Pollen, Nectar, and our honey bees by Edward Hill
Nectar is an important source of carbohydrates for bees, but not all plants produce it. Plants such as wheat, barley, oats and rice are wind pollinated and do not produce nectar. It is mainly flowering plants which are visited by a pollinating insect that produce nectar.
Some flowering plants are hermaphroditic; botanists call them dioecious. This means that they have both male and female sexual parts within the same flower. Plants such as dandelions, tomatoes, cucumbers, pumpkins and chestnut trees are dioecious.
The advantage here is that they do not require insects to pollinate them, as gravity or wind will transfer pollen from the male stamens to the female style. A visit by an insect to collect nectar from a flower will improve the chances of it being pollinated, as the visiting insect will knock pollen off the male stamens onto the female stigma.
With plants things are not always simple. Some plants have either separate male or female parts on different plants and require pollen transfer from one plant to another of the same species. Some plants have separate male and female flowers on the same plant*. plants can change their sex over time – they will open first as male, then become female, eg. some Acer species.
The transfer of pollen from the male part of a plant to a female part is called Pollination. Fertilisation is the next stage, and this only occurs when the pollen grain travels down the style to fuse with the ovary of a plant that is receptive. Apple pollen will only fertilise apples, pollen from pear trees will not pollinate apples, and pollen from pears will not fertilise apples or damsons.
Apples usually have five ovaries and each one must be pollinated for a perfect fruit to develop; if they are not, this is referred to as incomplete pollination and you will get a misshapen or lopsided fruit. It is still edible to you and me, but probably under some EU law they cannot be sold. Poor pollination of flowers on trees in an orchard will result in both poor yield and low-quality fruit for the grower.
Pollination is important for plants as they are immobile. To produce seed to propagate themselves, they require their pollen to be dispersed and transferred between plants of the same kind. Depending on what you read, over 80% of flowering plants are said to require pollination from either insects or animals for fertilisation to occur. The largest group of pollinators in our part of the world are bees, either Honey bees, Bumblebees or Solitary bees.
In agriculture, these insects have a large role to play. A lot of fruit like apples, strawberries and tomatoes, vegetables, and oil-producing crops like OSR improve their seed set if visited by a pollinating insect.
Back to the start – nectar is produced by plants and is collected by insects in this part of the world; in other countries bats, moths and birds visit flowering plants for nectar. They visit plants to obtain nectar as a food source.
Beekeepers believe that plants evolved a means of attracting insects to visit and collect pollen by developing nectaries, which are secretory organs that produce scent and sweet sugary substances as a reward.
Further research may indicate that nectar is the reward and not the attractant. There are other things to take into account like colour, shape, size and scent of flowers which are also attractants. If there is no reward, then maybe insects would stop visiting.
Nectary glands are located in various places, depending on the flower. They can usually be found inside the flower at the base, but they can also be found on the petals, sepals, stamens, anthers or ovaries. Some plants have their nectary glands on the outside of the plant; these are called extra floral nectaries, and can be found on Elderberry, Portuguese laurel and Passion flowers to name a few.
These nectaries are believed to have developed here to attract ants, which deter other browsing animals from eating the plants.
As a beekeeper, nectar is important as a source of carbohydrates for our bees, but more importantly it is the main ingredient of honey.
So, what is it? It is mostly water containing a number of dissolved substances which include sugars, vitamins, organic acids, aromatic compounds and plant enzymes. It is the sugars which are important to bees. Glucose, fructose and sucrose are secreted in the plant sap.
Plants contain different proportions of these sugars, for example OSR and dandelions contain high levels of glucose, while apples and pears are high in fructose.
When a flower is pollinated, it usually stops secreting nectar, although other flowers on the same plant that have not been pollinated will go on secreting nectar. Almonds are unusual in that they still secrete nectar after pollination; it is suspected that it has something to do with feeding the developing fruit.
There are a number of factors such as wind, humidity, soil conditions, time of day, soil type, temperature, and age of the plant that affect the secretion of nectar.
Temperatures are important for the secretion of nectar. Blackberries seem to produce nectar in low temperatures, while lime trees need warm days and nights. Exotic plants from Mediterranean countries growing in Ireland require high temperatures to produce nectar.
Wind and Humidity: High winds and low humidity may cause nectar to dry up and prevent pollinators from flying.
Soil moisture: This is usually dependent on the time of year, but plants that are under stress and wilting will not secrete nectar, while deep rooted plants like trees can cope with extremes. However, the effects of constant flooding or drought for long periods will not be noticed for a few years afterwards.
Time of day: Nectar is usually secreted later in the day when water has been evaporated from the flowers. Pollinators often visit flowers for pollen early in the day and collect nectar from the same plants later.
Soil: A lot of plants will grow on most soils, but some have specific requirements for them to flourish. Both heather and bilberry require acid soils, but blackberries will grow well on either acid or alkaline soils, and clovers grow well on lime-based soils.
Age of plant: young plants in their juvenile stage growing in their preferred soil type produce the best nectar. Heather is an example of this; young plants which grow and produce flowers after burning produce the best nectar, while old woody plants with few flowers produce less.
Trees such as apples which are pruned and kept in a juvenile state with lots of flowers produce nectar, while older neglected trees produce fewer flowers and less nectar.
Shading: Plants require sunshine and warm temperatures to secrete nectar; those growing in shade can sometimes struggle. Having said that, some plants are only suitable for growing in the shade (these are mostly garden plants).
Topography: Plants growing in waterlogged conditions can struggle, but some like alder and willow which are visited by pollinating insects can thrive in these conditions. Others like heather which can grow on steep south facing slopes are said to produce lots of nectar. Frost pockets can delay or stop the secretion of nectar by plants.
Without flowers many plants would not be able to reproduce, and without secreting nectar as they would not attract pollinating insects; thus they would be unable to transfer their pollen to reproduce.
Without flowering plants secreting nectar we as beekeepers will not have a honey crop to collect at the end of the season. Therefore, it is important to locate our honey bees where they have access to a supply of both nectar and pollen throughout the season for our colonies to thrive.
(*Note: Chestnuts produce male and female parts on the same flower, but they also produce male and female flowers on separate spikes)