The science of nectar

Nectar is that sweet reward that flowering plants provide animals in exchange for their services as pollinators. It sounds incredibly simple on one level – much like rewarding a dog with a treat after it obeys a command. However, dig a little deeper and you realise that the reproductive success of the plant is dependent on very subtle yet complex characteristics of this substance – including when it’s produced and how much is produced, as well as its very composition.
Flowering plants will optimise the characteristics of their nectar in order to influence the foraging behaviours of pollinators and ultimately improve their reproductive fitness. The characteristics of the nectar not only determine which pollinators are attracted and when they come, but how frequently they visit and how long they stay. Suddenly one realises that there is an extremely complex system of regulatory mechanisms behind nectar secretion, which have not only influenced the evolution of flowering plants, but of the pollinators themselves.
Red admiral butterfly close up
Red Admiral butterfly (Vanessa atalanta) drinking nectar. 
Photo credit: Shelby Temple.

Nectar isn’t just about sugar

But before we get into the evolution, let’s first consider what nectar is, because as it turns out it’s not just about sugar – there are a number of things in nectar that are important for pollinators.
There is no denying, however, that carbohydrates – sugars such as glucose, sucrose and fructose – are usually the main constituent of nectar. Nectar will be anywhere between 7 to 70 % carbohydrates per water weight [1]. Other sugars might also be present in small amounts as well as sugar alcohols, such as sorbitol. It is these sugars that are the primary energy source for nectar consumers.
Amino acids and proteins are the next most abundant solute in nectar after the sugars. There are essential and non-essential amino acids, which are the building blocks for proteins and there are some non-protein amino acids that are constituents of enzymes and preservatives. It is thought that the amino acid and protein content of nectar may play a role in the taste preferences of insects [1], presumably related to their nutritional needs.
The water content of nectar may also be an important reward for pollinators, particularly in dry habitats.
Nectar also contains important ions, such as potassium, as well as antioxidants, trace amounts of lipids and some secondary compounds that seem to be associated with resistance to herbivory. 
Macro photography bee
A bee gathering its nectar reward in the Botanic Garden.
Photo credit: Shelby Temple.
Many species have also been shown to have antimicrobial compounds in their nectar, which prevents microbes from growing in the nectar as well as inhibiting florally transmitted diseases [2].
Terpinoids, which are the volatile organic compounds that give flowers their scent, also accumulate in the nectar.
The composition and consistency of nectar is extremely variable as it is tuned to the needs of the nectarivores (it’s a word…really). Flowers frequented by hummingbirds, for example, generally produce nectar in small amounts with high sugar content, while those frequented by more generalist passerine birds produce dilute nectar in large quantities. There has been some evidence that honeybees have a preference for warmer nectar that’s less viscous, regardless of the sugar concentration [3]. Bats also seem to prefer less viscous nectar, though will preferentially select more dilute nectar as the water content is extremely important for their rehydration.

Not all nectar is produced in the flower

Nectar is produced in glands known as nectaries. The glands are commonly found at the base of flowers, where they produce nectar as a reward for pollinators. However, there are also extrafloral nectaries located elsewhere on the plant, often on the leaves or petiole – the stalk that attaches the leaf blade to the stem. These nectaries provide a reward for mutualistic animals, almost exclusively ants, which benefit the plant. The ants help protect certain plant species by getting rid of the eggs of herbivorous insects deposited on the foliage and in return they feast on the nutrient rich nectar secreted by the extrafloral nectaries.
Extrafloral nectaries might be particularly critical at certain times in the plant’s lifecycle. For example, there are often nectaries located on the pedicelthat secrete nectar when the flowers are in bud. This attracts ants, which help protect the vulnerable flower buds from herbivorous insects and improves the reproductive success of the plant [4].
Unlike nectar produced in the flower, nectar produced in the extrafloral nectaries is far less variable as it is attracting mostly ants.

