Pollination of Seed Crops by Insects

GEORGE E. BOHART AND FRANK E. TODD.

IN A PLANT that is adapted for self-pollination, the male spores often are transferred from the anther of the plant to its stigma by gravity or movement of parts within the flower.

If the floral parts are so arranged as to prevent or limit sharply the ability of the plant to pollinate itself, the pollen has to be transferred by such external agencies as wind and insects in order for fertilization the union of the sex cells to take place.

Plants adapted for cross-pollination that is, the transfer of the pollen from a different plant depend entirely on external agencies.

Many plants are adapted for both self- and cross-pollination in different flowers or at different periods in the life cycle of the flower. For such plants, cross-pollination usually results in more seeds of better quality, but automatic self-pollination is insurance that some seeds will form even if external agencies fail.

Plants with specific adaptations for automatic self-pollination are almost always self-fertile that is, the sperms and eggs of the same plant are compatible. Self-fertility is also present in some plants that depend on external agencies for pollination.

Many plants that are adapted primarily for cross-pollination are more or less self-sterile. They must have external agents of pollination and external sources of pollen to produce seeds in any quantity.

Plant breeders in the past often selected plants for breeding for their self-fertile, self-pollinating characteristics.

Varieties developed from such selections tend to remain true to type, and they do not depend very much on external agents of pollination. Their principal drawback is the loss of hybrid vigor. The more recent emphasis on inherent yielding capacity has led to reinvestigation of many self-pollinated crops to determine the advantages and possibilities of growing hybrid seed.

WIND, BIRDS, and insects are the main external agents of pollination. Birds are of little importance to commercial crops, especially in temperate regions.

Wind is the principal pollinating agent of corn and other grasses, coniferous trees, nut trees (except almonds), and a scattering of other crops (for example, castorbeans, spinach, and beets).

Most other plants are self-pollinated or insect pollinated, or both. Peas, beans, tomatoes, and lettuce are examples of plants that produce good crops of seed without help from external agents of pollination.

WE GIVE a partial list of seed crops that require insect pollination or at least benefit from it. Not included are ornamentals, medicinals, and most spices, many of which are also insect pollinated. The stars indicate crops in which insect pollination usually increases yields of seed but is not essential for commercial production.

Some plants in the list (onion, for example) produce no seeds without insects. Others, like cotton, produce nearly as many seeds without insects as with them, but varieties within species often vary in this respect. Most are somewhere between these extremes.

When the production of hybrid seed is developed for more crops, the list will be increased, because nearly every crop that is not pollinated by wind requires insects for the production of hybrid seed.

Many thousands of kinds of insects visit flowers. Nearly all of them can pollinate at least a few plants. Most of them, because of small size, scarcity, specialized food habits, or for other reasons, however, are unimportant as crop pollinators.

A large variety of insects visit umbelliferous flowers, but even on these flowers, which are general in their pollination adaptations, a small percentage of the visitors accomplish most of the pollination.

The honey bee is by far the most important insect pollinator. Indeed, for commercial crops in temperate regions it is more important than all other species combined. It occupies this preeminent position by virtue of a number of valuable qualifications.

Because it produces honey and beeswax, the honey bee is maintained at a high population level in many agricultural areas. It can be readily increased and moved to satisfy pollination requirements. It is widely adapted and successfully fends for itself in most parts of the world.

As a species, it visits an extremely wide variety of flowers wider than any other insect. As an individual on a single foraging trip, it usually confines itself to one species of plant.

It is large enough and hairy enough to accumulate many grains of pollen and to touch the stigma in most of the flowers it visits. Because it must store enough pollen and nectar to carry the larval and adult bees through the winter and other dearth periods, it is unusually industrious in its flower-visiting habits.

But as one might expect, the honey bees are not always fully dependable pollinators. Because they generally seek the richest sources of pollen and nectar, they may neglect the crop requiring pollination and go to richer food sources that are blooming at the same time.

Honey bees sometimes visit seed crops for nectar alone. On many crops, nectar collectors are less efficient pollinators than pollen collectors. An individual honey bee usually works within a small radius after it starts foraging. This presents a problem in the production of hybrid seeds when a grower wants to use as few pollen parent plants as possible.

Although the honey bee is probably the most important pollinator of every crop in our list, other species have a valuable supplementary role. On a few crops, such as alfalfa and cotton, other species of bees may be more efficient individual pollinators. On a few crops (carrot and squash), other insects sometimes are more abundant.

WILD SPECIES of bees rank next to honey bees as pollinators of seed crops. Relatively few of the many kinds are sufficiently abundant on seed crops to be of great importance. The principal advantage of most wild bees is that they visit flowers primarily for pollen and have no tendency to take the nectar without effecting pollination.

Bumble bees usually are excellent pollinators of the crops they visit, but a few of the shorter tongued species cut holes in the bases of red clover and vetch flowers and in that way prevent pollination.

Wild bees usually have a much narrower range of hosts than honey bees. If the favored host is a crop that requires insect pollination, the reduced competition from the other plants is beneficial.

Many wild bees ignore cultivated crops, preferring a narrow selection of native plants. Most wild bees are nonsocial and require large areas of uncultivated land to develop effective populations. Some solitary bees, like the alkali bee, however, are gregarious and can build up immense numbers in a few acres of ground.

Insects other than bees are important on only a few of the seed crops in the list.

Flies of various families and sphecid wasps are important pollinators of umbelliferous and Allium crops and to a lesser extent of cruciferous crops. Such insects generally can be considered abundant only when all species are combined. They are usually most abundant on small seed fields surrounded by uncultivated terrain.