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Pollination of Crops

By S. E. MCGREGOR
(Deceased.)

BEEKEEPING IN THE UNITED STATES
AGRICULTURE HANDBOOK NUMBER 335
Revised October 1980
Pages 107 – 117

Farmers can provide the best agronomic practices-proper seedbed preparation, fertilization, soil moisture, cultivation, pest control, and harvesting methods-and fail to obtain a bountiful harvest, if they neglect to provide for pollination. Many of their fruit crops, legumes, vegetables, and oilseed crops depend upon pollinating insects for set of fruit or seeds. Therein lies the basis for the most important contribution made by any insect to agriculture-pollination by the honey bee.

Flowering and Fruiting in Plants

Some knowledge of the flower structure and fruit setting is necessary to understand the entire pollination process. All flowers have the same basic pattern, but there are many variations. The peach blossom, the tassel and ear of corn, and the sunflower head appear remarkably different, but all have the same basic parts.

Typically, the flower (fig. 1) is composed of the sexual organs, protected by the usually colorful delicate petals that may form a sort of tube or crownlike corolla. These are supported and partially protected by the usually green and more durable sepals, collectively called the calyx. The calyx and corolla combined are referred to as the perianth. There may be leaflike bracts just below the sepals.

FIGURE 1.-Generalized longitudinal section of a cotton flower (Gossypium spp.), approximately X 2, showing nectaries, pollen-laden anthers, and growth of a pollen tube (further enlarged) down the style to the ovary and into an ovule.

The male parts of the sexual organs are the stamens, and there may be one to several dozen in a flower. The stamens usually consist of hairlike filaments bearing the pollen-producing anthers on the outer ends. At the appropriate time, the anthers dehisce or split open and disgorge the male element, the numerous microscopic and usually yellow grains of pollen.

The female part of the flower is the pistil, consisting of the ovary, with one to numerous ovules or potential seeds, and extending from the ovary is the style, with the pollen-receptive portion, the stigma, on or near the tip. The pistil may be composed of one or more carpels or sections. Typically, the ovary, with its style and stigma, surrounded by the stamens, occupies the central area of the flower. Nectar usually is secreted at the base of the pistil, inside the corolla (fig. 2). N ectaries also may occur outside the corolla. These usually are referred to as extrafloral nectaries and do not contribute to pollination.

FIGURE 2.-Longitudinal section of a Bing cherry flower, approximately x 7, showing nectariferous area and nectar droplets.

The ovules produce the seeds, and the ovary develops into the fruit. Usually one ovule must be fertilized for each seed that develops. If no seeds are produced, the fruit is unlikely to develop although a few fruits (certain cucumbers and citrus varieties) develop parthenocarpically (without being pollinated). If an insufficient number of seeds develop, the fruit is likely to be asymmetrical or otherwise not fully developed.

In general, the sooner pollination can occur after a flower opens, the greater the likelihood that fertilization of the ovules and seed development will occur. As time elapses, the pollen may be lost to insect foragers, wind, gravity, damage by heat, moisture, or drying out. Also, processes may set in that result in the shedding of the fruit.

Pollination and Fertilization

Usually, we think of pollination in the combined sense of transfer of pollen and set of fruit or seed. Actually, two sets of factors are involved: (1) transfer of viable pollen from the anther to a receptive stigma, and (2) sprouting of the pollen grain and growth of the pollen tube down the style into the ovary, and ultimately the union of male nuclei of the pollen grain with female germ cells in the ovule that results in seed development. Pollination is of no value without fertilization.

Sterility, Fertility, and Compatibility

Most flowers have both male and female functional parts. Some plants, however, such as asparagus (fig. 3), coriander (fig. 4), dill (fig. 5), or squash (fig. 6) may have only male flowers, in which the ovaries are nonfunctional, or female flowers, in which the anthers are nonfunctional. In others, the stigma may not be receptive when the pollen is available within the flower (fig. 7). In such flowers, the pollen must be transferred from the male flower to the female. If the flower has both functional parts and is receptive to its own pollen, it is said to be self-fertile. If the flower is not fertilized by its own pollen, but is fertilized only when pollen comes from another plant or variety it is referred to as self-sterile. For example, both the ‘Red Delicious’ and ‘Golden Delicious’ apple are largely self-sterile, but when interplanted and cross-pollinated each will fertilize the other and good production of fruit is obtained. They are cross-compatible. Varieties that will not cross-fertilize are said to be cross-incompatible.

A plant may be self-fertile but not self-pollinating. A pollinating agent may be necessary to transfer the pollen from the anthers to the stigma.

FIGURE 3.-Longitudinal section of asparagus flower, with fully developed ovary and stigma (A) and anthers (B), approximately x 17.

