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INSECT
POLLINATION (cont’d.)
(Contact) Pollination of Legumes
Legume plants
are vital to world agriculture as they help to improve livestock and the
soil. Large amounts of legume seed
are required annually for planting, especially when intervals between crop
rotations become shorter. Improved
varieties with higher yields are always difficult to obtain so that good
pollination of existing varieties is essential. The average pounds per acre and maximum yields obtainable for
several common varieties have been estimated as follows: Alsike 33 (1000), Winter clover 58 (600), Red clover 65 (700), Alfalfa 97 (1200), Hairy vetch 125 (700), Sweet clover 160 (600), Lespedeza 193 (500).
Poor weather, inadequate irrigation, insect pests, diseases and
inefficient harvesting have caused the lower yields. However, inadequate pollination is the
most common cause albeit insect pests frequently inflict drastic reductions. The pollination requirements for
legume crops vary according to whether they are self-fertile or
self-sterile. For self-pollinating varieties
bees are of little value for peas, soybeans, peanuts, snap beans and
subterranean clover. But bees have
some value for lespedeza, lima beans and vetches. In the not self-pollinating varieties some outside agent like
bees is required. Varieties included
are strawberry clover, lotus, and crimson clover and white sweet clover. For self-sterile varieties, which require
cross-pollination) insects (usually bees) are required. Examples are yellow sweet clover, white
Dutch clover, alsike clover, alfalfa and red clover. Legume crops can be very
attractive to pollinators, especially Hymenoptera, for both their pollen and
nectar. They are capable of yielding
good honey crops. Examples include
lima beans, Dutch clover, alsike clover, purple and hairy vetch, yellow sweet
clover and white sweet clover. Crops
that are attractive for their pollen but less so for nectar are ladino
clover, crimson clover and red clover.
Crops very attractive for their nectar but less so for pollen are
alfalfa primarily. Those crops that
are only slightly attractive are snap beans, soybeans, peas, lotus, Hungarian
vetch, subterranean clover and peanuts. There are special problems with some important legume varieties. In alfalfa, for example, the blossoms must
be tripped in order for pollination to occur. To result in the required cross-pollination bees must carry out
the tripping. In Western North
America honeybees, alkali bees, leafcutter bees and bumblebees
are of greatest importance. Although
honeybees are usually abundant, they are rather inefficient as pollinators
when compared with alkali bees. Their
large numbers tends to offset this deficiency. Among the Alkali bees,
Nomia
melanderia is very common. Leafcutter bees
(Megachile spp.)
are usually present but in low numbers.
Bumblebees (Bombus spp.) occur locally in
moderate abundance. The problem
with honeybees is that they visit alfalfa more often for nectar than for
pollen. When they are searching for
nectar they learn to avoid the tripping apparatus, so that only about one percent
of the visits results in tripping.
When they are looking specifically for pollen they can be very
efficient, however. For high yields
to occur it is often necessary to have more bees on the field than are
required to make a honey crop, which is costly for the beekeeper. Problems that occur with wild bees
is that they almost always go to alfalfa primarily for pollen. Many species are very rapid in their
movements and trip at about twice the rate of pollen-collecting
honeybees. But they are not abundant
enough on most fields to do a thorough pollination, and their reliability is
low because their numbers fluctuate from season to season. It is desirable for farmers to cooperate
with neighbors in their region especially in the acquisition of beekeeping
services, which often require remuneration because of low honey yields. With the exception of the sweet
clovers that are attractive to wasps and bees, bees are the only reliable
pollinators for commercial legume crops.
Many hundreds of species may be involved. And although they are usually highly efficient, their numbers
are rarely high enough to do satisfactory pollination. Wild bees are best for such crops as red
clover and alfalfa. There are a few
species of bumblebee that re detrimental to pollination of vetch and red
clover because they cut into the bases of the corolla tubes. This can ruin red clover seed production
in some areas. Insect Pollination
of Fruit Crops
Most of the commercially grown fruits
require insect pollination. Instead
of seed production the goal here is to obtain the edible fruit. Among the important nut crops only almonds
depend on insects for pollination, most of the others rely on wind. Most fruit crops require pollination and
seed formation for their fruit to develop, but there are some, e.g. navel
oranges, where development is wholly parthenocarpic. The Rosaceae is an important group
where pollination requires special attention. Included here are the almonds, berries, pome fruits and stone
fruits. For commerce that are mostly
propagated vegetatively. Therefore, a
commercial variety is basically one genotype, with the rare exception in
which plants were grown from mutant buds.
The Rosaceae are rarely auto-self pollinating and even if they are
self-fruitful, they require insects to transfer the pollen from the anthers
to the stigma. The phrase
“self-fruitful” is more accurate here than self-fertile because the fruit or
seed that results from pollination does not have to be fertile. Some varieties are completely
self-fruitful, some are only partially so and many are mostly
self-unfruitful. If there is any
reduction in productivity when a variety has been self-fertilized it is
probably in the self-unfruitful category.
