Crop Pollination (14)
Pollination is the most important contribution bees make to human economies. The value of honey and beeswax pales in comparison to the value of fruits, vegetables, seeds, oils, and fibres whose yields are optimized by pollinating bees. There was a time when it was relatively easy to overlook this benefit, and it may be possible still in particular areas and cropping systems in which there are large and sustainable populations of bees, whether managed or naturally occurring. In such places the rich background of pollinators means that pollination rarely is a limiting factor in crop production.
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Benefits of Bee Pollination (2)
For many important crops, good bee pollination translates into higher yield, larger fruit, higher quality fruit, and faster ripening fruit. These benefits translate not only into optimized incomes for growers, but ultimately into a large and diverse food supply that promotes human health and wellbeing.
For crop pollination purposes, bees are more easily distinguished as either honey bees or non-honey bees.
‘Pollen bees’ is a recently-coined term intended to include all pollinating bees that are not honey bees. Bees in this group concentrate on collecting pollen compared to the honey bees that collect large quantities of nectar. The term pollen bees is aesthetically more pleasing than other choices (i.e. ‘non-honey bees’), but it is ambiguous because both honey bees and pollen bees collect pollen and pollinate crops. Plus, the life histories and management of pollen bees are so diverse that the term quickly loses descriptive usefulness. We prefer the terms ‘honey bees’, ‘non-honey bees’ (includes managed and non-managed species), and ‘non-managed bees’ (includes wild populations of honey bees).
Most non-honey bees have relatively simple life cycles and make simple burrows in wood, grass thatch, hollow stems, or soil and produce, at most, only a few offspring. One exception across much of the temperate developed world is the social bumble bees that make complex comb nests and colonies with several hundred individuals. Culturing and management methods for some non-honey bees are well developed and covered in ' '.
Compared to honey bees, non-honey bees can pollinate certain crops more efficiently because of their distinctive behaviours, morphology, or life habits (Kuhn and Ambrose, 1984; Cane and Payne, 1990). Bumble bees and others sonicate or buzz-pollinate blossoms by shaking pollen from the flower with high-frequency muscle vibrations; this improves pollination efficiency in relatively closed flower structures such as blueberry and tomato. Some non-honey bees have longer tongues than honey bees and this enables them to pollinate tubular flowers, such as red clover, more effectively. The strongly seasonal life history of many solitary bees serves in some cases to enhance their effectiveness as pollinators. These bees have a simple life cycle in which adults emerge, fly, mate, and provision brood cells during the few weeks of peak bloom in their area. Thus many solitary bees are specialists for those plants blooming during their brief flight season, and this specialization works in favour of the grower if those plants are flowering crops. Social honey bees and bumble bees, on the other hand, visit many flowering plants over the course of a season. They are generalists, not specialists, and they are more easily lured away from the crop of interest. The urgency with which solitary bees must work during their short active season also works in favour of the grower; compared to honey bees, non-honey bees often work longer hours, work faster, visit more blossoms per day, and fly more readily during inclement weather.
Solitary bees are generally less likely to sting than are the social honey bees and bumble bees. Bee handlers who propagate solitary pollinators such as alkali bees, alfalfa leafcutting bees, and orchard mason bees routinely do so with little or no sting-protective clothing. There are also disadvantages and uncertainties with non-honey bees and non-managed bees. First, non-honey bees pollinate several crops more efficiently than honey bees on a per-bee basis, but no study has accounted for the overwhelming population advantage of honey bee colonies. Honey bees produce the largest colony population sizes of any bee species. Conceivably, one honey bee colony, with thousands of inefficient (for argument’s sake) pollinators could match or exceed many nests of efficient, but solitary, bees (see Corbet et al., 1991, for an experimental design to determine the pollination value of a given bee species). Populations of non-managed bees are often too small to support commercial pollination needs (Morrissette et al., 1985; Parker et al., 1987; Scott-Dupree and Winston, 1987) or vary considerably between years and geographic regions (Cane and Payne, 1993). Just like honey bees, non-honey bees have diseases, predators, and parasites that limit their natural populations or, for managed populations, must be controlled by the bee-keeper. Nevertheless, where they occur in sufficient numbers, whether through favourable habitat, mass introductions, or culturing, non-honey bees can replace or supplement honey bees for commercial pollination in some crops.
