The importance of bees in crop pollination

Pollinator protection is a hot topic at the moment, and deservedly so. In order to feed a global population of 7.7 billion people, we must control unwanted pests that damage food crops, without killing the insects that help produce the food; this predominantly means bees. Various claims are purported as to the proportion of our food that derives from bee pollination, whether that is 30% or 50% or 90% it is largely immaterial; the key consideration is that bees must be protected along with the food source.

In this context, it is essential to include not only honeybees but also the wild bees, the largely unseen contributors to pollination, such as bumblebees and solitary bees. Nature has evolved over millions of years to ensure that bees transfer pollen and pollinate the plant and receive nectar and pollen in compensation.

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What is the Process of Pollination?

Pollination is the movement of pollen from the male parts of a flower (stamens), to the female part of a flower (stigma). The process may take place in the same flower, or a different flower on the same plant, or a flower on a different plant, and could be carried out by the wind, mammals, birds, but primarily by insects.

When the pollen contacts the sticky surface of the stigma, it may germinate and create a pollen tube which reaches an ovary of the flower where it fuses with an ovule to create a seed. This, in turn, stimulates the formation of the fruit.

Previously, with small fields and rich habitats, the pollination was done by wild bees. Nowadays, with huge fields and depleted insect populations, it is necessary to actively manage the pollination, which is extensively conducted by the honeybee.

Variety and Complexity of Pollination of Crops

To illustrate some of the diversity and complexities of pollination of crops, consider the following examples:

1. Apples

Pollination of the apple is the archetype of crop pollination. Pollen from a tree of one variety is carried to a tree of a different variety. Each flower has five ovaries, and each ovary has two ovules; complete pollination of apples occurs when ten seeds are formed. The presence of a seed produces hormones that stimulate the transport of resources, which make the fruit plump, juicy, colorful and tasty. Even with fewer than ten seeds in an apple, it can be attractive and nutritious, but with fewer seeds, the fruit is less hardy. Growers generally control the amount of pollination carefully, as over-pollination may produce an excess of fruit, which needs to be thinned.

2. Bananas

Conversely, the banana needs no pollination whatsoever, this fruit is produced by the mechanism of parthenocarpy, i.e., the production of fruit without any seeds. Other crops which can set parthenocarpic fruit (but not necessarily) are figs, pineapples, grapes and seedless citrus such as mandarins.

3. Berry Fruits

Berry fruits such as raspberries and strawberries need a considerable amount of pollination. For example, a single strawberry may have 300 stigmas, and a raspberry 90. Thorough pollination of the crop will produce fruit of uniform color, softness, shape, and sweetness, whereas incomplete pollination produces misshapen, discolored fruit with hard, undeveloped patches.

4. Almonds

The commercial pollination of almonds is one of the most extraordinary events in agriculture, with 2 million beehives moved from every corner of the U.S. to California to pollinate 400,000 hectares of orchard. The almond tree is unusual in that it is almost impossible to over-pollinate it. In essence, with no bees, the crop will be effectively zero; with an abundance of bees, the crop will be more bountiful.

The dilemma that growers and beekeepers encounter is, almost all the available commercial bee colonies in the U.S. are used to pollinate almonds. Thus facilitating the likely spread of diseases and parasites to the entire bee colony. Another serious bout of Colony Collapse Disorder will threaten both the crop and livelihoods of farmers and beekeepers.

One approach is to use a solitary bee called the blue orchard bee () alongside honeybees. The contrasting biologies and behaviors of the different bee species complement each other and yield better results than the use of a single species. However, it is a long process to develop the necessary techniques to manage both wild solitary bees with commercially managed honeybees.

5. Kiwi Fruit

As mentioned previously, bees are rewarded for their role in pollination in terms of nectar, but sometimes however, this fails to happen. For example, kiwi fruit grows as vines with both male and female flowers. Both male and female flowers produce pollen, but the pollen from the female is sterile and less nutritious, and no nectar is produced.

As is obvious from cutting open a ripe kiwi fruit, there are many hundreds of seeds, meaning that a large number of pollen grains, between 1,000 and 2,000, must be transferred for every fruit. With poor rewards and the reluctance of bees to move between male and female flowers, the grower falls back on a policy of inundating the field with bees. Moreover, bees will abandon kiwis, if more attractive forage is available.

6. Greenhouse Crops

Over the last 40 years, the production of blueberries, tomatoes, eggplant, and bell peppers has been transformed by the adoption of bumblebees as commercial pollinators, with these plants now grown in massive glasshouses. The flowers of these plants need vigorous intervention to open other cones and shake the pollen off the stamens and down through a tight corolla onto the stigma. This is achieved via technique known as buzz pollination whereby a bee hangs onto the underside of the flower and contracts their flight muscles to create strong vibrations. This behavior is common throughout bumble bees and solitary bees yet absent in honey bees. It is thought that approximately 9% of the flowers of the world are primarily pollinated using buzz pollination.

7. Oil Seed Crops

On the whole, oilseed crops, particularly oilseed rape are not pollinated commercially as the plant is capable of self-pollination by wind agitation. When grown for seed, or bred for high-value oil extraction, however, the grower may hire honeybee colonies to assist pollination.

8. Seed Production

It is easy to overlook the key aspect of seed production in maintaining our food supplies. The quality of seed is directly related to the quality of produce. For example, not only is the size of the individual seed important insofar as larger seeds have more nutrient reserves, but the rapidity of the act of pollination affects the quality of the produce. When pollination is quick and efficient, the flower shuts down and drops quickly, limiting the period that the flower parts are exposed to molds. A seed produced in these circumstances will harbor less botrytis for example. The result is not visible in the seedling, the mature plant, or its cauliflower; only when it is in the supermarket does this become apparent, a botrytis-free vegetable will have a longer shelf life – a big advantage for both the retailer and the grower.

9. Onion and Carrot Seed

Unlike the previous example of brassica seed, onions and carrots are notoriously difficult to pollinate for seed. Onions have been bred to suit the human palate, resulting in a high potassium content in the vegetable. This is also expressed in the nectar and pollen; which honeybees find abhorrent. The only successful method in producing onion seed is to overwhelm the crop with bees, before replacing them several days later with new colonies.

Carrots are difficult to pollinate for a different reason – they are morphologically a fly-pollinated plant, so the reward for honeybees is limited.

10. F1 Hybrid Seed Production

Only by careful breeding of the separate maternal and paternal lines and controlling the movement of pollen can F1 hybrid seeds be produced. These are very much the prize in the eyes of seed producers as they are, despite the extra price, the pathway to larger, and healthier produce.

11. Meat and Milk

These are unusual candidates for a discussion on pollination, but nevertheless, bees play a full supporting role.

The first example is that of New Zealand in the 19th century, a country suited to sheep rearing, but which did not reach its potential until after 1885 when four species of bumblebees were exported from the UK and released. These long-tongued bees were able to reach deep down into the flowers of red clover and hence produce clover seed. This provided self-perpetuating rich forage for livestock, and in essence, underpinned the economy of the country and still contributing today.

A second example is alfalfa (or lucerne) which is grown globally as cattle fodder. The U.S. has a thriving industry in alfalfa seed production, which is dependent on a small solitary bee, supplied in millions to growers. Honeybees tend to dislike the flower because it is ‘spring-loaded’ and knocks them on the head when the flower is ‘tripped’; consequently, honeybees tend to look for alternative forage very quickly. The much smaller alfalfa leaf-cutter bee (Megachile rotundata) is not affected in the same way. Another solitary bee used in some suitable locations is the alkali bee (Nomia melanderi).

Summary

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