FIGURE 4.-Longitudinal section of coriander flower, approximately x 40: A, Staminate stage; B, pistillate stage.

FIGURE 5.-Longitudinal section of dill flower, approximately x 20: A, Female; B, male.

FIGURE 6.-Longitudinal section of reproductive portions of acorn squash flowers, approximately x 2: A, Staminate or male flower; B, pistillate or female flower.

The avocado flower is an unusual example of the need for cross-pollination by bees (fig. 7). It opens twice on subsequent days. The first day the stigma is receptive to pollen but none is released by the anthers. After a few hours, the flower closes. The next day, it opens again when the stigma is no longer receptive to pollen, but the anthers release sticky clumps of pollen. Again the flower soon closes, never to reopen; therefore, it cannot be self-pollinated.

FIGURE 7.-Longitudinal section of 'Fuerte' avocado flower, approximately x 18: A, Stage 1: Stigma receptive, but stamens bent outward and anthers not dehisced; B, stage 2 the second day with stigma no longer receptive but stamens upright and anthers dehisced.

Still more unusual is the characteristic of some varieties of avocado flowers to open for the first time in the morning of the first day and in the afternoon of the second day. In other varieties, the flowers open in the afternoon of the first day and in the morning of the following day. Only when growers interplant two such varieties, where pollen is always available when stigmas are receptive, and they provide bees to serve as the cross-pollinating agents, are they capable of harvesting the maximum set of fruit.

In addition to the volume of the crop produced through adequate pollination, another value lies in the effect of pollination on quality and efficiency of crop production. Inadequate pollination can result in reduced yields, delayed yield, and a high percentage of culls or inferior fruits. With ample pollination, growers may be able to set their blooms before frost can damage them, set their crop before insects attack, and harvest ahead of inclement weather. Earliness of set is an often overlooked but important phase in the crop economy.

Hybrid Vigor and Bee Pollination

The value of pollination on the succeeding generation of crops also is frequently overlooked. The value of hybrid seed in not reflected until the next generation. Vigor of sprouting and emerging from the soil often is a vital factor in the plant’s early survival. Other responses to hybrid vigor include earliness of development, plant health, and greater and more uniform production of fruit or seed.

When two unlike varieties are cross-bred, the offspring frequently is more robust in some characteristic than either parent. This strengthening effect is referred to as hybrid vigor (heterosis).

Years ago, scientists learned that the offspring of two inbred lines of corn was more productive than either parent. Then they learned that when this offspring was crossed with another such offspring of two other inbred lines, ever greater production was obtained. As an example, the offspring of variety A crossed with variety B, or A x B, crossed with C x D, is known as a 4-way cross. Presently, most of the corn produced in the United States is derived in this way, and is generally referred to as hybrid corn.

Corn is wind-pollinated; therefore, breeders plant certain rows of variety A between rows of variety B. Then, by removing the pollen-producing tassels of one variety, production of seed can only result from pollen carried by wind from the tassels of the other. This system works well on corn because the tassels can be deftly removed before pollen is released.

In most of our other crops, the male and female parts are in the same, usually small, flower. In some crops, however, breeders have developed methods of producing and maintaining male-sterile lines-selections that produce no pollen. Then alternate rows, or groups of rows, are planted to normal lines and others to male-sterile lines and all the fruit or seed obtained from the male-sterile line is hybrid. The hybrid may be superior to the parents in productiveness, uniformity, earliness, resistance to diseases or insects, or other factors.

Growers currently are producing hybrid onions, carrots, cucumbers, sunflowers, and several other crops. Research is under way on the production of hybrid cotton, soybeans, and alfalfa. All of these are insect-pollinated crops and will need to use colonies of bees or other pollinating insects in their production.

Pollinating Agents

Although this handbook principally concerns honey bees and beekeeping, pollinating agents other than honey bees should be mentioned and their relative value considered.

Wind probably is the most important pollinating agent, insofar as it benefits our existence. Most of the forest trees, practically all the grasses and grains, with the exception of some that are completely self-pollinated, and many weeds are wind-pollinated.

The flowers of most wind-pollinated plants are either male or female. The male flowers produce an abundance of pollen to be carried by the wind. The female flowers usually have large stigmatic areas to receive the pollen. Corn is a good example of a wind-pollinated crop.

Birds of several different species feed upon nectar, pollen, or insects in some flowers and serve as pollinators. None is of significance in pollinating our cultivated crops. Their visits are confined largely to deep-throated, usually showy wild flowers.

Insects of many species visit flowers and pollinate them. These include bees, wasps, moths, butterflies, beetles, thrips, and midges. Bees are the most efficient and the only dependable pollinators, because they visit flowers methodically to collect nectar and pollen and do not destroy the flower or the plant in the process. Various species of bees, including the managed wild bees (see section on Management of Wild Bees), are highly efficient. An estimated 80 percent of our insect pollination is done by bees.