There are about 25,000 described species of bees in the world. Most are solitary species in which females single-handedly make a nest and produce the next generation of fertile offspring. Most solitary species produce only one or two generations per year.
Social species live together as a colony of related individuals in which there is: (i) cooperative care of the young; (ii) more-or-less infertile female workers; and (iii) offspring that stay at the nest to help their mother produce more siblings.
In social species, individuals of the same sex occur in different sizes and forms called castes. The queen is a female fully equipped to mate and lay fertilized eggs that become female workers or queens. Workers are females that do not mate but on occasion do lay unfertilized eggs that become males. Workers do most of the house cleaning, brood nursing, temperature regulating, foraging, and defending for the colony. Male bees, sometimes called drones, are visibly distinguishable from workers and queens; their only known function is to mate with queens. Both solitary and social species can be important crop pollinators.
Bees belong to the insect order Hymenoptera which also includes the sawflies, ants, and wasps. Unlike other hymenopterans, bees tend to specialize on exclusively vegetarian diets. Both immature bees and adults eat plant-derived pollen for protein and nectar for energy. Some social species feed their young glandular secretions produced by nurse bees, conceptually like lactating mammals. But even these secretions are metabolically derived from pollen and nectar. Some social bees with long-lived colonies dehydrate nectar into honey, a process which preserves the nectar for long-term storage.
Bees have body features that further distinguish them from other insects. Many of the body hairs on bees are finely branched so that pollen grains cling to them readily. Most bees have external body structures specialized for carrying pollen. With some, a segment of each hind leg has a structure called the corbiculum or pollen basket for holding loads of pollen while the bee is foraging. Others carry pollen on long hairs attached to their hind legs. One group carries pollen loads on the underside of the abdomen. Bees’ pollen-carrying capabilities and flower-visiting habit make them one of the most important crop pollinators worldwide.
Developing immature bees go through a complete metamorphosis. Individual bees start life as a single egg laid by their mother. After a few days the egg hatches into a larva (plural larvae) which is a grub-like, rapidly-growing feeding stage. As they grow, larvae shed their skin several times by a process called moulting to advance to the next larger stage, or instar. Eggs are laid and larvae develop in cells varying in complexity, ranging from hexagonal beeswax cells to simple dead-ends in earthen tunnels. The mother or siblings provision each larval cell with food. Some species add pollen and nectar to the cell regularly as the larva needs it; others feed it all at once as a large, moist lump at the time the egg is laid. Because bee larvae literally live in their food, defaecation is a problem. Larvae solve this by postponing defaecation until their feeding career is over. When it completes its feeding period, the larva defaecates, stretches out (then called a prepupa), and transforms into a pupa (plural pupae) which is a quiet stage during which larval tissues are reorganized into those of an adult. Finally, the pupa moults into an adult, complete with six legs and four wings, and breaks out of its cell. Species vary in the amount of time immatures spend in each stage.
Female bees control the sex of their offspring. They store sperm from their matings in the spermatheca, an organ connected to the oviduct which is the passage down which eggs pass during oviposition. Females have muscular control over the spermatheca. By opening it and releasing sperm on to a passing egg, the female can fertilize the egg and the result is a female. Unfertilized eggs result in males. This ability to regulate sex of offspring is important in solitary tunnel-nesting bees because they tend to lay male eggs near the nest -entrance so males can precede the females in spring emergence. For social species it is important to time male production according to seasonal food availability.
The adult stage of bees is dedicated to dispersal and reproduction. Bees do this with a variety of life strategies and nesting habits, ranging from solitary to social, from simple burrows to elaborate comb nests.
