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ENTOMOLOGY LEAFLET: DEWEY M. CARON

Dept of Entomology & Applied Ecology




A flowering plant springs from a seed and grows. Given favorable conditions a bud develops and from it a showy flower emerges. Then pollination must occur. As the flower fades a fruit with seeds is produced. With seed dispersal one of natures most fascinating and vital cycles, plant reproduction, has run its course.

There are about 250,000 species of flowering plants on earth that require pollination. Wind, gravity, water, birds, bats, and insects are the forces that accomplish pollination. Plants that produce a pollen that is light and easily blown about by wind are referred to as wind-pollinated plants. Pine trees and corn are two examples of wind- pollinated plants. A large number of plants produce pollen that is heavy and sticky and not blown easily from flower to flower. These require other agents, such as insects, to transfer pollen and are insect pollinated plants.

The focal point of pollination is the flower. In order to understand insect pollination one must become familiar with the general features of a flower.

The outermost part of a flower, which is usually green and originally sheathed the bud, is called the calyx, each scale of which is called a sepal. Inside the calyx is the corolla which is made up of petals. They may be separate or fused and are usually brightly colored of one or more hues. The petals are the insect attractor with their brightness and pattern of colors.

The sexual organs of flowers occur within the calyx and corolla. The male sex organs are the stamens. They vary greatly in number and arrangement with different species of flowers. Each stamen has a slender stalk or filament with a sac-like anther at the top. It is here at the anther where the pollen grains are produced.

The innermost part of the flower, the pistil, contains the female sex organs. It consists of three parts; the stigma, style, and ovary. The stigma is the end portion where pollen is received. It is usually sticky when performing its function of capturing pollen grains. The ovary, the most important part of the flower, contains the ovules. Some flowers produce a single ovule in the pistil whereas others may produce over 1,000. After fertilization, the ovules become the seed and the ovary, and sometimes adhering parts, become the fruit.

This is the basic arrangement of a complete or perfect flower. If the male portion (anthers and pollen) are absent then the flower is a FEMALE or PISTILLATE flower. The opposite situation exists when pollen is produced normally but the female portion (the pistil) is absent or altered and to be nonfunctional. This type flower is termed MALE or STAMINATE.

Plants that produce pollen on one portion of the plant while the pistil is located elsewhere are termed MONOECIOUS. Corn is monoecious - pollen is produced at the tassel on top of the plant and the pistil - known as the silk - is where the ears develop. Cucumber is another example of a monoecious plant. A second flower type, termed DIOECIOUS, is when the sexes are on separate plants. Holly is a good example of a dioecious plant.

Most flowers secrete nectar, a sugary, scented liquid to help attract insects to the flower in addition to pollen. This is usually offered deep within the flower near the base where petals originate from around the ovary. In seeking nectar or gathering pollen, the insect accidentally brushes against anthers so that pollen grains are transferred to the stigma. Pollination is not a deliberate behavior but an accidental one performed by the insects as they go about collecting food.

To produce a seed or fruit, pollen must move from the anther to a receptive stigma. This is POLLINATION. When the proper compatible pollen adheres to the stigma, it germinates and a pollen tube grows through the stigma and the style to the ovary. FERTILIZATION takes place in the ovary when the nucleus of the pollen or male germ cell unties with the nucleus of the ovule or female germ cell. Now a seed is produced.

POLLINATION - transfer of pollen from flower anther to stigma

SELF-POLLINATION -pollen comes from same flower or same plant or from plants of identical genetic material. Flower must be SELF-FERTILE or SELF-Compatible.

Cross-Pollination -pollen transferred from one flower to another. Flower must be CROSS-Compatible

FERTILIZATION -union of male nucleus of pollen grain (passed through pollen tube) with female nucleus of ovule.

Cross pollinated flowers must have transfer of pollen from one flower to the next. Even self-pollinated plants may benefit from cross- pollination with hybrid vigor, more rapid and complete growth, or in other ways. Male sterility has been found in many crops and plant breeders have been alert in finding and exploiting this characteristic to develop hybrids. Once developed, the production of hybrids is then dependent on cross pollination.