From a horticultural viewpoint self-fruitful varieties have three
advantages over self-unfruitful varieties.
They do not have to be interplanted with other varieties, they require
fewer pollinating insects, and the entire blooming cycle is effective for
pollination as the flowers are perfect and their blooming period is in
synchrony. Self-unfruitful varieties present
several problems. Different varieties
must be interplanted in order to provide a foreign pollen source. The flowering of the pollinizer variety
has to be in synchrony with that of the pollinated. The pollinizer must be genetically compatible with the
pollinated, and ideally the pollinizer should have fruit of marketable
value. The latter also requires that the
pollinizer be readily pollinated by the primary commercial variety (=
reciprocal pollination). With interplanting of varieties
they must be close enough so that insect pollinators will include both
varieties on a single foraging flight.
When compared with self-fruit varieties, pollinating insects need to
be present or provided for in greater numbers because only flower visits
subsequent to visits on other varieties are effective. Placing pollen in beehives can alleviate
some of the problems. Fruitfulness among members of the
Rosaceae varies with the species and variety. Apples are mostly self-unfruitful, while pears are partially
self-unfruitful. Plums can be either
partially or wholly self-unfruitful.
Peaches and nectarines are mostly self-fruitful, but some have either
scarce or poor pollen. In almonds
most varieties are self-unfruitful.
Apricots are mostly self-fruitful.
Sour cherries are at least partially self-fruitful but yields are
increased with crossing. Sweet cherries
are mostly self-unfruitful.
Raspberries are partially self-unfruitful, while blackberries,
loganberries and dewberries are mostly self-fruitful. Strawberries can be either self-fruitful
or self-unfruitful. Decisions on how to interplant in
cases of self-unfruitful varieties naturally involve both horticulture and
entomology. The economic value of the
pollinizer variety must be considered.
There are problems with cultivation and harvesting two or more
interplanted varieties. The species and
number of pollinating insects and how much territory will individuals cover
while foraging are of primary concern.
These considerations vary for different crops and in different
localities. In order to determine how many
pollinating insects are required requires many considerations. Are the varieties self-fruitful or
unfruitful? What is the type of
interplanting in the case of self-unfruitful varieties? During the hours of foraging what kind of
weather might be expected. What is
the number of flowers that need to be pollinated because small fruits need to
set a greater percentage of fruit?
What kind of competition is expected with other blossoms in the area
during the critical hours for effective pollination? What are the number and variety of
pollinating insects already present in the area and do they persist year
after year. What is the efficiency of
the insects provided, which would usually involve honeybees? What is the distance of the crop from
apiaries? Usually hives are placed in
groups within a planting. In general
practice as many pollinating insects as possible are provided. Some plantings within or adjacent to wild
country already have an abundance of wild bees, but their presence should be
known to prevail over a number of years if honeybees are not introduced by
beekeepers. A special problem can arise with
the production of large fruits, especially those that are self-fruitful. Here there may be too much pollination
where the fruit will be small and of low market value. Blossom thinning and thinning of
developing fruit or the reduction of the number of pollinating insects are
techniques that can be deployed. Insect Pollinators for Rosaceous Fruit
Crops Rosaceous fruit crops are generally attractive
to honeybees, queen
bumblebees and many early season solitary bees, syrphid flies,
bombyliid flies and to some extent blow flies. Certain fruit varieties such as pears are especially attractive
to blow flies and syrphid flies, while plums are attractive to solitary
bees. Syrphid flies may cover more
trees and be especially valuable in cross-pollination. The efficiency of a particular insect
species varies, however. Several genera of solitary bees
are effective pollinators when orchards are adjacent to wild hilly
terrain. Included are the genera Andrena,
Anthophora, Halictus
and Tetralonia. Other genera are more common under special
conditions. Syrphid flies can be most abundant where there are large aphid
infestations that serve as food for their offspring or shallow polluted
water, which is the preferred medium of some syrphid fly larvae. Blowflies are common near refuse dumps,
slaughterhouses or wherever there is abundant carrion. Honeybees may be successfully deployed in
most fruit growing areas but it is sometimes difficult to obtain them. This is because most Rosaceae are not
especially good honey plants and a beekeeper would rather place his hives in
dandelion or some other early spring flour source. This is true especially if the orchards need to be heavily
stocked. Beekeepers are also
reluctant to place hives near orchards that will be treated with
insecticides. Some areas are not
suitable climatically for honeybees during pollination time. In certain areas of North America and
northern Europe bumblebees are the only insects capable of foraging
effectively during the blossoming season. Gathering stamens prior to their
opening can yield pollen from pollinizer varieties. These can be stored under deepfreeze until the variety to be
pollinated blooms. They can then be
placed in the entrances to hives in special traps that force the bees to pass
through it on their way to the field.
