Natural History Society which details instructions to help people look for robbery of red campion flowers and send in their findings.

The great indoors

Charlton has field studies in agricultural environments under way, but in the winter, when new bumblebee queens are hibernating, working in the laboratory had distinct advantages in controlling the conditions.

"The biggest advantage is the weather. Too cold and bees and even flowers may not appear at the right time; too windy or wet and the insects won't fly; too hot and dry and the plants can suffer," says Charlton, adding that working in the lab also allows control of daylight and temperature so experiments can be run even in the winter.

However, he says that you can't ask all of the same questions you could outdoors. "The lab is an artificial environment. This influences the questions you can ask and the interpretations you can make."

References

  1. Bumblebees: Behaviour, Ecology, and Conservation. Second Edition, Oxford University Press (external link)
  2. Oilseed rape crops distort plant–pollinator interactions (external link)

Contact

Arran Frood

tel: 01793 413329
fax: 01793 413382