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Do alkaloids in nectar enhance pollination?

Sarah Barlow & Phil Stevenson
29 August 2014
Blog team: 

Sarah Barlow (Plant Ecologist) and Phil Stevenson (Natural Products Chemist), from Kew’s Jodrell Laboratory, report recent research on the role of floral alkaloids in the pollination of monkshood (Aconitum spp.)

Plants synthesise an array of chemicals to defend themselves against attack by herbivorous insects, but some of these defence compounds also occur in nectar and pollen – the food reward for insect pollinators. We know surprisingly little about what effect these compounds have on pollinators, or if they have an ecological role, but are beginning to develop some theories for such counterintuitive chemistry. Given that insects pollinate 75% of crops and up to 90% of wild flowering plants (Vanbergen et al., 2013), understanding the ecological functioning of floral reward chemicals has important implications for conserving and managing pollinators and the vital pollination services they provide.

Photo of the bumblebee Bombus terrestris feeding on Rhododendron

Bombus terrestris feeding on nectar containing grayanotoxin from Rhododendron flowers (Photo: P.C. Stevenson)

A possible explanation for the presence of these defensive compounds in nectar is that they enhance pollinator behaviour. For example, Phil’s recent research in collaboration with Newcastle University has shown that caffeine occurs in the nectar of Citrus and Coffea species, where it improves honeybee memory for the floral scents associated with food rewards. This increases revisits by the bees to the same plant species and therefore makes pollen transfer more efficient (Wright et al., 2013). It has also been suggested that nectar chemicals might protect bees against disease and pathogens (Manson et al., 2010). However, the same kinds of compounds can also cause mortality in bees - for example, the orchard mason bee (Osmia lignaria) was killed when fed nectar equivalent doses of the compound zygacine from flowers of deathcamas (Zigadenus) (Irwin et al., 2014).

Understanding the effects of floral chemicals on pollinators is therefore important, as they can be detrimental or beneficial to pollinators.These effects could be amplified where they interact with other environmental or anthropogenic pressures. A recent review, co-authored by Phil and collaborators on Insect Pollinators Initiative projects, reported that pollinators are facing multiple threats, in particular habitat loss, pesticides and the spread of pests and diseases.The report concluded that the interactive or additive effects of multiple pressures are more likely to explain pollinator declines and colony collapses than any single factor (Vanbergen et al., 2013).

Aconitum and Bombus interactions

In our continuing work to unravel the ecological role of floral chemicals, funded by the British Ecological Society and BBSRC, we are testing the hypothesis that diterpenoid alkaloids in the nectar and flowers of species of Aconitum (monkshood) deter nectar robbers (insects that ‘steal’ nectar without pollinating the flower), thus securing the services of legitimate pollinators.  

Photo of Aconitum flowers

Aconitum flowers have deep nectaries concealed beneath the hood (galea) (Photos: R. Fang)

The deep nectaries of the flowers of Aconitum species make them accessible only to bumblebees, and specifically those species with long tongues.  Some short-tongued species appear to steal the nectar by biting through the flower’s hood (galea) to gain access to nectar but without transferring pollen. This was observed by Phil on flowers of Aconitum lycoctonum, whereas A. napellus growing nearby was apparently undamaged, suggesting that bee foraging behaviour may be influenced by differences in floral chemistry between species. 

Photo of short-tongued bumblebees stealing nectar from Aconitum flowers

Short-tongued bumblebees such as Bombus terrestris sometimes chew through the galea of Aconitum flowers to rob nectar without providing a pollination service to flowers (Photos: S.E. Barlow)

Photo of bite holes in Aconitum flowers made by the bumblebee Bombus terrestris

Bite-holes in the galea of Aconitum flowers made by Bombus terrestris to steal nectar (Photos: S.E. Barlow)

Specialist pollinators source food from only a few species of plants and so are more likely to transfer pollen successfully (to flowers of the same species) than generalists that might visit many different species. A recent study suggested that alkaloids in A. septentrionale might protect nectar for specialist pollinators and be repellent to generalists (Gosselin et al., 2013). These intriguing observations led us to ask, could herbivore defence chemicals in Aconitum nectar discourage generalists in favour of adapted specialists?  

Combining field and laboratory studies – a new approach using novel motion-detection technology

We combined field and laboratory studies using a new, automated digital monitoring system (called Rana) along with manual observations to record bees visiting flowers, together with chemical analyses of nectar and flowers. This also provided an opportunity to show that the new Rana technology was able to answer an ecological question and introduce it to the wider scientific community.

Rana software activates a small, autofocus camera when it detects concentrated movements and combines consecutive “motion” frames into time-compressed, date- and time-stamped movies. The system is controlled remotely, allowing the user to “stand-off” to control settings, view the live video stream and download data (movies) in the field. It continuously monitors all insect visits to flowers throughout the day – far more convenient than spending days sitting in the field and making continuous observations. In addition, because the movies are time-compressed, the data is easy to analyse – typically, daily monitoring (7am – 9pm) is condensed into minutes of recorded motion footage.   

Photo of Sarah Barlow using the new Rana motion-detection system to monitor bees visiting Aconitum in Kew

Sarah Barlow using the new Rana motion-detection system to monitor bees visiting Aconitum species in Kew Gardens (Photo: O.M. Grace)

Photo of Sarah Barlow controlling Rana remotely via a web interface

Rana is controlled remotely via a web interface using a USB tethering connection to a smart phone (Photo: S.E. Barlow)

Is floral chemistry driving observed bee behaviour?