Darwin’s orchid: a classic example of the coevolution of flowering plants and their pollinators

Producing nectar may use up to 37% of a plant’s available energy [5]. This means that producing it comes with some cost to the plant, but these costs are clearly outweighed by the benefits of attracting pollinators that are far more efficient than relying on wind or water.
The evolution of flowering plants and their pollinators is the most frequently used example of coevolution – the physical characteristics of both flower and animal evolving to become more specialised. It was around 120 million years ago that honeybees developed longer tongues than their short-tongued ancestors in order to access the nectar reward flowers had started to produce. Their social structure became more complex and they became fuzzier and developed pollen baskets in order to carry protein-rich pollen, but also facilitating their role as pollinators.
Darwin’s orchid in bloom at the Botanic
Garden last year. Photo credit: Andy Winfield.
The flowers also changed shape in response to the preferences of their pollinators. The most classic of these examples is Darwin’s orchid (Angraecum sesquipedale) with a flower depth of 20 to 35 centimetres. The Madagascar orchid was named after Darwin because he proposed, based on its shape alone, that it had to be pollinated by an insect with a proboscis of lengths unheard of at the time. Forty years later, Morgan’s sphinx moth (Xanthopan morganii), was discovered with an unusually large proboscis…and it was indeed the pollinator of this orchid.
It is also thought that nectar chemistry itself has evolved in response to pollinators. As mentioned earlier, bats prefer nectar with low sugar concentrations and as a result bat pollinated plants from very diverse and distantly related taxonomic groups have evolved nectar with low sugar concentrations.

Deceit and robbing

Not all flowers use nectar – some have non-rewarding flowers. Around 30-40% of species within the orchid family do not produce rewarding nectar in their flowers [6] and instead use different methods to attract pollinators. Orchid flowers may look like another species that provides nectar or they may mimic shelters or brood-sites or even pollinators themselves in order to draw the attentions of individuals looking for a place to shelter or for a potential mate (such as in bumble-bee orchids).
Just as plants have found ways to get pollinated without producing nectar, some animals have found ways to get nectar yet avoid being pollinators. Some flower visitors – known as nectar robbers – will avoid the normal route to the nectar, usually avoiding the floral opening all together and pierce or bite the flower elsewhere to extract the nectar directly without coming into contact with any of the reproductive parts.
For many years it was thought that nectar robbers had a negative or neutral effect on the plants, but over the last couple of decades, research has shown they can also have a positive effect on the plant. Firstly, some nectar robbers do ultimately end up pollinating the plants. Secondly, their presence can modify the behaviours of the pollinators. For example, if flowers have less nectar (because the robbers have extracted some) then pollinators will visit more flowers, increase their foraging range, travel further distances and spend less time at each flower – all of which could improve cross pollination and increase genetic diversity. Maloof et al [7] provide a good review on this topic.  
There has been extensive research done on the characteristics of nectar and its relationship with pollinators. More recent research, however, is starting to unravel the mechanisms by which plants produce nectar – identifying some of the pathways sugars are transported within the plant and concentrated in their nectar [8]. There is still lots to learn.


[1] Pacini E, Nicolson SW (2007). Chapter 1: Introduction, In: Nicolson SW, Nepi M, Pacini E (Eds.) Nectaries and Nectar. Springer: The Netherlands. ISBN: 978-1-4020-5936-0. (pages 8-10).
[2] Sasu MA, Wall KL, Stephenson AG (2010). Antimicrobial nectar inhibits a florally transmitted pathogen of a wild Cucurbita pepo (Cucurbitaceae). American Journal of Botany 97 (6): 1025-1030. (link)
[3] Nicolson SW, de Veer L, Köhler A, Pirk CWW. Honeybees prefer warmer nectar and less viscous nectar, regardless of sugar concentration (link).
[4] Bentley BL (1977). The protective function of ants visiting the extrafloral nectaries of Bixa orellana (Bixaceae). J. Ecol. 65 (1): 27.38.
[5] Pyke GH (1991). What does it cost a plant to produce floral nectar? Nature 350: 58-59. doi: 10.1038/350058a0
[6] Johnson SD, Hobbhahn N, Bytebier B (2013). Ancestral deceit and labile evolution of nectar production in the African orchid genus Disa. Biol. Lett. 9 (5): 20130500. doi: 10.1098/rsbl.2013.0500.
[8] Lin IW et al.(2014). Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9. Nature 508: 546-549. doi: 10.1038/nature13082

Bee and Pollination Festival Was Buzzing

Standing in the marquee at the Bee and Pollination Festival felt as close to being in a hive as one could imagine as it was absolutely buzzing with activity! As I weaved my way between the stalls I caught little pieces of conversations going on –“…beekeeping is not easy…first thing you need to do is take a course…”, “…which shutter speed should you use if you’re trying to get the wings of a bee in flight…” and “…have you heard of colony collapse disorder…” . My 5-year old was busy making a giant paper bee, while my husband spoke to the folks at the allotment display and I was admittedly taste testing some of the amazing local varieties of honey. There was something for everyone.

