Pollen and forage
Text of lecture presented to the Belfast Beekeepers Association
9th January 2012
I’m a novice beekeeper and having taken up interests in the past, I soon found that a bit of formal training and joining my local association or guild was a guaranteed way of gaining knowledge rapidly and in a manner with which I could cope. On joining our Assoc in the autumn of 2008, I soon discovered that beekeeping is a combination of art, craft and science and that beekeepers appear to be busy people with numerous other interests, some of which enhance the beekeeping art. There are some who blend their carpentry skills and make kit. Others are keen cooks or brewers and use honey as an ingredient. Some beekeepers are interested in genetics and enjoy purifying strains of bees or spend their summer evenings in the company of huge numbers of apideas.
My additional interests that enhance my beekeeping activities are mostly linked to plants. Ever since I was a wee girl, I’ve wanted to work with plants. At about age 10 years, I discovered that people who work with plants are called botanists. I hadn’t a clue what a botanist did but I was determined to become one – and I did. My first degree was in botany and throughout my training, I came across pollen in numerous disciplines. It was the my training in past environmental studies, properly known as palaeoecology that introduced me to the science of pollen analysis or palynology and that led to my doctorate, in which I used pollen analytical techniques to reconstruct early historic farmed landscapes in the north of Ireland. I held the professorial Chair in Palaeoecology at queen’s until my retirement 16 months ago. So I’m better acquainted with the pollen than with the beekeeping!
As beekeepers, we know that pollen is essential for the growth and maintenance of the health of our colonies. On the first of the spring days, there are few sights that cheer the heart of the beekeeper more than watching the girls bring in loads of pollen. Using our knowledge of the plants in flower and the colour of the pollen loads allows us to speculate on the sources of the pollen.
What is pollen?
The word pollen is derived from Latin and means fine dust or flour and it has been used since antiquity to describe the yellow material found in flowers. The pollen is made up of individual grains and these are male reproductive cells produced by the Conifers, for example pine or spruce and the higher plants or Angiosperms, for example daisies and primroses. They are the plant equivalent of sperm but unlike sperm cells that can swim and therefore move by themselves, pollen has no independent powers of movement and must rely on a vector to move it from the plant the produced it to another of the same species so that pollination and then fertilisation can take place. The pollen grain makes a sealed box that protects the living pollen grain contents on its travels but this box must be able to absorb water and have some sort of opening so that the pollen grain contents can grow out on the journey to fertilisation.
Vectors include wind, water, bats, birds and a wide range of insects of which flies and bees are important as movers of pollen. Some plants such as the rare bee orchid are shaped to attract specific insects although such plants are rare in the native Irish flora. Here’s a tip. Bee orchids grow at Killard Nature Reserve, beyond Strangford village. They are at their best on the Saturday on which the Queen’s official birthday falls. You’ll see them at their best that weekend.
Unlike many animals that have their sexes represented in separate individuals, many of the higher plants are hermaphrodite – they have both sexes on the same plant and usually in the same flower with the pollen being produced in organs called stamens. These comprise two parts, the filament or stalk topped by the anther which is the pollen producing organ. Once the pollen it contains is mature, the anther splits and the pollen is exposed. Some insects are capable of biting open an immature anther to get to the protein-rich pollen it contains and honey bees are able to do that. A reason why honey bees are supreme pollinators, from a flowers perspective is that they are flower faithful. They remain with the same species of flower and that behaviour increases the chances of pollination and fertilisation. This is an intimate relationship that may be as much as 85 million years old.
Shape and size
In general individual grains are made up of a protein-rich centre protected by an outer wall. This outer wall is made up of a rather waxy coating of various complex chemicals as it known as the Pollenkit. This substance acts as a sun screen that protects the inner living contents from solar damage. It is the thick layer of oily pollenkit on lily pollen that can stain clothing. Within it is the outer grain wall proper, comprised of a very robust and chemically inert natural polymer called Sporopollenin. This material can withstand attack by powerful acids, even hydrofluoric acid although the grains don’t encounter such chemically powerful forces under natural conditions. The grains are remarkably resistant to microbial decay. This layer holds the various pigments that give pollen its characteristic colours; often yellow or orange but also green or pinkish. Coloured pollen is usually associated with insect pollination with wind pollinated plants having pale or colourless pollen. The inner grain wall is made up of cellulose, as are the vast majority of plant cell walls and some pectin, the material that lets jam set.
Individual grains are usually microscopic. Only the largest are just discernible to the human eye. The grains are measured in 1,000,000 of a metre, a unit known as a micron. It is easier to grasp the small size of many pollen grains knowing that a micron is 1000 0f a millimetre and that many of the pollen grains of our native Irish plants fall into the range on 25-40 microns. The smallest pollen grain in the Irish flora is of forget-me-not, at about 10 microns with pine pollen at the larger end at about 80 microns. Some cereal pollen can be up to 100 microns and it is the individual grains of those that are just visible to the naked eye.There is no relationship between the size of the adult parent plant and the size of the pollen grain it produces. The pollen of oak and of buttercup is the same size and also these two pollen types have grains so similar in morphology that they are difficult to tell apart.
