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
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
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
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
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
,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,
sprouts,plums and prunes,pears,pimento,nutmeg,niger,passion fruit,peaches,cole
gooseberry or kiwi,cicermilkvetch ,cabbage,macadamia,mango, cherries,honeyball,honeydew,
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
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
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
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