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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.


Collected and Stored Pollen


          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.


Examinations & Exercises


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