Pollen varies in shape with some grains being spherical, some shaped like a rugby ball and others looking like a portion of Toblerone chocolate. Almost all have elaborate patterns on their outer walls and the vast majority also have pores, furrows or both. It is all of these characteristics that enable us to distinguish one pollen type from another.
I was at Clive de Bruin’s lecture last spring and at the end of his talk he took questions. One member of the audience asked him if she could tell which plants had been foraged by her bees by looking at the pollen in the honey. He began his reply by saying ‘pollen analysis is easy’. I felt a chill! When one is learning to identify pollen using a microscope, the first day or so is not too demanding as one is started on the pollen of the native trees and these all look sufficiently different to make the early stages of learning manageable BUT get to day 3 and that is the beginning of the end of your life!!!
Once one starts to tackle the pollen of the native herbaceous plants, the task becomes fiendishly difficult as the characteristics that distinguish one pollen species from another are subtle and one needs to be properly skilled with a microscope to see the characteristics. Couple that with the bewildering terminology used by the identification keys and one gets a measure of the task. In my lab at Queen’s there are students and staff who do ancient landscape reconstruction by analysing partially fossilised dead pollen preserved in peat or lake mud. One never goes through the door without someone saying ‘would you have a look at this grain’ any idea what it is!!??
Spiky pollen myths
There are some wonderful myths about pollen grains, the commonest being that many pollen types have very spiky surfaces that let the grains cling to the pollinating insect. It isn’t true!! Many of the nectar-producing plants have pollen that hasn’t as much as a pimple on the grains let alone a spike. The last plant of the year to be foraged by our bees is ivy and it has a surface covered in a microscopic network. Similarly willow which isn’t pollinated by bees, it is pollinated by wind but in the late winter and early spring, our bees find it a most valuable source of protein-rich food for raising the first of the new brood. Its pollen grains are much smaller than those of ivy but they too have a network surface and no spikes. Willow has male and female plants so if planting willow for early bee forage, make sure it is from male plants that you take cuttings.
A pollen forage plant that is prolific in the British Isles, especially Ireland is the whin or gorse, depending on the part of the country from which you hail. Whin is a plant that thrives in our Atlantic climatic conditions and it is prolific here and also in New Zealand where it was introduced in the early 19th century and is now an invasive alien species. In Europe, whin becomes rarer as one works inland and in Poland it is a rare species. Whin flowers aren’t rich in nectar but they are rich in pollen and they are in flower when the brood is expanding and swarming imminent. Whin also has a smooth wall that is on the thin side.
Perversely, it is the very numerous grains of the daisy and thistle family that have the spiky grains. In Irish beekeeping year, we are delighted to see the first of the dandelions in flower as we expect a good nectar flow and an increase in colony size following. But here is the perverse information. In spite of their spiky pollen grains, dandelions are not pollinated by bees. Dandelions pollinate themselves. It is a scientific mystery why a plant that produces copious nectar has no use for a pollinating insect.
Another native plant beloved of bees and therefore beekeepers is the blackberry that gives the great nectar flow in the early summer. It has a fairly smooth pollen grain and it too pollinates itself. These plants exhibit Apomixis. Blackberries are in the plant family to which the roses belong, the Rosaceae and in this family there are a large number of species important as crops, for example, apples, pears, almonds, peaches, cherries, raspberry and strawberry. These species have open flowers but don’t produce much nectar per flower, meaning that a bee must visit a large number of flowers to get a full load of nectar. That maximises pollination. On the other hand, the simple structure of the flowers means that the bees doesn’t have to push away big heavy layers of petals so obtaining lots of small nectar packages isn’t impeded by the flower.
Honey bees’ use of pollen
The primitive and solitary bees often have hairy bodies and this may be an adaptation to let pollen cling to the body of the bee. In contrast, the body of the honey bee is adapted for pollen collection as the back legs are adapted to carry large loads of pollen. The legs of the insect is covered with stiff bristles which are used to comb the pollen off its head and thorax and transfer it to its back legs. On these are a series of hairy rakes along with a hinged joint called the pollen press. This press can squeeze the loose dust into a compact cake. Ultimately, the pollen is passed on to the smooth surface of the mid leg and held there by the fringe of long hairs that make up the pollen basket.
Pollen has to be carried to the nest as it is there that most of it will be used. It has been estimated recently that up to 60% of the contents of the pollen grain may be protein, a value higher than earlier studies indicated. Bees have a poor ability to digest the wall of the pollen grain. Some species appear to extend the live of the bees, such members of the genus to which the blackberry belongs whilst others do not have the same qualities, an example being that of pollen of the daisy family. Pollen varies in its digestibility and this may be influenced by such factors as thickness of the pollen grain wall and access to the nutritious internal contents via the pores and furrows in the grains.
It is the nurse bees that have the well developed ability to digest pollen (they have copious proteolytic digestive enzymes) which is an essential component of food for brood and royal jelly. Protein-rich materials are also distributed to the other members of the colony once pollen has been digested by the nurse bees. The foraging bees have little ability to digest pollen.