The University of Bristol Botanic Garden hosted the 4th annual Bee and Pollination Festival on the 7th and 8th of September, and as usual, people came out by the hundreds to celebrate bees and other pollinators that perform an essential ecosystem service. We do love our bees!
Nawbash helps a young bee enthusiast spot the queen.

Becoming a beekeeper

I happened to be looking at the display frame set up by the Bristol Beekeepers Association when a gentleman sidled up to Sue Jones, one of the beekeepers on hand to chat with the public, to ask about how one gets into beekeeping.
Sue quickly lays it on the line for him by saying “beekeeping is not easy”, but she quickly adds that some courses and hands-on experience are the first steps one needs to take to get into beekeeping. She’s not trying to discourage anyone from beekeeping, she, like anyone who has kept bees, knows that it is not something you enter into lightly.
Morgan (left) and Zippy (right) making a giant paper bee.

Nawbash Mohammed is another beekeeper on hand and so I begin to speak with her about being a beekeeper. The Bristol Beekeepers Association runs beginner beekeeping courses during the winter to cover the theory about beekeeping. The courses run over three Saturdays. Then, over the spring and summer, the Association offers practical education to cover all the hands-on aspects of beekeeping from opening a hive to handling the frames.

Nawbash has been a beekeeper since 1997, first in Iraq and then when she moved to Bristol in 2011. I ask her about beekeeping in Iraq and how it differs from keeping bees here in the UK. “The principles are all the same,” she says, “but it’s just very different weather and the honey has a very different taste”.  Nawbash describes for me the extremely unique taste of a premium honey in Iraq, made from the nectar collected from mountain flowers. I have to admit to her that this serene image of bees moving from flower to flower in mountain meadows is not the image that comes to mind when I think of Iraq – but I suppose it is the evening news that I have to thank for that. She admits there were additional challenges associated with being in a country fraught with war and political strife, but for the most part the challenges were the same facing beekeepers around the world – disease, mite infestation and colony collapse disorder.
Some of the honey on display and for sale at the festival.

Feeling thoroughly enlightened, I head over to buy a jar of Henleaze honey as I live in the neighbouring community – can’t get more local than that! “That’s my honey!” Nawbash laughs as she sees the jar I pick! Local, and I’ve met the beekeeper…brilliant!

The full experience

After speaking with Nawbash, I found my son and husband floating about the delicious cakes on display at the Bramble Farm table. Bramble Farm is a small landshare farm in Bristol. They keep sheep, pigs and turkeys and grow lots of veg, most of which goes to support the families that share in the upkeep in the farm, but any extra is sold at events such as this.
At the foot of the table lies a basket of some of the largest courgettes I’ve ever seen! However, it is some decadent chocolate cake that has caught the attention of my family! A little hint of fresh mint in the chocolate – delicious!
The apple press that kids took turns operating.

I stop briefly to look at the schedule of activities for the day as I don’t want to miss the demonstration hive talk. In doing so, I start to chat to a woman who is one of nearly one hundred volunteers that are there helping make the weekend’s events run smoothly. Jen Ellington is a committee member of Friends of the Garden as well as one of the Welcome Lodge Volunteers. She’s also opening up her garden next month as part of the Friends’ Open Gardens Programme – each year the Friends open their gardens, large or small, to raise funds for the Botanic Garden. Last year, Jen’s Gardyn raised over £400. Not bad considering her garden is only 15’ x 31’! However, don’t judge the garden by its size as it sounds as though there is plenty to see in this space. “We can’t go out anymore,” said Jen, “so we’re going up – I’m claiming my airspace!”  Within two minutes of listening to Jen’s description of her little garden haven, I’m hooked – so stay tuned to hear more as I will definitely be attending the open garden! The tour is Sunday, 6th October from 2-5pm and you don’t have to be a Friend of the Garden to attend. The address is 4 Wroxham Drive, Little Stoke.

People gather around the demonstration hive.
Next we join the crowd outside the tent that is watching some children work a small apple press to make fresh apple juice. My son obviously has to have a go…after all, we need to wash down the chocolate cake!
We quickly make our way over to the display hive where the beekeeper is taking apart the hive to show the crowd the combs and what it’s like to work a hive.  As the smoke from his smoker rises up through the crowd, the beekeeper explains that “everything runs on pheromones in the hive. As soon as I open the hive, alarm pheromones will be released saying there’s an intruder. The smoke masks those pheromones. So you don’t ever let your smoker go out” – sage advice to any budding beekeepers in the audience.