Honey Bees and Pollination

Modern agriculture has come to depend greatly upon honey bees to fulfill its pollination needs (fig. 8). This insect has several valuable qualifications for this role (fig. 9). Beekeepers maintain honey bees at a high population level in most agricultural areas of the United States for the honey and wax they produce. The colonies easily can be concentrated when and where needed to satisfy pollination requirements, and by using techniques developed for honey production their numbers can be increased in a relatively short time. The honey bee is adapted to many climates and can successfully revert to its original wild state in most parts of the world, quickly becoming part of the natural reservoir of pollinators.

FIGURE 8.-Honey bee colonies in apple orchard.

FIGURE 9.-Intense flight activity at the hive entrance usually indicates that the colonies are populous or "strong."

Beekeeping in Relation to Pollination

When growers conclude that their crops must be insect-pollinated but do not have sufficient pollinators to do the job, they may decide to rent colonies of honey bees from a beekeeper. There may be a local contractor with whom growers can deal-one who acts as contact agent on behalf of a few neighboring beekeepers. Most of the bee rentals, however, are personal arrangements between growers and local beekeepers. Unfortunately, growers and beekeepers consider the rental arrangement from entirely different points of view.

Growers may consider only the fees they are paying and the potential value of the bees to their crops, along with having the beekeeper, the beekeeper’s vehicles and crew, and the bees on their premises. Growers want to buy a service, pay for it, then go about their other duties.

Beekeepers consider the value of the bees to themselves before, during, and after the pollination contract is concluded. They weigh the advantage of the rental fee against the possibility of a reduced honey crop and the possibility of better forage than in their permanent location. (Beekeepers are always looking for better locations.) They also consider the constantly threatening adverse effect of pesticides, and the danger of damage to the bees and equipment in making the move. The bees usually are moved at night, and this is hard work with many chances for accidents on the roads or in the fields. When the colonies are moved from a location, another beekeeper may take it over. Exposure to bee diseases increases when colonies, particularly from many beekeepers, are concentrated in one area. And finally, problems associated with collecting the fee after the service is rendered frequently develop. These disadvantages discourage many beekeepers from renting bees to growers.

Crops Dependent Upon or Benefited by Insect Pollination

The following crops are dependent upon or benefited by insect pollination. Unfortunately, the information on their pollination needs is scanty or based upon earlier popular varieties.

For some, insect dependence is absolute; for others, the benefit ranges from scant to great. For most of them, there is a great need for study of current varieties under different environmental conditions to determine the precise dependency on insect pollinators.

Fruit and nut crops Vegetable crops Forage crops 1 Oilseed crops 1 Other
Acerola Artichoke 1 Alfalfa Cotton Buckwheat 1
Almond Asparagus 1 Alsike clover. Flax Cacao
Apple Balsam pear Arrowleaf clover. Peanut Cashew
Apricot Broccoli 1 Ball clover. Rape Chicory 1
Blackberry Brussels sprouts. 1 Berseem clover. Soybean Clove
Blueberry Cabbage 1 Black medic Safflower Coffee
Chestnut Cantaloupe Broadbean Sunflower Kola
Chinese gooseberry. Cardoon 1 Cicer milkvetch. Tung Lupines 1
Coconut Carrot 1 Crimson clover. Tea
Crabapple Casaba melon Crownvetch Many garden flowers. 1
Cranberry Cauliflower 1 Kenaf
Currant Celeriac 1 Kidneyvetch
Feijoa Celery 1 Kudzu
Gooseberry Chayote Lespedeza
Grape (some kinds) Chervil 1 Mung bean
Grapefruit Chive 1 Persian clover.
Guava Coriander 1 Pigeon pea
Jujube Cowpea Red clover
Lemon Crenshaw Rose clover
Litchi Cucumber Sainfoin
Loquat Dill 1 Scarlet runner bean.
Macadamia Eggplant Strawberry clover.
Maney sapote Endive 1 Sweetclover
Mango Fennel 1 Sweetvetch
Nectarine Honeydew Trefoil
Olive Kale 1 Vetch
Orange (some kinds). Leek 1 White clover
Papaw Lima bean Zigzag clover
Papaya Muskmelon
Passion fruit. Mustard 1
Peach Onion 1
Pear Parsley 1
Persimmon Parsnip 1
Pomegranate Pepper
Plum Persian melon.
Prune Pimento
Quince Pumpkin
Raspberry Radish 1
Tangelo Rutabaga 1
Tangerine Squash
Temple orange Turnip 1
Vegetable sponge.
Welsh onion. 1

1 For production of seed.