One of the most recent, controversial, and potentially revolutionary areas of agricultural research is the development of genetically engineered, or so-called transgenic crops. Genetically engineered organisms are ones in which genes from different species have been
inserted in such a way as to permit those genes to express their characteristics in the host organism. In crop plants the focus has been on inserted genes from the bacterium Bacillus thuringiensis that trigger production of insecticidal compounds in the tissues of the host
plant. The number of different transgenes that have been used to confer insect resistance in crops approaches, but only B. thuringiensis transgenes have been commercialized. Transgenes also have been used to confer herbicide tolerance, drought or salt tolerance, or to alter the nutritional qualities of the crop. The relevance of pollinators to transgenic crop technology rests on two issues – the potential harm to pollinators posed by transgenic crops, especially insecticidal ones, and the potential harm to the environment caused by pollinators spreading transgenes into wild plant populations.
Transgenic insect resistance is seen to have numerous advantages over conventional insecticides. It provides more targeted delivery of a toxin to the pest, greater resilience of the toxin to weather or other forms of biodegradation, reduced exposure risk to the applicator, and reduced use of conventional broad-spectrum insecticides and their associated risks to the environment. But there has been concern that engineered toxins, whether in the plant’s tissues or in its nectar and pollen, could be detrimental to bees. Fortunately, that risk seems small at this time. A sizeable research record has shown that B. thuringiensis toxins, whether conventional or engineered, are generally benign to bees. The risk from other candidate engineered toxins, namely the pesticidal proteins chitinase, glucanase, and cowpea trypsin inhibitor.
Another area of concern with this new technology has been the potential of transgenes ‘escaping’ from engineered crops into wild plant populations with unknown and perhaps detrimental effects. Chief among these concerns is the potential for transgenes for herbicide tolerance becoming incorporated into weedy species, thus making them more difficult to control. Bees, because of their pollen-collecting habit and catholic flower preferences, are seen as primary vehicles for the spread of such transgenes. One solution to the problem may be a buffer zone of conventional crop plants grown around the transgenic ones. Pollinators visiting the transgenic plants may subsequently deposit much of their plant-available pollen on to conventional crop flowers in the buffer zone before leaving the area to visit other plant species or to return to the nest. It is unlikely that buffer zones alone will solve the problem in all cases. In one study, honey bee colonies were placed at a distance of 250 m from a field of transgenic herbicide-tolerant maize. The transgenic field was surrounded by a 3 m buffer zone of conventional maize. Of the pollen samples collected from the colonies, 52% contained the transgene for herbicide tolerance; thus the 3 m buffer zone did not prevent the spread of the transgene. There is evidence that most of the pollen from a particular plant is deposited by a bee forager on to the next few subsequent flowers visited, but some pollen can persist for up to the 20th subsequent flower. Ongoing research is concentrating on gene flow from transgenic crops, the competitiveness of transgenic plants, the efficacy of isolation distances, and the interactions of bee foraging behaviour with pollen movement among plants.
Ecologic theory supports the notion that bee visitation, pollination and seed yield. Likewise, low nectar quality and associated low levels of bee visitation are limiting factors in fruit-set in avocado. There is a clear and positive relationship among nectar sugar concentration, frequency of bee visitation, and resulting seed number in watermelon. Thus, theoretical work and supporting field studies strongly suggest that it is in the best interest of farmers to grow crop plants that are attractive to bees.
Nectar production is affected by ambient conditions and culturing practices. Nectar production is relatively high under conditions of low nitrogen supply, moderate growth, and high levels of sugar in tissues. It is lower under conditions of abundant nitrogen supply, high vegetative growth, and low sugar levels. Nectar production is generally higher in sunny weather because sugars accumulate in plant tissues during photosynthesis. These sugars may reach a surplus and be excreted as nectar if plants are not growing maximally. However, if nitrogen is available it encourages plant growth which diverts stored sugars into proteins and other products necessary for producing tissues. Thus, nectar production tends to be lower in plants that are growing rapidly. (In perennial plants that bloom before leaves unfold in spring, nectar production relies on sugars stored in tissues from the previous season.) The model of nectar production presented above is not universally applicable; for example, nectar production in the Mediterranean herbaceous perennial Ballota acetabulosa is relatively unresponsive to measured changes in solar irradiation.