Flower pollinating agents: Wind, Bats, Monkeys, Kangaroos, Insects, Water, Man, Snails & slugs, Spiders & mites ,Birds

Man is a very expensive pollinating agent. We perform hand application of pollen by brush or duster. Commercially, vanilla and date crops are hand pollinated. Man also uses his technical knowledge to accomplish pollination, as for example mechanically shaking tomatoes in greenhouses to distribute pollen.

Of the pollinating agents wind and insects are the major agents in pollination of most flowers. Virtually every major group of insects has been implicated in the pollination of some plant.

Those insects identified as being involved in the production of one or more commercial crops are:ants, butterflies, moths, aphids, flies, thrips, bees ,midges, wasps, beetles, mosquitoes.

Pollination is essential for the commercial production of many fruit, vegetable and seed crops. It is estimated that about 90 crops in the United States depend upon insects, mainly honey bees, for pollination. This is some $10 billion in total crop value and represents about 1/3 of the diet of the U.S. citizen. This U.S. Department of Agriculture (USDA) list identifies those crops requiring assistance of insect pollinators.

A1falfa, Clovers minor, Gooseberries, Pumpkins, and squash,allspice, Cranberries ,Herbs(spices) Crimson clover, JuJube Quinine, Almonds, Crownvetch, Kola-nut, Radish,Apples, Raspberries, Cucumbers, Lavander, Avocado ,Currants, Litchi, Red clover ,Berseem ,Cut-flower,Longan, Rutabagas,Blackberries,Mango,Sunflower, Buckwheat,Eggplants,Muskmelons,Cocoa,Sweetvetch, Cantaloupe,Sweetclovers,Tea,Crenshaw,Cardamon,Casaba,Broccoli, Cardamon,trefalis,welsh onion,watermelon,leek,fennel,dill,coriander,collards,cauliflower,cabbage,brussel sprouts,plums and prunes,pears,pimento,nutmeg,niger,passion fruit,peaches,cole crops,chives,chicory,carrots,caraway,asparagus,turnips,tung,tea,tangerine, tangelo,pummelo,chinese gooseberry or kiwi,cicermilkvetch ,cabbage,macadamia,mango, cherries,honeyball,honeydew, pears..etc.

Bees as pollinators

The honey bee Apis mellifera is the most widely dependable pollinator of insect pollinated plants. The need for honey bees as pollinators is increasing, and this trend is likely to continue or even accelerate in the next few years. The demand for controlled insect pollination is being created by several modern agricultural practices. The number of bees has been decreasing by 1% per year for the last twenty years. The use of insecticides, herbicides, mowing of pastures and roadsides, and the overgrazing of pasture land has contributed to the decline in populations of wild bees. Also large scale monoculture demands massive numbers of pollinators for the one or three week bloom period but provides nothing to support bees for the remainder of the season.

Five reasons the honey bee is our most valuable pollinator are:

1. Perennial colony

2. Nectar & pollen their only food

3. Plumose body hairs

4. Flower-constant foraging behavior

5. Populations can be manipulated

Three other bees important on a more limited scale in planned crop pollination are:

1) Bumble bees, Bombus sp. efficient in pollinating flowers with deep, narrow corolla tubes such as red clover.

2) The alfalfa leafcutter bee, Megachile rotundata, a valuable pollinator of seed alfalfa, and

3) the alkali bee, Nomia melanderi, an alfalfa seed pollinator in somewhat cooler, windier windier weather--than the the alfalfa leafcutter bee.

Providing Honey Bee Colonies for Pollination

Providing honey bee colonies to pollinate crops is by far the easiest and best method of providing planned pollination. There are approximately 3.5 million acres of crops in the United States that depend upon honey bees for pollination - some 80% of the total.

It is important for beekeepers to provide strong colonies to the grower who rents bee hives. Strong colonies not only supply more field bees, but the bees continue to forage when a weak colony will not. Strong colonies do little pollinating below 55'F and weak ones do little below 65'F. A strong colony will forage in stronger winds than a weak one. Strong colonies need proportionally fewer bees to perform hive duties and thus they are able to have more field bees.

Bee colony strength can be determined by the amount of brood in the col on . Colonies should have at least 1200 square inches of brood to be considered adequate. Rental fees are standardized on a colony basis but since some colonies are 2 or 3 times as strong, it may eventually be necessary for a beekeeper to grade colonies before delivery and vary the rental fee accordingly.

