Crop Pollination

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

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.

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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.