13 November, 2018

How much do you really know about the world of pollination?

By Matt Allan

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Pollinator protection is a hot topic at the moment, and deservedly so, given the need to feed 7.6 billion people. The challenge we all face is how to control the pests that damage food crops without killing the insects that help produce the food. By insects, I mean of course predominantly, but not exclusively bees.

CPC-honeybee-on-purple-flower-insights-featured-img-1108x1108pxVarious claims are bandied about as to the proportion of our food that derives from bee pollination. Whether that is 30% or 50% or 90% is largely immaterial; the point is that our bees must be supported, and in this context it is essential to include not only our familiar honeybees, but also the wild bees, the largely unseen contributors to pollination, i.e. bumble bees and the almost invisible solitary bees.

The bargain that has been negotiated between plants and bees over millions of years is that the bees move the pollen as required and receive as reward nectar and some pollen.

The importance of pollinators lies in two different but overlapping areas. The first is the maintenance of our natural habitat. The second, which is the theme of this piece, is the value in commercial production of food crops.

To illustrate some of the varieties and complexities of pollination of crops, let us look at some examples.

What is the process of pollination?

Pollination means the movement of pollen from the male parts (stamens) of a flower, to the female part (stigma) of a flower.

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 wind; by mammals; by 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 is fuses with an ovule to create a seed. This in turn stimulates the formation of a fruit.

In bygone times, 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 pollinators (which means in most cases, that we fall back on the easiest, the honeybees). For more information about bee studies, you can read the Development in Life Sciences article ‘Innovations in ecotoxicology: it really is the bee’s knees’ .

The importance of pollinators lies in two different but
overlapping areas; the maintenance of our natural habitat
and commercial production of food crops.

Variety and complexity of pollination of crops

1. Apples

This is the text book crop. 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 occurs when ten seeds are formed. The presence of a seed produces hormones which stimulate the transport of resources which make the fruit plump, juicy, colourful and tasty.

Even with fewer than ten seeds in an apple, it can be attractive and nutritious, but the fewer seeds, the less successful the fruit is. Growers generally control the amount of pollination carefully, as over-pollination may produce an excess of fruit which needs to be thinned.

2. Bananas

The opposite of a text book crop, 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, grapes and seedless citrus such as mandarins.

3. Berry fruits

Berry fruits such as raspberries and strawberries need through pollination. For example, a single strawberry may have 300 stigmas, and a raspberry 90. Again, good pollination will produce fruit of uniform colour, softness, shape and sweetness, whereas incomplete pollination produces misshapen, miscoloured 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 USA 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 abundant bees the crop will be more and more successful.

The tightrope that growers and beekeepers totter along is that almost all the available colonies are taken to pollinate almonds, where diseases and parasites from all over the continent are liberally shared. Another serious bout of colony collapse disorder will threaten many livelihoods.

One approach is to use a solitary bee called the blue orchard bee (Osmia lignaria) alongside honeybees. The different biologies and behaviours of the different bee species complement each other and yield better results than one species on its own. It is however a long process to develop the techniques of managing solitary bees to match the expertise of honey beekeepers.

5. Kiwi fruit

As mentioned previously, plants reward bees for moving pollen, but sometimes the deal breaks down. For example, kiwi fruit grow as vines with female flowers and vines with male flowers. Both male and female flowers produce pollen, but the pollen from the female is sterile and less nutritious. No nectar is produced.

As is obvious from cutting open a ripe kiwi fruit, there are many hundreds of seed. This means 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 overstocking the yard. In addition bees will abandon kiwis if more attractive forage is available.

6. Greenhouse crops

In 40 years, the production of tomatoes, aubergines and peppers has been transformed by the adoption of bumble bees as commercial pollinators.

These plants are advantageously grown in glasshouses. The flowers rely on vigorous intervention to shake the pollen off the stamens and down through a tight corolla onto the stigma (or a bee hanging onto the underside of the flower).

Bumble bees have a technique known as buzz pollination which achieves precisely this. Now bumble bee supply is a very large global business.

7. Oil seed crops

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

8. Seed production

It is easy to overlook the key position of seed production in maintaining our food supplies.

The quality of seed is reflected in the quality of produce.

For example, not only is the size of the individual seed important insofar as a larger seed has more nutrient reserves, but the rapidity of the act of pollination affects the value of the produce. When pollination is poor or limited, a flower remains open for a long period.

When pollination is quick, the flower shuts down and drops quickly, limiting the period that the flower parts are exposed to moulds.

A seed produced in these circumstances will harbour less botrytis for example. The result is not visible in the seedling, or the mature plant, or say the cauliflower.

When it is on the supermarket display however, a botrytis-free vegetable may last one or two days more on the shelf before it has to be disposed of – a big advantage for 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.

Why?

For a start, 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 apparently find abhorrent.

The only method so far successful in producing onion seed is to swamp the field with bees, then removing them a few days later when they flatly refuse to work the onions, and replacing them with other 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

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

11. Meat and milk

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

The first example is that of New Zealand in the 19th century, a country obviously suited to sheep rearing, but which did not reach its potential until after 1885 when four species of bumble bees 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.

The second example is that of alfalfa (or lucerne) which is grown around the globe as cattle fodder. The USA has a thriving industry in alfalfa seed production, which is dependent on a small solitary bee, supplied in millions to the growers.

Honeybees tend to dislike the flower because it is spring-loaded and smacks them on the head when the flower is ‘tripped’ After a period of this abuse, honeybees tend to look for alternative forage.

The much smaller alfalfa leaf-cutter bee (Megachile rotundata) does not suffer in the same way. Another solitary bee used in some suitable locations is the alkali bee Nomia melanderi.

Take home messages

The above examples illustrate the variety of crops, the variety of techniques, and the variety of bee species that need to be handled to provide the pollination services we need for today, and an indication of the needs of tomorrow.

There is no one solution to our pollination needs.

For sure we will need to be innovative and bold, and subtle in our management of the environment so that wild pollinators are integral to our strategies. In the meantime we need to ensure that the plant protection products we use are subject to real and rigorous scrutiny.

For more information about bee studies, you can read the Development in Life Sciences article ‘Innovations in ecotoxicology: it really is the bee’s knees’ .

Regulations and registrations: your guide to successful ecotoxicology studies on bees

 

About the author 

Matt Allan, Director at Atlantic Pollination Ltd, is a pollination specialist who has been involved professionally with bees for over 30 years, working in the fields of education, advisory services, manufacture, rearing, and pollination, across Europe and the USA. Matt shares insights from the ecotoxicology arena and, specifically, on bee studies, where he reveals some highlights from the projects he is collaborating on with Envigo. If you would like more information about Atlantic Pollination's services

please contact Matthew Allan. His other blog posts include;

Category // Crop protection, ecotoxicology, "pollination", Matt Allan, bees