This practice can reduce the dependency on interplanting and the
pollen may often be purchased on the open market. Synchronization of the pollinizer with the pollinated variety
is also not necessary, and fewer colonies of bees are required because the
bees are not bound in their cross-pollination by their limited foraging
area. However, it is necessary to add
fresh pollen frequently and sometimes the bees learn to avoid the trap or
they carry the pollen back into the hive.
The pollen must be checked periodically for a viability of at least 50
percent. Collected and stored pollen can be
diluted with Lycopodium
and dusted with different kinds of apparatus onto the bloom clusters. Although rapid, this method can be
somewhat unreliable. Aircraft dusting
is wasteful of pollen and usually gives very poor results. Pollen may be transferred manually by
means of a small brush, but it is a tedious and slow process albeit more
reliable than dusting. Pollination of Nonrosaceous Fruit
Varieties In grapes insects may assist in
pollen transfer even though most commercial varieties are self-fruitful. Muscadine and Muscat grapes usually have
sterile pollen so that they require interplantings of male plants or plants
with good pollen. Honeybees and some
solitary bees and many kinds of lies may be attracted to grapes, but in some
areas pollinating insects are not very abundant. Blueberries are self-unfruitful
and require bees for pollen transfer.
They are frequently harvested from wild areas where solitary bees are
plentiful. On the other hand,
cranberries are self-fruitful, and although they may be auto-self pollinating,
honeybees have been moved into cranberry areas with reported large increases
in yields. There are few problems in citrus orchards
with pollination. Navel oranges
produce seedless fruit without pollination.
Most commercial seeded types are self-fruitful but require
pollination. Little pollination may
be required because after one ovule is pollinated other seeds are produced
apogamically from mother tissue and well-shaped fruits result. Orange blossoms are very attractive to
bees and excellent quality honey crops can be produced. The common edible black mission fig produces parthenocarpic
fruit with abortive seeds so it does not require pollination. The preferred Smyrna fig and several other
varieties must be pollinated by another variety, however. The only insects that can pollinate figs
are small wasps of the genus Blastophaga. These wasps develop in special gall
flowers of a type of fig known as Capri. They emerge
from these and accumulate pollen as they exit the fruit. They then enter the “eye” of a Smyrna fig and attempt to oviposit, unsuccessfully, in
the long styles of the female blossoms.
This activity effects pollination.
In commercial plantings the Capri figs containing wasps have been
gathered and hung on the branches of the Smyrna variety. Newer varieties of edible figs are
available that avoid this procedure. Some Special Considerations for
Pollination For certain industries, e.g.,
flower-growing, it is a standard practice to rent honeybee hives for
pollination. Yields can be increased
in some crops that would normally not require pollinators. For example, cotton is largely
self-fertile and capable of self-pollination, though unpollinated ovules
produce a twisted boll that is undesirable.
Therefore, the introductions of pollinating insects give a higher more
desirable yield. To produce hybrid
cottonseed cross-pollination is required, but honeybees do not pollinate
cotton flowers efficiently because extra-floral nectarines are present which
attract the bees more strongly. The
bees do not gather pollen from them either. Honeybees are the principal
pollinators of melon crops. Melon
sweetness and earlier maturity are increased by intensive pollination. In glasshouses the ambient air is
very quiet, so that it is necessary to fan flower blossoms to make them
attractive to honeybees and bumblebees.
Cucumber crops are ruined by pollination so that bees are excluded in
this culture. With the cut-flower trade pollination
is avoided because pollinated flowers lose their attractiveness. In alfalfa pollination once
tripping has occurred some smaller bee species might gather pollen and effect
cross pollination. A membrane covers
the stigma before tripping. This
membrane is ruptured with tripping and either cross- or self-pollination
occurs. Bees are the primary trippers
of alfalfa, but Scolitid wasps will trip
alfalfa in the Southwestern United States.
Cantharid beetles will trip alfalfa in the Great Plains area. The nectar concentration is lower when
alfalfa is grown in humid areas. Bees
quickly learn how to reach the nectar in alfalfa without tripping so that a
large number of bee colonies are required for effective pollination. These must be scattered throughout the
field. Honeybees that re bent on
pollen-collecting can visit about eight flowers per minute of which up to 80
percent will be tripped. When bees
are searching for nectar they may visit 18 flowers per minute but will trip
only about one-percent of them.
Honeybees in the Southwestern United States can collect more pollen
because there is less competition from other plants in the dry climate. Alkali bees are the most successful
pollinators in the Northwestern and Inter-Mountain area of the United States. |
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