There is evidence that nectar production also may be affected by plant genetics. There are measurable differences in nectar production within crop genera or species, as in pepper, cranberry, and watermelon, and sometimes these differences are apparent even when environmental or cultural effects are controlled. These studies suggest that nectar production is at least partly under genetic control and could be increased by selective plant breeding.
The time is right to give renewed attention to increasing nectar production in the world’s most important bee-pollinated crops. This is especially justified given the evidence for a generally decreasing pool of available bee pollinators. Nectar production has received comparatively little attention from crop breeders, agronomists, and horticulturists. Low nectar production may not be a problem in areas with abundant bee populations, but where bees are scarce a nectar-poor crop will have trouble competing with weeds for the
limited number of pollinators.
Bees use signals from plants to identify worthwhile visits. In some species the flowers remain open, intact and turgid until they are pollinated, after which they are no longer attractive to pollinators. The negative cues involved in this include cessation of nectar and scent production, change in colour, wilting, permanent flower closure, and petal drop. Even receptive inflorescences can vary in their attraction to bees. In general, inflorescences with a larger number of open flowers have higher nectar rewards, and bees preferentially land on those inflorescences. Once landed on an inflorescence, bees prefer wide, relatively shallow flowers, presumably because the nectar is more accessible to evaporation which concentrates it and increases the energetic profit of the visit.
These observations from ecological studies form the basis of a practical crop pollination recommendation. It is advisable for growers to delay the introduction of bee hives in an orchard until the crop has already begun a modest amount of flowering. This practice will provide bees with an abundance of floral signals that will encourage them to concentrate on the crop instead of non-target plants in the area. It is customary in apple to delay hive introduction until the crop is at about 5% bloom.
Bees, having encountered a patch of profitable flowers, tend to forage in a more-or-less straight line. This behaviour limits the chance of a bee revisiting a flower recently emptied of its nectar. The most straightforward implication for crop pollination involves those crops in which a main variety is inter-planted with one or more pollenizer varieties to ensure cross-pollination. Optimal foraging theory would suggest that the main varieties and pollenizers should be planted in the same orchard row to increase chances of bees cross-pollinating them.
From a pollination perspective, bee foraging activity is generally more efficient in flower patches that are rich in nectar and pollen. It has been shown that animals forced to forage in resource-poor habitats tend to spend more time at each food site than do animals in rich habitats (Pyke et al., 1977). It is advantageous for insects to be moving rapidly between flowers, accomplishing a high rate of pollination, rather than lingering for relatively long periods on the few flowers in a patch that are yielding nectar. Southwick et al. (1981) demonstrated that bee visitation rates increased in flower patches with increasing number of nectar-bearing flowers, nectar volume, and sugar concentration of nectar.
Not only do resource-rich plantings encourage rapid bee visitation between flowers, but they encourage pollinators to stay in that patch. It was shown that bumble bees and honey bees that have just visited highly-rewarding flowers fly shorter distances before visiting another flower than do bees that have just visited less rewarding flowers (Pyke, 1978; Waddington, 1980). This behaviour increases the likelihood of the bee encountering another rewarding flower in a site which is shown to be profitable.
Collectively, these studies make a strong argument for improving the nectar and pollen production output of our important bee-pollinated crops. Optimal foraging theory predicts that if nectar output of a crop is relatively high, bees pollinate more efficiently because they visit more flowers in a given period of time. Conversely, if the crop is nectar-poor, bees forage more slowly and visit fewer flowers.
