The Bees' Needs (part 2)
In my previous post I looked at pollen - now it's time to consider nectar and how important it is for bees.
Nectar is the sugary fluid that is secreted by plant nectaries. Nectaries can be categorized as either ‘floral’, which means they are found within the flower, or ‘extra-floral’ which are located outside the flower, usually, but not always, at the point where a leaf joins the stem. For pollen-bearing plants, floral nectaries are of the greatest importance because the presence of nectar attracts pollinating insects to the flower. Floral nectaries are usually found at the base of the ovary which means that the insect has to brush past the pollen-bearing stamens to get to the nectar. In effect, nectar is the insects’ reward for being ‘hijacked’ by the flower to distribute its pollen.
The amount and frequency of nectar that is produced varies from flower species to flower species. Even within each species the amount of nectar secreted can fluctuate enormously depending on factors such as the robustness of the plant and the age of the flower. Environmental issues also play a part, such as temperature and humidity.
What is nectar?
Nectar is the main source of carbohydrate for bees. It is basically liquid sugar in the form of sucrose (a disaccharide), glucose and fructose (both monosaccharides), in varying amounts. Nectar also contains vitamins (especially vitamin C and some in the vitamin B complex), minerals (such as potassium and calcium), amino acids, and other substances.
Is all nectar the same?
There are basically two types of nectar. The first contains chiefly fructose and glucose, with fructose making up the greater part. This nectar is generally associated with open, or cup-shaped flowers which, for honeybees, are easy to reach.
Second, there is nectar whose sugar content is predominantly sucrose; this type of nectar is usually found in tube-shaped flowers. The nectaries of these flowers may not be as easy to get to for honeybees, especially if the nectar is in a reservoir at the bottom of a corolla tube (a group of petals forming a tube).
Research has shown that bees prefer nectar where the ratio of each of the sugars, namely sucrose, glucose and fructose, is the same, but this ratio seldom occurs in nature: here bees usually target nectar with a ratio of 2:1:1. (Interestingly, the higher the fructose content, the longer the resulting honey will remain liquid). The sugar content of nectar varies between 3% and 80%: for example, Borago officinalis (borage) nectar contains about 25%; Trifolium repens (white clover) up to 40%; and Origanum vulgare (oregano) reaches almost 80%.
How much nectar do honeybees need?
Each colony of honeybees needs about 120kg of nectar a year: 70kg is consumed during summer months as immediate food and for energy requirements; the balance is converted into approximately 20kg honey to be used as store food during the winter months.
What happens to nectar in the hive?
Nectar has to be processed before it is turned into honey. During processing the sucrose (diasaccharide) content of the nectar is converted into glucose and fructose (monosaccharides). This is a chemical process which is partly carried out inside the forager bee’s honey sac, (also called honey stomach, or honey crop) during her flight from the foraging site to the hive. (The honey sac is used to store and partly process nectar, rather than digest pollen and nectar which is carried out in a separate ‘proper’ stomach. The honey sac is also used to carry water back to the hive.) When she reaches the hive the nectar is passed to a ‘receiver’ bee who ingests it, swallows it and regurgitates it many times, and so the conversion process continues. Each time the nectar is regurgitated, the water content decreases. When the sugar conversion is complete the droplet of nectar is placed in a cell in the comb along with other droplets. At this stage the moisture content may still be too high, so other worker bees within the hive will fan their wings to aid evaporation until the water content is below 19%. This is the point at which naturally occurring yeasts in the honey cannot grow and therefore the honey will not ferment. The cell is now sealed with a wax cap. This capped honey is what is known as mature or ripe honey, and this is what the beekeeper harvests from the hive.
A typical nectar load is between 25mg and 40mg, so, as we saw with the pollen, a huge number of nectar foraging flights have to be made. The majority of foraging bees collect nectar, although 17% will collect both pollen and nectar. It has also been reported that bees will change their allegiance from a pollen crop to a nectar crop but never the other way round.
That's just about it for nectar! Keep an eye out for my next post about bees and their water requirements.
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