Pollination is necessary for all seed and fruit production by flowering plants. Transfer of pollen among flowers to allow their reproduction is a vital mechanism for maintaining life on earth. People harvest the seeds of some crops for food; examples are oilseed crops, nuts, legumes such as beans and peas, and cereals such as rice and maize. Other crops provide fruit that develops with the seed, for example citrus fruits, mangoes and tomatoes. Seed is required for the production of the next generation of crops and allows plant-breeding pro-grammes to improve varieties.
Plant reproduction requires the transfer of pollen from the anthers, or male part of a flower, to the stigmas, or female part of a flower, either on the same plant or on a separate plant that may be some distance away. After thousands of years of evolution and adaptation to local environments, each plant species has specific requirements for this important transfer of pollen; many depend on foraging insects to transfer pollen among flowers. Many species of insects visit flowers to seek nectar or pollen; while doing so most will transfer a few pollen grains, thus contributing to pollination. Honeybees are highly efficient pollinating insects, because:
FIGURE 4 Everywhere in the world where there are flowering plants - there are bees, and over 25 000 species have so far been described.
· they have hairy bodies that easily pick up thousands of grains of pollen as they move about inside flowers;
· they visit only one species during each foraging trip;
· each foraging bee not only collects sufficient food for its own requirements, but continuously forages for nectar and pollen to supply the daily food needs of the colony.
During a single day, one bee may visit several thousand flowers of one plant species, collecting nectar and pollen and continuously transferring pollen grains from one flower to another.
Cross-pollination occurs when an insect moves pollen from one plant to another. It is needed when plant sexes are segregated on different plants, such as melons, or by different periods of flowering of the same plant, such as avocado. Many varieties of fruit trees need cross-pollination; they should be planted so that pollinizer trees are near the main crop trees. Production of hybrid seed crops on a commercial scale creates a special need for cross-pollination by insects: a large population of pollinating insects is needed to carry pollen from rows of male plants to rows of female plants.
Crops vary in the extent to which they benefit from insect cross-pollination. Some crops, such as field beans and mangoes, are self-pollinating but give better yields if pollinated by insects. Many, such as passion fruit, cowpea, sesame, litchi, mustard and cashew, give a substantially increased yield when pollinated by insects. Others such as sunflowers, clover, beans, almonds and melons are completely dependent on pollination by insects and otherwise will not produce crops.
Adequate insect pollination affects both the quantity and quality of crops: uneven, small fruit often indicate insufficient pollination. Adequate pollination by insects also ensures that early flowers set seed. This results in a uniform and early harvest and gives the crop the maximum length of time to ripen.
FIGURE 5 Coffee in Yemen. Yield improves significantly when optimally pollinated by bees; beekeepers obtain good honey crops from the abundant nectar produced by coffee flowers.
Pollination can be as important in crop production as water or fertilizer. With the use of improved cultivars and irrigation, pollination can be the limiting factor. The pollination requirements of all major temperate zone crops are well known. In countries with highly mechanized agriculture, the use of bees for pollination increased greatly during the twentieth century and became an integral part of crop production. Less research has been done on the pollination requirements of crops grown in the tropics.
Many other species of bees and pollinating insects living in the wild are highly important for pollination, in addition to honeybees living in the wild or managed in hives by beekeepers. Many factors have caused a decline in the numbers of these insects available for crop pollination. The most serious threat to pollinating insects is the use of insecticides. Herbicides, grazing or cutting of roadside verges and other destruction of flowering plants remove food sources for pollinating insects. Intensive land cultivation and destruction of hedges, banks and rough verges further reduces the habitat for nesting and hibernation sites for bees.
It is in everyone's interest to maintain strong populations of honeybees and other pollinating insects. This means increasing people's awareness of the value of insect pollination, stopping unnecessary pesticide use and increasing forage for bees by including nectar-bearing bushes and trees in planting schemes. Farmers can contribute to the protection of honeybees and their habitats as follows.
· Select and use insecticides with great care; if wild pollinating insects are destroyed, there is a risk of decreased crop yields in the future.
· Never use insecticides when flowers are open; foraging insects work on open blossoms and are killed by sprays at this time; if insecticides must be used, spray at a time of day when crop flowers are closed.
· Allow wild plants to flower on wasteland, because they will help to support populations of foraging insects.
· Make habitats more acceptable for nesting and hibernation of pollinating insects.
Paradoxically, intensive agricultural practices diminish the numbers of wild pollinators and at the same time increase the need for them. Larger fields increase the need for pollination while a crop is flowering, yet decrease the ability of the local insect population to pollinate adequately. The tendency to concentrate particular crops in certain areas intensifies this situation, because when the major crop is not in flower there may be insufficient forage from other sources.
In temperate countries, large-scale monocultures have increased the need for pollination, yet have decreased the populations of wild pollinators. A similar dilemma is arising in the tropics, where there has been an increase in mechanized farming and an accompanying increase in field size. In the tropics, however, crop flowering is more prolonged and less intensive than in temperate regions. Where growing conditions are favourable, the same crop species may occur in a sequence of growth stages. Many fruit trees flower and fruit throughout the year, albeit more abundantly at certain periods, and therefore forage for bees may be present at all times.
Increased monoculture in the tropics means that flowering will be more concentrated, so large pollinator populations will be needed for shorter periods. Although pollen sources that allow cross-pollination are naturally present in small mixed farms, special provisions for crop pollination are necessary for large areas of a uniform crop (Free, 1999).
FIGURE 6 Market stall in Egypt. Adequate pollination increases the quality and quantity of many cash crops.