The apple (Malus domestica) flower occurs in a cluster of flowers at the end of a 1-3-year-old woody shoot called a spur. The flower has five stigmas that join into a style that leads to the ovary. The ovary has five divisions, each with two ovules, which means that a fully-pollinated fruit will have ten seeds (the variety `Northern Spy' has four ovules per division for a maximum of 20 seeds). Surrounding the style are 20-25 pollen-bearing stamens. Nectar is excreted at the base of the central style. Five pinkish-white petals surround the sexual parts.

Each flower cluster has a primary bud called the king bloom that opens first and produces the best fruit. Production of nectar and pollen is marginal to good, and bees readily visit the blossoms.

  Apple Pollination Requirements

At least 6-7 ovules must be fertilized. If this threshold is not met then the fruit will be misshapen (Brault and de Oliveira, 1995), small, or may not stay on the tree until harvest (McGregor, 1976). Inadequate pollination can also reduce calcium concentrations in fruit (Volz et al., 1996) which can predispose the fruit to storage problems (Ferguson and Watkins, 1989).

Most apple varieties require cross-pollination with another compatible vartiety. Some exceptions to this are the varieties 'Newtown' and, to a lesser extent, `Golden Delicious' and `Rome Beauty'. Many varieties show a degree of self-fruitfulness, but not enough to allow solid-block plantings. So orchardists must interplant main varieties with compatible pollenizer varieties. Generally, closely-related varieties - as, for example, McIntosh, Early McIntosh, Cortland, and Macoun - do not cross-pollinate each other well. Likewise, spur types do not pollinate the parent variety well. The bloom periods of the main and pollenizer varieties must overlap. To optimize pollination, it is necessary to plant both early- and late-blooming pollenizers so that the main variety blooms in between. In that way, ample pollen will be available for the early-blooming king bloom on the main variety, and if frost kills the king blooms the late-blooming pollenizers will provide pollen for those flowers that remain.

Some apple varieties have sterile pollen. These varieties willingly receive pollen from other varieties and produce fruit, but they cannot be used as pollenizers. Table 17.1 lists some apple varieties, their pollen viabilities, and relative bloom intervals for the southeastern U.S. (see original for Northwestern U.S.). Nursery stock catalogues also list main varieties and compatible pollenizers.

Insects are the only practical pollinators of apple. A mechanical power pollen duster device was shown to have no effect on fruit set, fruit size, seed number, or yield (Schupp et al., 1997).

Growers can use flowering crabapples as pollenizers instead of another commercial variety. This is warranted if other candidate pollenizers produce inferior fruit, take up too much orchard space, have conflicting pesticide requirements, or produce fruit that pickers cannot distinguish from the main variety. The flower color of a crab variety should match the color of the main variety since bees do not readily switch to a different-colored blossom during a foraging trip (Mayer et al., 1989). Crabapples can be planted in existing space between main variety trees or grafted on to them; this makes crabapples an attractive remedy for old orchards that were not interplanted with pollenizers.

One can cut bouquets of flowering crabapples (or of any pollenizer) and place them in barrels of water between rows during bloom. Such bouquets should be cut only when the king bloom is open, and they should be made large. It is helpful to change water when blooms begin to wilt and to place bouquets in sunny, still areas.

Because crabapples bloom on 1-year-old wood, crabapples should be pruned immediately after bloom in order to get them ready for next year. Some crabapples are more susceptible to virus diseases, but whole-tree crabapple plantings, rather than grafts, can reduce this problem. If crabs are grafted into virus-infected main variety trees, the grafts may fail because of virus-induced graft incompatibility.

Bees prefer to work up and down rows rather than across rows. This is especially true in dense plantings and during even the lightest wind. One must consider this when planning the arrangement of main and pollenizer varieties in an orchard. With orchard plan 1, every other tree is a pollenizer ; this maximizes the number of pollenizers, but it is practical only if there is a market for the pollenizer. All other plans compromise some degree of pollination efficiency in favor of convenience at harvest. With plan 2, every third tree in every third row is a pollenizer; this ensures that every tree of the main variety is next to, diagonal to, or across from a pollenizer on one side. Plan 3 calls for a solid planting of the pollenizer every fourth row; this leaves one row of main variety by itself and is practical only if the pollenizer has market value. Plan 4 calls for two pollenizer rows next to four rows of the main variety; this is the least efficient design. Main varieties and their pollenizers should not be planted in the same row, as in plans 1 and 2, if they cannot withstand the same chemical regimen. Instead, pollenizers should be planted in their own rows so that they can be treated separately, as in plans 3 and 4. Crabapple pollenizers can be pruned for tall growth to take up little or no extra space. In this manner, they can be planted as every sixth to ninth tree every row.

Honey bees are the most important pollinator of apple in North America (Table 17.5). Honey bees usually work apple blossoms readily, but apple is not always the richest available forage and competing bloom can be a problem. Although honey bees pollinate apple well, they are not the most efficient apple pollinator. They sometimes rob an apple flower of its nectar without pollinating it; this happens most often with the `Delicious' apple variety. Honey bees make fewer contacts with the sexual column of the apple flower, compared to certain solitary bees.