Always something new to see

Ethel standing proud in a temporary position for the festival.

Of course, we can’t leave without touring the rest of the garden and I must say that the warm days of summer have made things all rather lush.  As my son stares into the pond outside, looking at what seems to be hundreds of dragonfly larvae, I admire the grape vines laden with fruit.

I also notice that Ethel, the giant willow moa bird sculpture, is finished and is on display!
There is a potted orchard, which is new – apples, blueberries, pears, figs, olives, plums and other edible delights line one of the pathways – another reminder of why we should celebrate pollinators!
Down near the glasshouses, Writhlington School has an extraordinary orchid display and inside the glasshouses the lotus plants are in bloom.
A pollinator at work in the garden.

I’ve said it before, but there really is always something new to see in the garden with every season. This weekend marked my second Bee and Pollination Festival and the anniversary of this blog. Having written the blog for the Garden for a year now, I also get to have some insight into some of the plans for the garden and new displays that are on the horizon. I have to say that I have been impressed to no end at how quickly things seem to turn from idea to reality in this garden. Of course, the staff and volunteers that are there every day doing the grunt work behind it all, may feel differently, but for someone who is there every few weeks, things seem to move at an incredible pace. It’s been a wonderful year and I look forward to sharing more about the people, plants, events and research that goes on in this beautiful garden.

There's plenty of room at the bee hotel

Andy gently pushes some moss out of the way to allow me to peer in. “See there,” he says, “they’ve moved some of this moss and built that wall – this is occupied”.  I’m staring into one of the rooms of the hotel trying not to invade the guests’ privacy, but also too curious to look away. The occupant seems to be out getting a meal or tucked away so as not to be seen by peeping eyes.
An occupied suite at the Botanic Garden’s Bee Hotel.
We are standing in the wildlife area of the Botanic Garden, behind a city skyline of wooden planks. We are staring intently into what might at first glance look like a very artistically and precisely stacked woodpile.  However, this is indeed the Garden’s bee hotel – the sign above it even says so – and there are guests!
The Garden had quite a bit of bamboo left over from the construction of the Chinese herb garden, as well as other materials from some coppicing they had done, and what better way to use them than to create habitat to encourage native bees.

There are over 250 species of native bees and about 90% of these are solitary

Bee-opolis – a city skyline of wooden planks also serves as
potential habitat for solitary bees
Though we often think of hives humming with tens of thousands of bees, most bees in the UK actually lead a solitary lifestyle. This means that a female bee will find or make a burrow where she will rear her larvae on her own.
The nest type and habitat requirement of each species is a little different. The British Science Association created a very informative videoin 2009 as part of their ‘Save Our Bees’ campaign, which talks about the habitat and nests for a variety of native bee species including leafcutter bees, masonry bees, tawny mining bees, cuckoo bumble bees  and carpenter bees.

There’s something for everyone at the bee hotel

The Bee Hotel
The bee hotel was constructed with many possible occupants in mind as well as their diverse habitat requirements.
On the ground floor of the hotel there is a section of large diameter plastic piping that has been sealed at both ends. Protruding out of the side of the pipe is a small piece of bamboo. This is ideal bumblebee habitat as the bamboo provides a narrow entrance but then opens up into the larger pipe.
There are 24 species of bumblebee in the UK, but only eight of these are common. Bumblebees build communal combs either underground or in long tussocky grass, with narrow entrances to discourage curious predators. This ground floor suite of the hotel fits the bill nicely.
As you move up from the ground floor you notice that a diversity of materials have been used including bricks and branches as well as bamboo.
Up closer to the penthouse suites it is mainly bamboo sections that have been stacked and the ends stuffed with moss. The one we are looking at has clear evidence that it has been occupied. Some of the moss has been moved aside and a mud wall has been constructed save for a small hole, about a ¼” in diameter, that is clearly the entrance.
The entire hotel is kept dry with a living roof with semperviren succulents and a skillfully woven willow ‘Bee Hotel’ sign to top it off.
Andy is clear that this is the Garden’s first foray into bee hotels, but this evidence that it’s being used within the first year of construction is encouraging. He is hoping that the diversity of habitats created within the hotel will attract lots of different insects, not just bees.

‘Hopefully it will encourage people to do something in their own garden’

There is global concern about the welfare of bee populations as loss of habitat and food sources, disease, and widespread use of toxic chemicals take their toll on these insects. The loss of bees has significant implications for food security as well as healthy ecosystems in general.
The staff at the Botanic Garden hope the Bee Hotel not only attracts insects, but also inspires visitors to the garden to construct habitats at home and help build resilience for our native pollinators.