The mechanisms used by bees to regulate the amount of pollen they need is complex and at present, poorly understood. Clearly, foragers do not keep on bringing in pollen unless it is going to be used and demand for pollen will depend, in part on the weather conditions and the laying capacity of the queen.
Forage is for nectar as well as pollen
Sugar flow rate and sugar concentration appear to be assessed as part of the decision making process for nectar forage. Individual bees do not collect equal amounts of pollen and nectar. They collect mostly one or the other, depending on the colony’s need.
Our native and introduced flora
In Ireland, most of the plants foraged by bees are herbaceous, we have a limited number of native trees that provide honey bees with anything worthwhile. Holly and ivy are native species that provide well for the honey bee. We value the honey from lime and horse chestnut but neither of these trees is an Irish native. Small leaved lime is native to Great Britain but not Ireland and the horse chestnut is native to continental Europe. We have a good range of smaller woody plants and numerous native herbaceous plants that provide well for our bees. Examples are clover, dandelion, thyme, hawthorn, knapweed, thistles, heather, rosebay willow herb. As beekeepers, we need to be aware that many of these species are under pressure.
I have my hives in a meadow on the County Down coast and it is one of the very few remaining in the area that hasn’t been put to the plough for silage production over the last couple of decades. I guard it with my life. The man who cuts my hay knows I don’t want the land to be sprayed or improved by ladling on fertiliser. East Ulster hay meadows aren’t particularly rich in species of plants that feed bees. Indeed they aren’t even particularly rich in species of grasses. My meadow appears to have a stable vegetation system. There are plenty of vetches amongst the grass in the spring and after the hay is cut, there’s usually a good flowering of the autumn hawksbit, one of those yellow dandelion-like plants in which our native flora is quite rich. We need to safe guard such places. There are more plant species native to GB than Ireland and a goodly number of these are good forage plants but beware!!! Most of the bee texts are written with a southern English slant – I can say the same of most British natural history texts – so keep in mind that we in Ireland aren’t so well furnished when taking advice from the written word.
Borage is a crop that feeds bees well. We see it being grown fairly commonly now. There are some of the new invasive aliens that are the bane of the conservationist’s life but a boon to the beekeeper. The Himalayan Balsam was unknown here outside of garden circumstances until the 1950s. It had been introduced as a garden ornamental in the 19th century and in the 1950, it began to expand its range throughout the British Isles. This handsome plant grows along water courses and takes over, to the detriment of species richness of plant and invertebrate alike. The flowers are a good source of nectar and pollen in the later summer. One might say that it is the only invasive alien that has any redeeming features. The great villain of the tribe of invasive alien plants is Rhododrendron, whose honey is poisonous. I don’t know how much one has to eat to feel the effects.
Garden flora in Northern Ireland is rich as our mild climate suits plants from the temperate regions of the world.
Other things one can do with pollen.
I have mentioned that ancient environments can be reconstructed using pollen fossilised in lake muds and peats. The grains become fossilised because they are protected from microbial decay as in cold wet environments there is next to no oxygen and decay can’t take place unless both water and oxygen are present. We have good records for the ancient vegetated landscape of the British Isles, Europe and much north America through studies of fossil pollen produced by plants that lived thousands of years ago. This information is of great value to people trying to conserve landscapes and it is also of value to climatologists. As plants respond to varying extents to climate, having a grasp of the vegetation of past systems allows climatologists to determine if systems today are similar to or greatly varying from those of the past.
Pollen can be used to determine the source of the plants that have been foraged to make honey and also to indicate where the bees have been feeding. Some years ago, I was asked if I would look at brushings off bees to ascertain if they had been feeding locally. I replied that I couldn’t give such specific information but that the information gained would help to discover if the bees had been exploiting plants native to our bit of Europe or had been feeding on plants not native to that part of the world, provided that they weren’t living close to a well stocked Botanic Garden! In the case in which I was involved, the pollen was like no other I’d seen and it was dominated by the pollen of the daisy family. I speculate that the bees had been working flowers in South Africa. The official’s face told me that I wasn’t too far off the mark.
Pollen analysis of forensic material has been used reliably in court cases. In such instances, the pollen analysts has to be very sure indeed of their identifications.
I think it was in An Bechaire that I read recently that a new EU directive will state that honey contains pollen rather than pollen being an intrinsic part of honey. I suppose in the strictest sense, pollen is not a component of honey as this substance is a natural product of nectar but how does one have honey that won’t contain pollen? I fear we have yet another example of pressing naturally produced foods into a mould best fitted by highly processed foods where every stage can be named and measured; a sad reflection on our diet and eating habits.
It is believed by some practitioners that hay fever symptoms can be relieved by eating local pollen or honey. I’ve not seen any studies that show that this practice is helpful and I must admit to being somewhat bamboozled by the claim as hay fever is usually caused by an allergic reaction to the pollen of wind pollinated plants, such as grasses and earlier in the season, some trees. Pollen derived from insect pollinated plants or honey has scant association with the pollen that causes the problems.
Global agricultural importance
It has been estimated that 10-15% of all human foodstuffs are pollinated by honey bees and the same amount for animal feedstuffs. The importance of forage and pollination to the global trade network can hardly be overestimated.