Researchers have increased apple visitation by honey bees with pheromone-based bee attractants (R.D. Fell, unpublished report; Mayer et al., 1989a; Currie et al., 1992b). Thus, emerging technology and honey bees' naturally large foraging populations may serve to partly compensate for their relative inefficiencies in apple.

1(2.5) Humphry-Baker (1975); Crane and Walker (1984); Ambrose (1990); Kevan (1988)

2 (5) Mayer et al. (1986)

0.25, 0.5, 1, 2 (0.6, 1.2, 2.5, 5) McGregor (1976)

1-2 (2.5-5) Levin (1986)

0.4-2 (1-5) Kevan (1988)

0.8-1.2 (2-3) British Columbia Ministry of Agriculture,

Fisheries, and Food (1994)

1.5-5 (4-12.5) Scott-Dupree et al. (1995)_____________

20-25 honey bees/tree/min Mayer et al. (1986)

75 honey bees leaving hive

entrance/ min Ambrose (1990); Mayer et al. (1986)

1 honey bee/1000 flowers Palmer-Jones and Clinch (1968)

6 honey bee visits/ flower Petkov and Panov (1967)

250 orchard bees/acre (618/ha) Torchio (1985)

2834 orchard bees/acre (7000/ha) Batra (1982)

As with any early-blooming crop, it can be difficult to get strong colonies in time for apple bloom. As much as possible, colonies should have large adult populations and plenty of brood. Mayer et al. (1986) and Ambrose (1990) recommend a minimum strength standard of six frames of brood covered by adult bees. Such a colony will have about 20,000 bees.

In smaller orchards, colonies should be placed in groups of 4-6 at 150-yard (137 m) intervals. With larger orchards, colonies should be placed in groups of 8-16 at 200-300-yard (183-275 m) intervals, starting about 100 yards (92 m) from the edges. Young trees, with fewer blossoms, are less attractive to bees than older trees, so in young orchards the grower may have to increase hive numbers in order to compensate (Mayer et al., 1986).

Colonies should not be kept at the apple orchards year-round. Instead, they should be moved in after about 5% of the orchard is in bloom or when the first king blooms open. Such a delay will encourage bees to focus on the crop rather than learn to visit competing plants.

Pollen dispensers (hive inserts) are devices that fit at the entrance of bee hives and hold pollen of desirable pollenizer varieties in such a way that bees dust themselves with the pollen as they leave the hive. Although dispensers have a questionable record (Jay, 1986; Mayer et al., 1986), many apple growers use them. Like crabapples, pollen dispensers are an attractive remedy for old solid-block orchards with no pollenizers planted nearby (Anonymous, 1983; Mayer and Johansen, 1988). In general, inserts are warranted when weather restricts the blooming of pollenizers and the activity of the bees.

It is necessary to use only pure, hand-collected apple pollen in dispensers; cured, pollen-laden anthers are even better (Mayer et al., 1986). Bee-collected pollen pellets will not pollinate apple flowers, even though they are easily obtained from pollen traps at hive entrances. Lycopodium powder is sometimes used to dilute pollen, but it agitates bees and is no longer recommended. High-quality pollen for inserts is available commercially. Pollen should be kept refrigerated until use, and inserts should be replenished with about a teaspoon of pollen every few hours while bees are actively flying. A bee density rate of two bee hives per acre (5/ha) is recommended if one is relying on inserts for pollination (Mayer et al., 1986).

Orchard mason bees (Osmia cornifons, O. lignaria lignaria, and O. lignaria propinqua) are potential apple pollinators. O. lignaria lignaria, and O. lignaria propinqua land directly on the anthers and stigma of the blossom, thus maximizing the chance of successful pollination (Torchio, 1985). Honey bees, on the other hand, sometimes land on the flower petals before approaching the sexual column. In a comparison of orchard bees and honey bees in Japan, orchard bees visited more apple flowers per minute and contacted the sexual column 26 times more frequently (Batra, 1982). Orchard bees are most promising in cases where honey bees are not available or in those varieties that honey bees work inefficiently, such as `Delicious'. In North Carolina, O. lignaria lignaria, O. lignaria propinqua, and O. cornifrons improved fruit-set, seed number, and fruit shape in 'Delicious' apples, even in areas of orchards that already had honey bee hives (Kuhn and Ambrose, 1984). In spite of these favorable studies, orchard mason bee management has not reached practical large-scale levels. Recurring problems include the timing of bee emergence with apple bloom, the dispersal of females away from the orchard after release, and disease contamination in nest materials.

