COMMERCIAL POLLINATION OF DECIDUOUS FRUIT
Commercial pollination is a critical component of integrated orchard management practice. Obtaining optimal fruit set requires the correct orchard design (selection and positioning of pollinisers), flower management (pruning and thinning), control of alternative forage, as well as the proper introduction and usage of adequate strength honeybee colonies.
There is little published information or scientific data on the commercial pollination of deciduous fruit crops in South Africa. Most practices are based on experience and anecdotal evidence. A current DFPT project evaluating basic principles of pollination is intended to remedy this situation. This article should be used only as a GUIDELINE, to be updated as more information becomes available.
Pollination is one step in a multi-stage process that culminates in the development of seed or fruit, and is the transfer of pollen from the male anther to the female stigma of the flower. The agents (pollinators) that bring about this transfer of pollen are varied and include insects, birds, mammals, wind, water and gravity. The two most important agents are insects and wind. Most fruit, seed and berry crops are dependent to a lesser or greater extent on insects, first and foremost honeybees, to ensure a fruit or seed set.
What Pollinates Deciduous Fruit Crops in South Africa?
Insects typically are responsible for about 85% of pollination, and honeybees for about 80% of insect pollination. Honeybees are highly adapted as successful pollinators with bodies covered in finely-branched hairs that develop static electricity sufficient for the collection of as many as 40 000 pollen grains on the body, pollen baskets to collect pollen, and a high demand for pollen and nectar. Both nectar-gathering and pollen-gathering honeybees are important for pollination, the latter more so. Approximately one third of the foods which make up our total diet comes directly or indirectly from honeybee-pollinated crop plants. The value of fruit, vegetables and seeds resulting directly from pollination is about 150 times the value of bee products. In South Africa the value added to crops by the use of honeybees in paid pollination is in excess of R4 billion per annum of which almost R2 billion is from deciduous fruit crops.
Many growers believe that “other pollinators” are sufficient for the pollination of their crops. This might well have been true in days gone by but now insects other than honeybees play only a supplementary role in pollination. Recent data indicates that between 94-98% of all insect visitors to deciduous fruit trees in the Boland were honeybees, and that honeybees made up more than 98% of actively pollinating insects. “Natural” pollinators are becoming increasingly scarce and unimportant, probably due to increasing habitat destruction and pesticide load. There are also no other “commercial” pollinators that can be used in South Africa, such as the leaf cutter bees or bumblebees. The deciduous fruit industry is almost totally dependent on a healthy honeybee population and a viable beekeeping industry.
Why commercial pollination?
There is extensive data on almost all deciduous fruit species and cultivars that indicate that the introduction of commercial honeybees for pollination improves fruit set, fruit weight and fruit quality. Successful pollination results in fertilization with the fertilized ovule developing into a seed comprising an embryo (derived from the fusion of one male nucleus with the female egg) and the endosperm. The endosperm nourishes the embryo during its development but also has a second and perhaps more important function, for it secretes hormones that control the growth of the embryo and the growth and shape of the fruit and the retention of the developing fruit. Seed hormones are particularly important in apples and pears, with inadequate pollination resulting in low seed numbers causing fruit drop and misshapen fruit. While it is not necessary to have ten seeds in every fruit, the more seeds produced the more export quality fruit produced, and the better the pollination the more seeds that are produced. There is generally a positive relationship between successful pollination and: (a) fruit set; (b) percentage of fruit that do not drop; (c) effectiveness of chemical thinning; (d) fruit size; (e) fruit shape; and (f) fruit quality and shelf-life.
Some basic principles regarding honeybee foraging behaviour are important in optimizing the performance of honeybee colonies in commercial pollination. Foremost among these is that honeybees forage on flowers only for a reward of pollen and/or nectar; pollination is a fortuitous consequence of the foraging. Honeybees are able to detect minor differences in the sugar concentration present in nectar, and in the protein content in pollen, and for the most part will forage on the most rewarding resource available. Nectar in apples and plums is relatively poor in both sugar content and quantity, with normally only enough nectar to sustain colonies and not enough for a honey surplus. Apple and plum nectar levels are inferior to those of common weeds such as Cape Weed (Gousblom) and Wild Radish (Ramenas). Pear flowers have extremely poor nectar, both in sugar content and quantity, and are not very attractive to foraging honeybees. All of apples, plums and pears have pollen that is readily collected by honeybees, but is not as attractive as that of common weeds. It is because the reward provided by deciduous fruit crops is less than that of common weeds that the practice of introducing naïve bees (colonies introduced during blossom) has become standard in commercial pollination. The rationale is that by introducing honeybee colonies when target crops are already 20% or more in bloom, that this will result in foragers working on the inferior-quality target crop for a number of days at least while other foragers locate better quality forage crops, to which the foragers will then switch. Recent results indicate that bees might not switch as readily as thought, and if confirmed, these practices (i.e. the use of naïve bees) might need to be reviewed.
