Wednesday 7 January 2015

The Military Orchid and the Bumblebee

Ryan has nearly finished collating all the records you sent in for the BSBI New Year Plant Hunt, and he and Tim are all set to start on the analyses, but in the meantime - here is something completely different, which Ryan prepared earlier! 

Bee with pollinaria attached to mouthparts
Image: R. M. Bateman
The mail-out of New Journal of Botany has been held up by the holidays, although it has now been published on-line (members only, sorry) and we understand that print copies will be mailed out any day now. 

But as a pre-Plant Hunt treat (because I knew we would be working him hard over New Year!), I gave Ryan an advance peek at one of the forthcoming NJB papers and he has very kindly prepared this summary for you:  

"The process of pollination is essential for both wild plants and crops to reproduce. In Britain it is mainly insects that pollinate our wildflowers and crops. Without these insects many rare wildflowers are vulnerable to not being able to reproduce successfully and are therefore liable to extinction. Pollination usually results in the setting of seeds which allows plants to move geographically - this is thought to be increasingly important as plants in Britain move northwards in response to warming temperatures and global climate change.  We currently know very little about the pollination ecology of some of Britain's rarest plants, even though this knowledge will be essential if we want to anticipate how these plants will respond to future climate change.

Richard and Ian orchid-hunting in Ireland
Image courtesy I. Denholm
"The Military Orchid (Orchis militaris L.) is one of Britain's rarest flowering plants and is anecdotally thought to be solely pollinated by small species of solitary bees; honey bees (Apis mellifera) and bumblebees (Bombus spp.) supposedly do not pollinate this orchid species. 

However, research presented in the latest issue of New Journal of Botany suggests that this may not be the case, according to a paper entitled 'Bumblebee-mediated pollination of English populations of the Military Orchid (Orchis militaris): its possible relevance to functional morphology, life history and climate change'. 

The paper is co-authored by Richard Bateman and Paula Rudall, both eminent botanists at Royal Botanic Gardens, Kew and frequent contributors to New Journal of Botany. Richard is also one of two BSBI Orchid Referees, the other being BSBI President Ian Denholm.

Richard with BSBI Irish Officer Maria Long
at BSBI Recorders' Conference 2013
Image: L. Farrell
 "Bateman and Rudall observed the pollination events involving a group of military orchids in Homefield Wood (Buckinghamshire) for one hour on the 31st May 2014. This population was used as it is well documented and is thought to be well established. Three slug damaged flowers were then collected in order to be digitally imaged using advanced microscopy techniques and a scanning electron microscope. 

"Studies elsewhere in Europe have indicated that the Military Orchid is mainly pollinated by solitary bees and honey bees, so the authors of this paper wanted to find out if this was the case in Britain. In Britain various people have recorded a wide variety of insect species visiting the flowers of Military Orchids, but until now we did not know which species were actually pollinating the flowers. In Bateman and Rudall's study they found that males of Bombus vestalis (Vestal Cuckoo Bumblebee) and female workers of Bombus pratorum (Early Bumblebee) pollinated the Military Orchids at Homefield Woods. The authors were unable to observe an unequivocal  pollination event when Military Orchid pollinaria (a mass of pollen grains) were observed not only stuck to the bumblebees but also deposited on stigmas. On average, each bumblebee spent a few minutes on each orchid spike, investigating many flowers on each inflorescence. 

Bee delving into flower
Image: R. M. Bateman
"The study suggests that pollinaria were often deposited on other flowers on the same flower spike, therefore not exhibiting cross pollination with other plants. The pollinaria attached to the pollen sacs on the hind legs of B. pratorum and the faces of both bumblebee species. B. vestalis does not possess pollen sacs on the hind legs so pollen is not collected there. The bumblebees were observed trying to remove the pollinaria from their bodies but no successful attempts were recorded. 

"The study also suggests that most pollinaria were probably deposited away from the stigma (female receptive organ) of the receptive plant so would not be successful in pollinating the plant. Therefore, although this study shows that it is likely that these two species of bumblebee pollinate Military Orchids, this was not proven.

David Pearman Celebratory Day, RBG Kew, Sept. 2014
Richard chatting to event organiser Jodey Peyton...
Image: BSBI Publicity Team
"The paper also suggests that there may be further implications to bumblebee pollination of Military Orchids, as bumblebees were not observed to visit these flowers frequently in previous studies, suggesting that this is an increasing phenomenon. This may in turn increase seed set of the orchids and expand their local populations. Another bumblebee species, B. hypnorum (Tree Bumblebee), has recently colonised Britain due to climate change.This bumblebee has a similar tongue length to those needed to feed on Military Orchids so it is possible that this species may visit these orchids in the future. The final implication of this paper is that it shows that pollinaria also attach to male bumblebees. Usually in pollination studies it is mainly worker bumblebees that are studied so this research suggests that maybe other bumblebee castes should be factored into pollination studies.

Jodey quickly launches Richard on 2nd career as wine waiter!
Image: BSBI Publicity Team
 "On a personal level, I really enjoyed reading this paper which combines my love for plants and pollinators and congratulate the authors for their sterling work. I am pleased to hear that the Military Orchids seem to be doing well and, as the site is fairly close to me, I look forward to visiting it next year. Hopefully future studies will quantify the pollination events that occur between these species and analyse the effect that novel pollinators may have on the populations of Military Orchids in response to climate change. You can read the full paper in the latest issue of New Journal of Botany. This journal is published 3 times a year and members of BSBI receive hard copies for free along with back issues online and a number of other membership benefits.  All the more reason to join BSBI!"

Many thanks to Ryan for the above and to Richard Bateman for commenting on Ryan's draft and allowing us to publish this summary of his paper. As soon as those Plant Hunt analyses are done, we'll share them with all of you - watch this space!


  1. Interesting! In 2001 I was involved with a project led by the late Bill Havers to get volunteers looking out for potential Military Orchid pollinators at Homefield Wood, with a rota of people watching the flowers for many hours over several days. As described above, at that time bumblebees seemed not to be playing a significant role, and our summary at the time was:
    "Following this year’s observations it seems clear that a range of solitary bee species, as well as the honey bee Apis mellifera, have the potential to pollinate O. militaris. Bumblebees are sometimes attracted to the flowers, but they seem to collect pollinia infrequently, and when they do the pollinia can become attached to various parts of their body, making them less likely to go on to successfully pollinate another flower. In contrast, the honey bees and solitary bees have consistently had pollinia positioned towards the front of the head, on or near the clypeus, where they appear to be ideally placed to effect subsequent pollination."

    So it is surprising that just a single hour of flower-watching produced so much evidence of bumblebee activity. I wonder if bee behaviour has changed, or whether the range of flower visitors is just broader than we assumed and that the ones seen vary from year to year?

    The other mystery is that as far as I know Military Orchid is believed not to produce nectar, so why do insects visit it regularly? Maybe it produces some other substance that they need.

    1. Hi Martin,

      Thank you very much for your comment. It is interesting isn't it! I wish we knew for sure. At least it gives an area for further research. I wouldn't have guessed that bumblebees would have been the main pollinators in favour of smaller species but like you say maybe things are changing. It would certainly be interesting to see what they do produce if not nectar.



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