Bee habitat doesn’t need to be complicated

Bamboo or branches cut into 10 inch lengths or so with 1/8-3/8” holes drilled 3-8 inches deep are ideal habitat for solitary bees. Bundle a group of these lengths together and hang them in a dry place and you’re done – simple. You can even use dried stems of raspberries, brambles and elder or other similarly sized hollow cane-like vegetation bundled together. 
The key, however, is that the material needs to be dry and it needs to stay dry. It is the wet rather than the cold of winter that can threaten these animals, so keeping the habitat dry is essential.

This is also a great project to do with children – from collecting the materials to maybe even some supervised drilling and then finally watching to see who moves in. I’m looking forward to building some habitats with my son…as well as watching the bee hotel at the Garden over the next year or so to see who has come to stay!

Photos of pollinators at work in the garden this week:

Through the eyes of bees

One of the many fabulous things about the Botanic Garden is that on any given day, you may find scientists out there conducting cutting edge research. There are currently at least seven research programs going on either directly or indirectly with the garden, making it not only a place of beauty, but also a place of scientific discovery.

I recently had the opportunity to speak with a researcher from the University of Bristol’s Ecology of Vision group, who was in the garden photographing flowers with the most bizarre looking camera. I generally take notice of cameras anyway, but this one could hardly be ignored. It was essentially a metal box perched atop a tripod with a lens protruding out one end and an abundance of wires to connect it to a laptop protruding out the other. It simply begged the question – “what is that and what are you doing with it?”
“It’s a POL camera,” said James Foster, a PhD student in the School of Biological Sciences, “we’re imaging the polarized light reflected off flowers. Humans don’t have polarization vision, so we use this camera to create an image that allows us to see what animals with polarized light sensitivity, such as bees, can see. We want to see what flowers are like from a bee’s point of view.”  
James Foster, a PhD student in the School of Biological Sciences,
is using a special camera, built by Dr. Shelby Temple
and Dr. Nick Roberts, to photograph flowers in the
Botanic Garden. The camera characterises and quantifies
all aspects of light polarization and will help researchers see
the flowers from the bee’s point of view.
If, like me, you aren’t confident in your understanding of polarized light, here are the basics. Light travels as a wave, oscillating as it moves through space. As it travels, the wave can oscillate in any number of orientations, up and down, side to side or any angle in between. Polarization refers to either oscillation orientation of the wave (the angle of polarization) or how many waves oscillate in the same direction (the degree of polarization).
Though humans aren’t sensitive to polarized light, many other species are, including many important plant pollinators such as bees.  It’s been known for about 60 years that bees use patterns of polarized light in the sky to navigate, but are they using polarized signals in other aspects of their daily behaviours?  This is the question Foster is trying to answer as part of his PhD.
“We know that it’s the upwards facing portion of the bee’s eye that is most sensitive to polarized light, so we’ve been looking specifically at downwards facing flowers,” said Foster. “Those that seem to be most popular with the bees are usually found as clusters on an inflorescence, often where there are less mature flowers at the top and more mature flowers at the bottom.  I don’t expect that the bee is using polarized light signals to identify the flower as a flower, but once it gets to the flowers it might be using those signals to optimize its foraging activities. For instance, it could influence the bee’s decision to stay on a lower, more mature flower that may be more depleted in nectar or move quickly up to younger flowers that may have more nectar.”
The research is still in its early stages, but if Foster can demonstrate differences in the polarized signals of mature flowers versus younger flowers that also relate to differences in nectar availability, this will be a first step in determining whether bees are using polarized signals for more than just navigation. 
Recent research conducted at the University of Bristol and the University of Cambridge, has shown that conical cells on the surface of the petals of many flowering plants help increase grip for visiting pollinators and are particularly important when the flowers are moving (as they often do in a naturally breezy world).
“Those conical cells would also reduce the degree of polarization of reflections coming off the flower,” added Foster. “What we predict is that there will be areas that will be rich in these conical cells, for gripping, but there will also be regions where the cells are completely flat and these areas will allow polarized reflections that may be important signals for pollinators.”
It’s a tricky thing trying to study a sensory world that we are essentially blind to. However, it probably means that you might see more of James wandering about the garden, POL camera in tow, trying to see the flowers through the eyes of bees.