It has been shown that honey bees are capable of flying several miles to forage if necessary, but they forage preferentially near their nest if the resource richness permits. This implies that honey bee colonies used for crop pollination should be placed relatively near the target crop. Interestingly, the matter is reversed for bumble bees which prefer to forage on resources 164–2070 ft (50–631 m) from their nest. However, this fact has little bearing on commercial bumble bee pollination because bumble bees are most often used in glasshouses in which their foraging range is artificially limited.
Many insects visit flowers to collect pollen as food. As they do this they pollinate the flowers. Most flowers offer sugary liquid nectar as a reward for these pollinating insects. Bees are especially effective insect pollinators because they eat pollen and nectar almost exclusively, visit many flowers of the same species during a single trip, and have hairy bodies that easily pick up pollen grains. There is a close association between flowering plants and bees. Bees pollinate over 16% of the world’s flowering plant species (Buchmann and Nabhan, 1996) and nearly 400 of its agricultural plants (Crane and Walker, 1984).
Ecology of Bee Pollination
Although bees and bee-pollinated flowering plants depend on each other, both operate selfishly. For each, there is a cost/benefit equation that must balance in its favour. Nectar and pollen production are costly to a plant and must be balanced for maximum return (that is, maximum chance of successful reproduction) for the energy spent to produce them. For example, individual flowers must contain enough nectar to attract pollinators, but little enough to keep pollinators motivated to visit many flowers, thus accomplishing pollination. Some plants accomplish this strategy by putting heavy nectar loads in only a few (5–8%) of their flowers. These constitute the so-called ‘lucky hits’ for a bee (Southwick et al., 1981) that motivate it to keep foraging on the plant. Flight and foraging activity are energetically costly to a bee and must be balanced against the calories derived from nectar and pollen. A large body of ecologic literature, called optimal foraging theory, predicts that foraging animals will forage efficiently, moving between food patches and lingering in food patches in such a way as to get the most return for their effort. When such hypotheses are tested in the field, they have often proved true. It is beyond the objectives of this book to treat this rich topic exhaustively, but we have selected for discussion a few principles of bee ecology that have implications for commercial crop pollination management.
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Not all flowering plants have the same pollination requirements.
Cross-pollination is the transfer of pollen from flowers of one plant to the flowers of a different plant or different variety. Many crops require or benefit from cross-pollination.
Self-fertile plants can develop seeds and fruit when pollen is transferred from anthers of a flower to the stigma of the same flower or different flower on the same plant. However, such plants are not necessarily self-pollinating. Insects still may be necessary or helpful in moving pollen to the stigmas. Inter-planting of varieties is not necessary but may be helpful; for example, many self-fertile crops, such as Swede rape (canola) and high-bush blueberry respond well to cross-pollination.
Self-sterile plants require pollen from a different plant or even a different variety. If the plant requires different varieties, the grower must interplant pollenizer varieties with the main variety. Cross-compatible varieties are receptive to each other’s pollen, whereas crossincompatible varieties are not. Seed and nursery stock catalogues usually provide tables that cross-list compatible varieties.
Monoecious plants have both male and female flowers on the same plant. Dioecious plants have only one sex of flower on the same plant, rendering cross-pollination obligatory.
Parthenocarpic plants develop fruit without requiring the pollination process, and that being the case, parthenocarpic fruits can be partially or completely seedless. There are plant growth regulators that can be applied to induce plants, even plants normally cross-pollinated, to develop fruit parthenocarpically. This is the case in rabbiteye blueberry with the growth regulator gibberellic acid which is applied in early spring to augment natural pollination. It is important to treat these chemicals as supplements for pollination, not its replacement.
Pollination
Pollination is the transfer of pollen from the male parts (anthers) of a flower to the female part (stigma) of the same or different flower. If the pollen is compatible, fertilization of the ovule and seed formation can occur. More seeds develop when large numbers of pollen grains are transferred. Seeds, in turn, stimulate surrounding ovary tissue to develop so that, for example, an apple with many seeds will be larger than one with fewer seeds. In this way, good pollination improves both fruit yield and size. Pollen may be transferred by wind, gravity, water, birds, bats, or insects, depending on the plant. Some flowering trees in the tropics are pollinated by monkeys (Gautier-Hion and Maisels, 1994), and in Japan at least one company grows and markets a fly that pollinates strawberries and other crops (Matsuka and Sakai, 1989). Worldwide, bees are the most important pollinators owing to their vegetarian diet, flower-visiting habit, and hairy bodies that readily pick up pollen grains.