Notwithstanding the ability of honeybees to assess forage quality and to rapidly direct foragers to more fruitful resources, individual honeybee foragers are remarkably flower constant and area constant. At any one time the field force of a honeybee colony will demonstrate a preference for a number of forage species, but individual foragers will tend to work only on a single forage species until that forage type becomes unavailable or unrewarding, and will only “switch off” this forage if markedly better forage becomes available. Individual honeybee foragers will also tend to work in a small forage area of about 10m2, returning to the same tree or to adjacent ones on successive foraging trips. This also means that in the repeated use of the same colonies for pollination, colonies should be moved a minimum of 3km, to prevent foragers returning to the old colony position.
In summary: Relatively few foragers from any colony will be active on the relatively poor quality deciduous fruit crop but those that there are will not readily leave. If they do, however, they will not easily return. All efforts at management to improve commercial pollination should bear these principles in mind.
Making arrangements for insect pollination is standard practice in the growing of many crops. Scientific information on the management practices to be followed is very limited and is largely based on experience and anecdotal evidence. Furthermore, there is very little local information available and most data is from temperate regions. The following are the suggested best management practices for commercial pollination in deciduous fruit orchards in South Africa, based on available information, and with the objective of optimal delivery of the correct pollen to the target crop.
When: It has generally been considered best practice to introduce honeybee colonies at 20-30% blossom. However, because flowers are often most receptive on the day that they open and are often only receptive for 2-3 days, and because recent data indicates that colonies require at least 24 hours to settle down and resume normal foraging after introduction, it is recommended that colonies be introduced at 10% blossom. There is also no value in keeping honeybee colonies in the orchard for longer than is needed, and colonies should be removed as soon as possible after full bloom, and by 30% petal drop at the latest.
Where: Conventional wisdom is that honeybee colonies should be placed in the full sun or semi-shade, protected from the wind, and placed on stands of some sort to keep them off the damp ground. Recent data suggests that this is less important than previously imagined. Common-sense should prevail. In extremely cold conditions, bees should be placed in the full sun; under very hot conditions, in the shade. Semi-shade is probably best for non-extreme conditions. The site should always be dry and out of the prevailing wind. It is apparent that intra-hive conditions, in particular ventilation, will have a significant impact on foraging rates. The type of hive material used (marine-ply, pine) and size of the colony entrance are most important. A poorly ventilated or overly insulated hive might hive difficulty keeping cool in the heat of the day, resulting in bees clustering outside the hive to keep cool, and this reduces foraging rates. Similarly for overly ventilated or under-insulated hives during the early morning or evenings. The condition and type of hives used should be considered in determining the optimum position to place the hives. Also take into account the movement of people, animals, vehicles and the planned spray programme in the placement of colonies. It is recommended that colonies should be placed singly or in small groups of 2/3 colonies, and evenly distributed around the orchard. Where possible, colonies should be placed near pollinizers, to facilitate cross-pollination. Colonies should be placed at the end of rows of trees, particularly with hedgerow systems, as bees forage down rows and are 10-30 times more likely to move to the next tree in the row than to move between rows. Where possible, place colonies away from the immediate edge of the orchard, as this helps in the dispersal of foragers throughout the orchard. Colonies, however, should be placed no further than 100m of the target crop as the numbers of foragers decreases rapidly with distance, especially in bad weather, and there should not be windbreaks between the colonies and the target crop. If rows are greater than 100m in length, coloniesshould be placed at both ends.
How: Honeybee colonies should be introduced at night whenever possible, or in bad weather conditions when no or a minimum of bees are foraging, and with traveling screens or other means of adequate ventilation to prevent overheating and to allow colonies to settle down and resume foraging as quickly as possible. Each time a colony is moved there are forager losses and a decrease in the strength of the colony and it is recommended that colonies are used not more than three times for deciduous fruit pollination during a season. How many: It makes little sense to have hard and fast rules as regards the numbers of honeybee colonies needed for the pollination of a particular crop, as the numbers of foragers needed is influenced by the cultivar type, the age of the trees, by the weather and by local conditions. There is little hard data on the number of colonies needed, or the direct impact of increasing colony numbers, and most recommendations are based on experience and assumptions. The critical factor is the numbers of blossoms in the orchard, and ensuring that sufficient honeybee foragers are present for multiple visits to each flower. Standard recommendations regarding the numbers of colonies needed are presented in Table 1, but it should be remembered that the more colonies present the more foragers there are available, and it is better for a crop to be too heavy than too light. Waves: It is recommended that difficult cultivars such as ‘Packham’s Triumph’receive two waves of honeybee colonies for pollination, the first at 10-20% blossom and the second at 60-70% blossom and no more than 7 dayslater. The rationale is that the “naïve foragers” of the first wave will work on the pear blossoms for only a few days and will then “switch-off” and begin working on more attractive alternative forage in the vicinity. Hence the second wave, to introduce a second set of naïve foragers to prolong the foraging on the pears. The value of sequential colony introductions is presently being carefully researched, and it is clear that honeybee foragers do not abandon pear blossoms as readily as was supposed. Results regarding the use of two waves in pear pollination obtained during the 2004 and 2005 seasons have been somewhat contradictory to established principles, and if confirmed, these practices might need to be revised.