Honey bees obtain greater rewards of nectar or pollen from some plants than others. Crops to be pollinated may be less attractive than other plants. It is therefore wise to move colonies of bees to crops during or just before their main flowering time. This will usually insure that a good pollination job is done before the bees discover competing flowers. Since individual bees will forage close to the colony location before moving greater distances, bee colonies used for pollination are usually placed in or just adjacent to the crop requiring pollination.

When renting colonies for pollination it is desirable to have a written agreement between the beekeeper and grower.

The contract should cover:

(1) number and strength of colonies to be used;

(2)plan of colony distribution in the field;

(3) time of delivery and removal;

(4) the beekeeper's right of entry to service the colonies;

(5) the degree of protection from pesticides;

(6) plan of payment of the rental fee;

(7) penalties for poor quality or service by the beekeeper or breach of promise by the grower; and

(8) rewards or bonuses for exceeding the minimum in quality service, protection, or time of financial settlement.

Pollination of Delaware Crops

The majority of honey bee colonies that are rented for crop pollination in the U.S. are used on five crops: alfalfa seed, almonds, apples (and other fruits), cucurbit vegetables and vegetable seed production.

In Delaware, honey bee colonies are used early season for fruit pollination of apples, pears, cherries, strawberries, blueberries and brambles. Later they are needed to produce cucurbits such as watermelon, melon and cucumber. Honey bees are also moved to soybean or lima bean fields by many beekeepers for honey crops. They assist in pollination but are not rented by the farmers.

For apples, Pyrus malus, pollination is the most critical annual event. Apples are grown in temperate regions and they flower early in the season. The weather is often unfavorable for bee flight and pollination. Even pollen tube growth, fertilization and other factors affecting fruit set may be adversely affected by the weather. Growers know they need to provide adequate numbers of honey bees that can do the job of pollination. One colony per acre is usually recommended to produce a maximum crop of fruit. When conditions are favorable, the apple crop must be thinned for efficient annual production. Excess fruit can be removed by the thinning process but once the bloom time passes there is no way to add fruit to a tree. The same applies to pear and cherries.

For blueberry, strawberry and the brambles, insect pollination is required to produce high yields of well-developed, full-fleshed berries. The berry flowers have many pistils and each must receive its grain of pollen so it can contribute to the size, shape and taste of the fruit to come. Poor pollination results in small, misshapen fruits of poor quality that are slow to ripen to size.

Vegetable crops assisted by insect pollination are beans, eggplant, pepper and lima beans. Cucurbits are totally dependent on insect pollination since most are monoecious in flower habit. Some of the features cucurbits have that indicate their dependence on insect pollination are:

1. Large showy yellow flowers, a highly attractive color to bees. Nectar is abundant in the large flowers. It is secreted from numerous pores of a large nectary. Cucurbit nectars are high in sucrose content and highly attractive to bees.

2. Although flower bloom often lasts for only 1 day, cucurbit plants produce numerous blooms. The plant is not a honey plant for the beekeeper, however, because the flower density is too low even in high density plantings. Not all the flowers need to be set to realize a crop.

3. Cucurbit pollen must be transferred by an insect because the pollen is heavy and sticky. Wind can't transfer cucurbit pollen.

4. For the plant pollen is self-fertile but not self- pollinating because cucurbits are monoecious with male and female bloom.

5. Cucurbit flowers need a lot of pollen as the ovule number may easily reach into the 100's. Some cucurbits, like watermelon with a 3lobed flower, must have pollen on all 3 lobes for perfect fruit formation. Ten,or more visits with the accidental transfer of pollen grain are needed to realize perfect fruit in the cucurbits. Inadequate pollination results in misshapen fruit that mature slowly.

Pollination or honey crop?

The interrelationship between honey bees and plants is a complex one. Although the chief value of honey bees is to perform their essential function of pollinating fruit, vegetable and seed crops, they are usually kept by beekeepers to produce a crop of honey for sale. A beekeeper's profit is influenced greatly by the yield of honey which he/she receives from each hive. Maintaining beekeepers in the honey- producing business also means maintaining the bee pollinators that are indispensable for the production of these Delaware fruit and vegetable crops. A healthy bee industry means healthy bees available for planned pollination.

this article was distributed freely among the attendees of the 1997 EAS Conference


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