The Flower and the Fruit
A flower is a plant organ designed for sexual reproduction. An inflorescence is an arrangement of flowers on a stem. There are several types of inflorescences – single flower, head, raceme, panicle, spike, and umbel. The main stem of an inflorescence is the peduncle, and the stem of any individual flower is the pedicel.
The outer whorl of petals is called the corolla (plural corollae) and is designed to protect the interior sexual parts, to exclude ineffective pollinators, to attract effective pollinators, and to direct effective pollinators towards the inside of the flower. In legume-type flowers, two anterior petals join to form a keel inside which are housed the sexual parts of the flower. Male parts of a flower are called the stamens, each made up of a slender filament holding an anther at the tip. When it is mature, the anther opens and releases pollen grains which contain the equivalent of animal sperm. Female parts of a flower are called the pistil, each made up of an ovary with ovules and a stalk-like style with a sticky stigma on top.
A flower with both stamens and pistil is called a perfect flower. Many plants have flowers that are imperfect, that is, only male or only female. Sometimes both types of imperfect flowers occur on the same plant. It is easy to identify imperfect female flowers in cucurbit crops (cantaloupe, cucumber, gourd, pumpkin, squash, watermelon) because of the large ovary at the flower’s base which later develops into a mature fruit.
When a pollen grain lands on a receptive stigma, it grows a pollen tube down the style to the ovary. Male genetic material passes down the pollen tube and fertilizes an ovule. Ovules become seeds and the surrounding ovary develops into the fruit. This process is called fruit-set.
A single pollination episode does not necessarily guarantee fruitset. Pollen in sufficient quantities must reach the stigma while its surface is receptive, the pollen must be compatible, and pollen tubes must successfully grow down to and penetrate the ovules. Many things can go wrong. Transfer of pollen can be poor if the pollinator population is low or bad weather keeps pollinators from foraging. Female flowers are often most receptive to pollen in early morning, and anything that disturbs morning bee visitation, such as rain, can impair pollination even if bees are active for the rest of the day. An absence of compatible blooming pollenizers (sources of pollen) can prevent good pollination in plants that require inter-varietal cross-pollination such as apples and rabbiteye blueberries.
Flower structures vary widely, and there are many types of fruit. A berry, such as tomato, has a fleshy outer wall surrounding one or more fairly small seeds. A pome, such as apple, has a fleshy outer wall surrounding a tough core with seeds. A drupe or stone fruit, such as peach, has a fleshy outer wall surrounding one stony seed. With aggregate fruits, such as strawberry and raspberry, many pistils develop together as a single mass. If pollination is poor in ovaries with more than one ovule (such as berries and pomes) or in multiple neighbouring ovaries (such as aggregate fruits), ovarian tissue develops only around those ovules that are fertilized. This is a cause of misshapen fruit in many crops.
Since good pollination increases fruit yield and quality, farmers have long been interested in this phenomenon. The civilizations of the ancient Middle East understood, at least in a practical sense, the importance of pollination. A bas relief from Assyria dating around 1500 BC shows mythological creatures manually cross-pollinating date palms. The prophet Amos in the 8th century BC was a ‘piercer of sycamores’, a practice still done today in which poorlypollinated figs are manually gashed to induce ripening. Today, 90% of worldwide national per capita food supplies are contributed by 82 commodities that can be assigned to plant species and by 28 general commodities (such as hydrogenated oils) that cannot be assigned to particular species. Bees are pollinators for 63 (77%) of the 82 species commodities, and they are the most important known pollinator for at least 39 (48%) (Prescott-Allen and PrescottAllen, 1990; Buchmann and Nabhan, 1996). The multiplicative value of bee pollination becomes apparent when one tallies bee-pollinated food plants and considers the large quantities that are converted to animal feeds and ultimately meat, egg, and dairy products. One wellworn, and probably accurate, estimate says that one-third of the human diet can be traced directly, or indirectly, to bee pollination (McGregor, 1976). This estimate is probably more accurate for human diets in developed countries.