Populations of non-managed soil nesting bees range from abundant to insignificant. Some soil nesters are good apple pollinators where they occur in large numbers. In Maryland, the introduced Andrea pilipes villosula is active during apple bloom. It forages in cool, damp weather, and works from before dawn to after dusk (Batra, 1994). Managed leafcutting bees (M.egachile rotundata) will visit apple blossums in Washington, but they fly only at higher temperatures and the bees must be incubated starting 21 days before bloom (which is not easy to predict).

Ambrose, J.T. (1990) Apple pollination. In: N.C. Apple Production Manual. North Carolina Agriculture Extension Service, AG-415.

Anonymous (1983) pollen inserts for apple pollination. Illinois Cooperative Extension Service Bulletin E-6.

Batra, S.W.T. (1982) The hornfaced bee for efficient pollination of small farm orchards. US Department of Agriculture, A.R.S. Miscellaneous publication 1422.

Barta, S.W.T. (1994) Anthophora pilipes villosula Sm. (Hymenoptera: Anthophoridae), a manageable Japanese bee that visits blueberries and apples during cool, rainy, spring weather. Proceedings of the Entomological Society of Washington 96, 98-119.

Brault, A-M. and de Oliveira, D. (1995) Seed number and an asymmetry index of 'McIntosh' apples. Hortscience 30, 44-46.

British Columbia Ministry of Agriculture, Fisheries and Food (1994) Tree Fruit Production Guide for Commercial Growers in Interior Districts. Victoria, British Columbia.

Crane, E. and Walker, P. (1984) Pollination Directory of World Crops. International Bee Research Association, London.

Currie, R.W., Winston, M.L., Slessor, K.N. and Mayer, D.F. (1992b) Effect of synthetic queen mandibular pheromone sprays on pollination of fruit crops by honey bees (Hymenoptera: Apidae). Journal of Economic Entomology 85, 1293-1299.

Ferguson, I.B. and Watkins, C.B. (1989) Bitter pit in apple fruit. Horticultural Review 11, 289-355.

Humphry-Baker, P. (1975) Pollination and Fruit Set in Tree Fruits. British Columbia Department of Agriculture, Victoria.

Jay, S.C. (1986) Spatial management of honey bees on crops. Annual Reviews of Entomology 31, 49-65.

Kevan, P.G. (1988) Pollination: Crops and Bees. Ontario Ministry of Agriculture and Food, Publication 72.

Kuhn, E.D. and Ambrose, J.T. (1984) Pollination of 'Delicious' apple by megachilid bees of the genus Osmia (Hymenoptera: Megachilidae). Journal of Kansas Entomological Society 57, 169-180.

Levin, M.D. (1986) Using honey bees to pollinate crops. US department of Agriculture, Leaflet 549.

Mayer, D.F. and Johansen, C.A. (1988) WSU research examines bee hive pollen dispensers. Good Fruit Grower 39, 32-33.

Mayer, D.F., Britt, R.L. and Lunden, J.D. (1989) Evaluation of BeeScent as a honeybee attractant. American Bee Journal 129, 41-42.

Mayer, D.F., C.A. Johansen and Lunden, J.D. (1989). Honeybee (Apis mellifera L.) foraging behavior on ornamental crab apple pollenizers and commercial apple cultivars. HortScience 24: 510-512.

Mayer, D.F., Johansen, C.A. and Burgett, D.M. (1986) Bee pollination of tree fruits. Pacific Northwest Extension Publication, PNW 0282.

Mayer, D.F., Johansen, C.A. and Lunden, J.D. (1989b) Honey bee (Apis mellifera L.) foraging behavior on ornamental crab apple pollenizers and commercial apple cultivars. HortScience 24, 510-512.

McGregor, S.E. (1976) Insect Pollination of Cultivated Crop Plants. US Department of Agriculture, Agriculture Handbook 496.

Palmer-Jones, T. and Clinch, P.G. (1968) Honey bees essential for pollination of apple trees. New Zealand Journal of Agriculture 11, 32-33.

Petkov, V.G. and Panov, V. (1967) Study on the efficiency of apple pollination by bees. In: 21st International Apiculture Congress Proceedings, College Park, Maryland, USA, pp.432-436.

Schupp, J.R., Koller, S.I. and Hosmer, W.D. (1997) Testing a power duster for pollination of 'McIntosh' apples. HortScience 32, 742.

Scott-Dupree, C., Winston, M., Hergert, G., Jay, S.C., Nelson, D., Gates, J., Termeer, B. and Otis, G. (eds) (1995) A Guide to Managing bees for Crop Pollination. Canadian Association of Professional Apiculturalists.

Torchio, P.F. (1985) Field experiments with the pollinator species, Osmia lignaria propinqua Cresson, in apple orchards: V, (1979-1980). Methods of introducing bees, nesting success, seed counts, fruit yields (Hymenoptera: Megachilidae). Journal of Kansas Entomological Society 58, 448-464.

Volz, R.K., Tustin, D.S. and Ferguson, I.B. (1996) Pollination effects on fruit mineral composition, seeds and cropping characteristics of 'Braeburn' apple trees. Scientia Horticulturae 66, 169-180.