Neighbouring patches of Aconitum lycoctonum and A. napellus growing in established beds at Kew provided the perfect study site for observing bees visiting flowers and for collecting nectar samples for direct comparison of their chemistry with recorded bee behaviours. Over the summer, while plants were flowering and bees were in full swing, we used two Rana motion-detection cameras to capture all insect visits to a marked flowering stem (raceme) of each Aconitum species for a full day. We also carried out manual observations in 2-hour sampling periods during each monitoring day to validate Rana movies. Nectar and flowers were collected from marked racemes at the end of each monitoring period and analysed by liquid chromatography- mass spectrometry (LC-MS). In total, Rana monitored flower-visiting insects to Aconitum species for 244 hours over four weeks and recorded c.500 discrete visits by bees to marked racemes.  

Photo of project placement student, Emily Marr, recording bees visiting Aconitum in Kew Gardens

Project placement student, Emily Marr, recording bees visiting Aconitum in Kew Gardens alongside the new, high-tech Rana monitoring system (Photo: S.E. Barlow)

Photo of bagged flowering stems

Marked racemes were bagged overnight prior to nectar sampling (Photo: S.E. Barlow)

The Aconitum study plants finished flowering in early August and our preliminary analyses from this first season of observations have already given some fascinating insights. As predicted, the principal legitimate visitor to both A. lycoctonum and A. napellus was the long-tongued bumblebee Bombus hortorum, accounting for over 90% of visits to each species. However, the total number of legitimate visits to A. lycoctonum was markedly higher than those to A. napellus

Bombus terrestris attempted to steal nectar from both Aconitum species, but bees would frequently give up after partly chewing through the flower. This behaviour was observed more frequently for A. napellus. Based on these observations we were eager to know if floral chemistry could be driving bee foraging behaviour. 

Photos of bumblebees visiting Aconitum flowers, taken by Rana

Bombus hortorum visiting Aconitum species, captured by Rana, a novel motion-detection monitoring system (Photos: S.E. Barlow)

Analysis of floral chemistry

Phil and Emily have since identified differences in the chemical profiles of A. lycoctonum and A. napellus nectar and flowers that point to a potential ecological role of diterpenoid alkaloids in driving the observed bee foraging behaviours. Nectar from A. napellus comprised a greater diversity of diterpenoid alkaloids, including a relatively high concentration of an alkaloid compound tentatively identified as aconitine – and which was absent from A. lycoctonum (shown in the chromatograms below). Furthermore, the concentration of aconitine was approximately 100 times greater in A. napellus petals than in the nectar, suggesting that Bombus terrestris may be chemically deterred when chewing through the galea rather than from the occurrence of these compounds in the nectar as originally thought. The nectar chemistry of A. napellus may reduce the visitation frequency of its legitimate pollinator, Bombus hortorum, too.  

Chromatograms of alkaloids detected in the nectar of Aconitum species

Chromatograms of alkaloids detected in the nectar of Aconitum lycoctonum and A. napellus. A greater diversity of diterpenoid alkaloids is present in the nectar of A. napellus. (Image: P.C. Stevenson/E. Marr)

Download a high resolution version of the chromatograms

Next steps

A recent study by Phil with collaborators at Trinity College Dublin has found that bees have poor acuity for detecting toxins in nectar at biologically relevant concentrations (Tiedeken et al., 2014). As a result of our recent findings, we now want to find out whether the same is true for aconitine. This work will be done later this year by Prof. Jeri Wright at Newcastle University using Bombus terrestris and following the method described for caffeine in Wright et al. (2013). We are also taking full advantage of Kew’s diverse living collection by repeating the field and laboratory studies using other, late-flowering Aconitum species growing at Kew, so come along to see Rana in action!  Once data analyses are complete we will be able to draw more informed conclusions about the possible ecological role of nectar and flower alkaloids in Aconitum-Bombus interactions. 

-Sarah & Phil-


References

Gosselin M., Michez D., Vanderplank M.Y., Roelants D., Glauser G. & Rasmont P. (2013). Does Aconitum septentrionale chemically protect floral rewards to the advantage of specialist bumblebees? Ecological Entomology 38: 400-407. Available online

Irwin R.E., Cook D., Richardson L.L., Manson J.S. & Gardner D.R. (2014). Secondary Compounds in Floral Rewards of Toxic Rangeland Plants: Impacts on Pollinators. Journal of Agricultural and Food Chemistry 62: 7335-7344. Available online

Manson J.S., Otterstatter M.C. & Thomson A.D. (2010). Consumption of a nectar alkaloid reduces pathogen load in bumble bees. Oecologia 162: 81-89. Available online

Tiedeken  E.J., Stout J.C., Stevenson P.C. & Wright G.A. (2014). Bumblebees are not deterred by ecologically relevant concentrations of nectar toxins. Journal of Experimental Biology 217: 1620-1625. Available online

Vanbergen A.J. & the Insect Pollinators Initiative. (2013).Threats to an ecosystem service: pressures on pollinators. Frontiers in Ecology and the Environment 11: 251-259. Available online

Wright G.A., Baker D.D., Palmer M.J., Stabler D., Mustard J.A., Power E.F., Borland A.M. & Stevenson P.C. (2013). Caffeine in floral nectar enhances a pollinator’s memory of reward. Science 339: 1202-1204. Available online

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