For fruit- or nut-bearing crops, pollination can be thought of as a grower’s last chance to increase yield. It is the degree and extent of pollination that dictates the maximum possible number of fruits. All post-pollination inputs, whether growth regulators, herbicides, fungicides, or insecticides, are generally designed not to increase yield but to conserve losses. Because of its yield-optimizing benefits, bee pollination can play an important role in maintaining a sustainable and profitable agriculture with minimized disruptions to the environment. Alterations in agricultural practices that significantly reduce yield rates have the danger of encouraging more wild lands to be converted into farmland to make up for reduced yields (Knutson et al., 1990). Good bee pollination and optimized crop yields are thus part of a sound environmental management policy.
Although wind-pollinated cereals make up the bulk of human diets, insect-pollinated crops often mean the difference between eating for survival or eating for pleasure. Insect pollinated crops are the delicacies one can easily take for granted. They are the low-acreage, high-value crops that pump millions of dollars into local agricultural economies. They are the forage plants that fuel livestock production. To gain an appreciation of bee pollination, one need only imagine life without beef steak, blueberry muffins, ice cream, pickles, apple dumplings, or watermelon. For many people in the world, such deprivations are not imaginary. If the gross disparities that exist in the world between rich and desperately poor, well-fed and not, are ever to be absolved, bee pollination will play a part. The area of bee-pollinated crops is increasing in many developed countries (Torchio, 1990a; Corbet et al., 1991). In Canada, over 17% of cultivated land is used for crops that depend entirely or in part on insect pollination (Richards, 1993). If developing countries follow suit, we can expect unprecedented growing demand for bee pollination in the 21st century.
The degree to which a particular crop needs insect pollination depends on the flower morphology, level of self-fertility exhibited by the plant, and arrangement of flowers on the plant or on neighbouring plants. Those crops are most dependent on insect pollination that have separate male and female flowers (so-called imperfect flowers), whether occurring on separate plants or on the same plant. In these cases insects, especially bees, are important pollen vectors, moving pollen from male to female flowers. There is a higher rate of self-pollination in plants with flowers housing both male and female sexual components (perfect flowers); however, bees often optimize pollination even in perfect flowers. Pollination in other crops, particularly the cereals, is accomplished by wind and gravity, and bees play only a minor role.
Finally, the economic value of bee pollination goes beyond production agriculture because bees pollinate more than just crop plants. All told, bees pollinate over 16% of the flowering plant species in the world (Buchmann and Nabhan, 1996). Bee pollination sustains native and introduced plants that control erosion, beautify human environments, and increase property values. Bees pollinate native plants which provide food for wildlife and have inherent value as members of local natural ecosystems. Although some believe that this generalization does not apply to the cosmopolitan honey bee, A. mellifera, which is an exotic species throughout most of its modern range, the bulk of experimental evidence suggests that introduced honey bees are only rarely a detrimental feature of local ecologies (Butz Huryn, 1997). In the absence of large-scale demonstrable negative impacts of introduced honey bees and considering their widely acknowledged value as pollinators of crop plants and their catholic plant preferences, it seems reasonable to anticipate that honey bees, even introduced populations, play an important role in sustaining natural plants and the animal communities that depend on them.