Competitive forage: Many weeds common in orchards, such as the Cape Weed (Gousblom), “bloublommetjies” (Echium) and Wild Radish (Ramenas), are highly attractive to honeybees and offer better rewards than do deciduous fruit flowers, especially pear blossom. These weeds as well as any other attractive bee forage should be removed to prevent them drawing foraging bees away from the target crop. Recent data suggests that it is not critically important that ALL alternative forage be removed because some foragers attend to the fruit blossoms even in presence of more attractive alternatives. Excessive alternative forage will present a problem, however, and alternative forage should be removed as far as is feasible. Chemical control of weeds can be practiced before blossom time, but thereafter mechanical control should be used.
Pesticides: In any discussion on the impact of pesticide application on honeybees during commercial pollination two factors should be remembered. Namely, that deciduous fruit flowers are relatively unattractive to honeybees and secondly, that honeybee foragers discouraged from foraging in an orchard because of the application of a pesticide will not readily return after they have “locked onto” an alternative forage source. It follows that while it is accepted that modern agriculture requires pest control, it is probable that all pesticide applications immediately before or during blossom time are disadvantageous to the bees used for pollination, or to the pollination process. Great care should be taken in the choice of pesticides, and in the decision to spray, and the question should also be asked: IS THIS ABSOLUTELY NECESSARY DURING BLOSSOM TIME? If pesticide applications are unavoidable, use the least toxic or repellant pesticide or formulation available, and apply at night and away from the honeybee colonies to have as little effect as possible. Try not to spray anything in the first 3 days after the introduction of honeybees, and do not keep colonies in orchards longer than necessary.
Different classes of pesticides have differing effects on honeybees and on pollination. All pesticides have been classified as either harmless to honeybees, moderately harmful to honeybees, or extremely harmful to honeybees. If there is uncertainty of the bee hazard classification of a remedy the registration holder of such remedy should be contacted. Products extremely toxic to foraging honeybees may cause a knockdown of foragers in the orchard, or mass mortality at the colonies. Large numbers of dead bees on the orchard floor, or large numbers of crawling bees around the colony entrance, are almost always the result of pesticide poisoning. The use of insect growth regulators or micro encapsulated insecticides may be equally deleterious to honeybees, and in all cases the use of such harmful products should not take place until honeybee colonies have been removed from the vicinity. Moderately harmful pesticides will typically cause some mortality in the colonies which will normally disappear after 24 hours, and can be used without removing the colonies from the orchard. Application should be at night to lessen the effect.
Products often regarded as harmless to honeybees (e.g. herbicides, fungicides, foliar sprays) may have a repellency effect, retarding the entry of foraging bees into the orchard and resulting in them failing to return. Practically anything can have a repellent effect on foraging bees, even the spraying ofwater. Although the honeybee colonies are seldom negatively affected by repellency, the result of repellency is a decrease in pollination efficacy and a reduction in fruit set.
Three final factors should be considered with regards to pesticide applications. Firstly, remember your neighbour. The application of a toxic pesticide may result in honeybee mortality in neighbouring orchards or properties, even if the colonies from the target orchard have been removed. Secondly, remember that successful pollination is the result of cooperation between grower and beekeeper, who should be informed timeously and in detail about all pesticide applications. Finally, be aware that spraying before and after bee introduction may cause damage to “natural” pollinators and beneficial, reducing future pollination potential.
Pollinizers: As almost all modern cultivars of deciduous fruit are self-incompatible, having the correct pollinisers in an orchard is critically important in setting an economic crop. The most critical factor in the choice of cultivar is overlapping flowering time; if anything, the polliniser should finish flowering just before the main cultivar finishes. As cultivars can vary greatly in terms of nectar quality and quantity, it is also important that the polliniser and the main cultivar are similarly attractive to honeybees, to facilitate cross pollination. The best planting pattern of the pollinisers in the orchard has been the subject of much debate and is presently being researched, but as yet there is insufficient data to deliver definitive answers, particularly with the hedgerow systems now being used. What is clear, however, is the tremendous local effect of pollinisers on fruit set. John Free (1962) demonstrated that the fruit set on the near-side of trees adjacent to a polliniser was far greater than the fruit set on the far-side of the same trees, proving that even a small increase in distance from the polliniser results in a significant decrease in fruit set. Similar results were obtained by Anderson (1985) in South Africa with fruit set on the near-side of the polliniser being 22% greater than that on the far-side. As distance from the polliniser is obviously critically important in fruit set pollinisers should be evenly distributed in the orchard as far as possible, and as abundant as is economically feasible. For orchards with insufficient or poorly positioned pollinisers, the only short-term solution is increasing the numbers of pollinators (honeybees) in the orchard during blossom. The extremely significant impact of distance from the polliniser on fruit set and the difficulty in providing sufficient pollinisers in an orchard suggest, however, that emphasis should be placed in developing more effective long-term methods in distributing polliniser pollen throughout the orchard.