Bumblebee research: A day in the life
Video reveals behavioural ecology of essential crop pollinators
25 August 2011
Scientists are engaged in a number of field studies to investigate the decline in pollinating insects such as bumblebees, which are essential components of both natural habitats and managed food production systems (see 'Halting the decline').
Many agricultural crops, such as strawberries and raspberries and sunflowers, have their flowers almost entirely pollinated by bees. Other crops, including broad beans and tomatoes in glasshouses, are exclusively pollinated by bees; losing bee pollinators could have a devastating effect on agricultural systems and have major impacts on rural economies, trade, and global food security.
BBSRC-funded PhD student Nicolas Charlton at Bristol University is looking into whether agricultural land management practices are having a detrimental effect on bumblebees. Specifically, if the large swathes of land growing a single flowering species, such as oilseed rape, is providing short term benefits to some species of bumblebee at the expense of others.
Bumblebees are important food crop pollinators.
Image: Debbie Harding
To do this, Charlton has taken to the great outdoors to observe and record the behavioural ecology of bees. "The best things about field work are that you're out of the office, seeing wildlife and nature first hand, and being in the place environment which you studying, he says. "It makes it relevant and gives you the opportunity to make possibly important discoveries and observations."
Crops and robbers
Charlton has been targeting a behaviour known as nectar robbing. Bumblebees fall into two categories: long–tongued and short–tongued, and each use their tongues to feed on sugar–rich nectar in flowers suited to their co–evolved mouthparts.
Too much of a good thing? Image: iStockphoto
But the short–tongued bumblebees have developed a sneaky behaviour to obtain the nectar in certain flowers usually reserved for the long–tongued species — they chew a hole in the side of the flower and make off with the booty inside, a behaviour known as 'nectar robbing'. Whilst this is good for the short–tongued bumblebees, the long–tongued species that would normally feed on the flowers are denied a crucial food resource.
But what's modern agriculture got to do with the private lives of bees? Because there is evidence that mass flowering crops, such as oil seed rape, are leading to an increase in nectar–robbing behaviour – which could then affect the diversity and abundance of long–tongued bumblebee species which may carry out crucial 'ecosystem services' such as pollination, for a number of wild flowers and agricultural species.
A study in Germany published in 2010 indicated 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 (ref 1). The researchers concluded that this was probably because of increased competition for the red clover. Similarly, although it did not look at nectar robbing directly, a 2011 study in Germany concluded that mass–flowering crops can threaten the pollination of concurrent wildflowers by diluting the numbers of pollinators (ref 2).
Hive of activity
Nectar robbing: a bumblebee has chewed through the petal, where indicated by the yellow arrow, to steal the nectar inside.
Image. J. Memmott
Unfortunately, Charlton says his results from last year were disappointing. "This was mainly due to a low number of bees earlier in the year, probably due to the long cold winter." He did learn that the most common bumble bee visitor to red campion was Bombus hortorum, the garden bumble bee, which has a very long tongue which allows it to feed easily from the deep flowers. "I also observed the buff–tailed bumble bee, Bombus terrestris, with its short tongue robbing."
The difficult field work conditions last year have led Charlton to try something different this year – experiments in the tube–like poly–tunnels that are a liked by some, maligned by others, feature of the UK's modern agricultural landscape. "This set–up allows me to control the numbers of plants and flowers easily, get very close to the bees, and the ability to mark and follow individuals while they forage," says Charlton. "The experiments are looking at two areas: nectar robbing behaviour in the presence and absence of oilseed rape, and how visits to strawberry flowers may be affected by the presence of wildflowers."
Halting the decline
Alongside the mysterious collapse and decline of honeybee populations, the UK has seen declines in bumblebee populations, with two of its 27 species extinct, and seven species threatened (ref 3).
It's important to look at individual factors such as land management (and how it may affect individual behaviours, such as nectar robbing), but bee declines (and that of honeybees) may well be multi–factorial. A study in the US showed declines in four bumble bee species by up to 96% and contractions in their geographic range by between one and three quarters. Comparing data from national surveys to museum reference species revealed that declining species often showed lower genetic diversity, as well as higher infection levels of a fungal pathogen, Nosema bombi (ref 4).
Moreover, a study on bumble bees from Sweden showed that, although abundance was not down, diversity was and that two short–tongued species B. terrestris and B. lapidarius, were found to dominate the landscape making up 89% of bumble bees sampled (ref 5). "Interestingly, both of these species can act as nectar robbers," says Charlton. "The declining species tend to be longer tongued species."
- Oilseed rape crops distort plant–pollinator interactions (external link)
- Expansion of mass-flowering crops leads to transient pollinator dilution and reduced wild plant pollination (external link)
- Bumblebees: Behaviour, Ecology, and Conservation. Second Edition, Oxford University Press (external link)
- Patterns of widespread decline in North American bumble bees
- Drastic historic shifts in bumble-bee community composition in Sweden (external link)
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