It is no exaggeration that the sheer abundance, high quality, and variety of food enjoyed today in much of the developed world – a bounty unmatched by any other period in history – derives in no small measure from bee pollination. The western honey bee (Apis mellifera L.) is arguably the most well-known bee pollinator of crops. Its native range is large, extending from northern Europe, through the Middle East, and all of verdant Africa. Beginning in the 17th century, European colonists began actively spreading this bee throughout much of the world. In the ensuing centuries A. mellifera has proven itself highly adaptable to a broad range of climatic conditions. Its adaptability, its tolerance of human management, and its honey-making habit have secured its place as humanity’s favourite bee. Large feral populations of honey bees became the norm in much of the world, populations that contributed significantly to crop pollination, with or without the knowledge or appreciation of the farmer. Today, many countries have large and sophisticated bee-keeping industries dedicated to the production of honey, other hive products, and pollination services
Five important species of honey bees are as follows.
- The rock bee, Apis dorsata (Apidae).
- The Indian hive bee, Apis cerana indica (Apidae).
- The little bee, Apis florea (Apidae).
- The European or Italian bee, Apis mellifera (Apidae).
- Dammer bee or stingless bee, Melipona irridipennis (Meliporidae).
The important features of these species are given below.
Rock bee (Apis dorsata)
They are giant bees found all over India in sub-mountainous regions up to an altitude of 2700 m. They construct single comb in open about 6 feet long and 3 feet deep .They shift the place of the colony often. Rock bees are ferocious and difficult to rear. They produce about 36 Kg honey per comb per year. These bees are the largest among the bees described.
Little bee (Apis florea)
They build single vertical combs. They also construct comb in open of the size of palm in branches of bushes, hedges, buildings, caves, empty cases etc. They produce about half a kilo of honey per year per hive. They are not rearable as they frequently change their place. The size of the bees is smallest among four Apis species described and smaller than Indian bee. They distribute only in plains and not in hills above 450 MSL.
Indian hive bee / Asian bee (Apis cerana indica)
They are the domesticated species, which construct multiple parallel combs with an average honey yield of 6-8 kg per colony per year. These bees are larger thanApis florae but smaller than Apis mellifera. They are more prone to swarming and absconding. They are native of India/Asia.
European bee / Italian bee (Apis mellifera)
They are also similar in habits to Indian bees, which build parallel combs. They are bigger than all other honeybees except Apis dorsata. The average production per colony is 25-40 kg. They have been imported from European countries (Italy). They are less prone to swarming and absconding.
Dammer Bee
Besides true honey bees, two species of stingless or dammer bees, viz. Melipona and Trigona occur in our country in abundance. These bees are much smaller than the true honey bees and build irregular combs of wax and resinous substances in crevices and hollow tree trunks. The stingless bees have the importance in the pollination of various food crops. They bite their enemies or intruders. It can be domesticated. But the honey yield per hive per year is only 100 gms.
Bee-keeping is a viable agricultural pursuit in developing countries, but the bee-keeping industries in many developed countries have contracted. World honey prices have been depressed the last few decades owing in part to the availability of other cheaper sweeteners. Parasitic varroa mites (Varroa sp.) and tracheal mites (Acarapis woodi) have spread from their native ranges and killed untold thousands of managed honey bee colonies and virtually eliminated feral populations in places.
One result of these hardships has been a renewed interest in the use of bumble bees and solitary bees as commercial pollinators. Only a few species of such alternative pollinators have been successfully cultured, so there is an emphasis on conserving their natural populations. There is great need for research in the conservation, culture, and use of these bees for pollination. Naturally-occurring bee populations are not always dependable for commercial pollination needs, owing to their uneven distribution or loss of their natural habitats and food plants. Rearing and managing methods for some non-honey bees are finely worked out and practical, but for others the rearing methods are poorly developed or protected as proprietary secrets.
One of our aims in this section of website is to promote an appreciation of all available bee pollinators. Pollinating bees, whether managed or naturally-occurring, are a valuable and limited resource. In this area of website, we concentrate on managing and conserving bees to optimize crop pollination. We cover honey bees, other managed bee species, and wild non-managed species. Each group has assets and liabilities from a plant grower’s point of view, but each deserves our best efforts to maintain its populations through good management or conservation.