Department of Agriculture, Western Australia

Bee pollination benefits for plums and prunes

Extract from Bulletin 4298: Honeybee pollination

Prunus domestica (European plums); Prunus salicina (Japanese plums). Prunus simonii (Asian plum), Prunus americana, P. angustifolia, P. hortulana and P. nigra (American plums), Prunus cerasifera, P. insititia, P. spinosa, P. divaricata (Turkey)

For pollination see also cherry - Prunus avium; Apricot - Prunus armenica; Peach and Nectarine- Prunus persica

General:

The crossing of Asian/Japanese plums with American species causes cultivars to be self-sterile or have a low rate of self-fertility. They give good fruit set when cross-pollinated (Ref. 6). European plums have incompatibility problems because of their hexaploidy (Ref. 6).

No parthenocarpy is known in stonefruit (that is, the ability to produce a fruit without pollination; for example, pears are parthenocarpic). Thus pollination is required, the transfer of compatible pollen to the stigma, germination of pollen grains, pollen tube growth into the ovule and subsequent double fertilisation of the egg and the two polar nuclei to produce the endosperm. Without fertilisation and the beginning of seed development, a flower will fail to set fruit.

In Italy, for example some growers use honey bees, other use bouquets in water-filled drums throughout the orchard, others buy pollen and spray it over their orchard and some rely on none of the above. It has produced extreme variability in yield across Italy ranging from 3 to 50 t/ha.

Plum exhibits a wide range of incompatibility traits, from complete self-fruitfulness to complete self-sterility. The most appreciated and highly demanded fruit from consumers are those from self-incompatible varieties - that is, plants that need cross-pollination, and this is usually by employing honey bees to transfer pollen from one variety to another. The highest rate of self-fertility was 43% in European plums and 7% in Japanese plums (Ref. 6). Establishing single cultivar orchards can be done only by using highly self-fertile cultivars. However, in some years even such cultivars may hardly set fruit and therefore the use of honey bees as an insurance is essential to maintain yield (Ref. 6).

Yields vary from year to year and is largely due to the conflict of acquiring honey bees (whose foraging performance relies on air temperature) and weather conditions at flowering time (usually windy, cold and wet and low sunlight).

Yields from self-fertile plums are not affected by limiting honey bee pollination (Ref. 3, Ref. 5).

Japanese plums - Two of the most popular varieties in the USA are Burbank and Abundance - both are self-unfruitful but will pollenise each other. The Japanese plum dominates stonefruit orchards in Western Australia. Most plums are self-sterile (self-unfruitful) and will require cross-pollination, pollen being shifted around the orchard by honey bees. These are low-chill plums and when planted at low density (300 trees/ha) in a subtropical environment can produce excessive vegetative growth, which competes with fruit development for nutrients. The use of the tree growth retardant, Paclobutrazol has been most successful in controlling tree vigour. The chemical can reduce tree size by 60% and increase fruit size by 48% and as a further consequence can advance fruit maturity by 10 days (Ref. 9).

Percentage fruit set in caged flowers See Ref. 6

European plums - All except USA varieties Damson and Stanley require cross-pollination (Damson and Stanley are self-fruitful). Have been tried in Western Australia but they do not flower very well.

Pomological characteristics:

See Ref. 8 (Turkey)

Cultivar	Black Amber	Santa Rosa	Beauty
Fruit wt (g)	51.8	26.5	30.9
Seed wt (g)	1.5	2.4	1.2
Seed ratio (%)	2.89	8.95	4.18
Fruit diam. (mm)	45.1	34.2	35.6
Fruit length (mm)	39.5	34.9	39.2
pH	3.48	3.76	3.72
Acidity (%)	3.84	1.36	4.24
TSS (%)	11.2	15	14.4

Fruit with a high refractometer value of 18° to 20° brix or more over three years is almost a guarantee for good tasting fruit. The sugar content (°brix) varies quite strongly per year due to variable tree loads and weather conditions - Acta Horticlturae 478 (1998).

Pollenisers:

The period of flowering is of importance for flower pollination. Cultivars with longer flowering periods ensure more efficient pollination in case of rainy days or other unfavourable conditions that occur during the period of pollination.

Cultivar (variety) Those in bold are dominant in Western Australia.	Requires polliniser?	Flowering commences	Suitable pollenisers (best)
Laroda	Yes, self-sterile	Early Sept	Beauty, Friar, Queen Rosa, Santa Rosa, Wickson.
Santa Rosa	No, but preferable, partially self-sterile	Early Sept	Beauty, Queen Rosa, Wickson, Narrabeen, Ruby Blood.
Amber Jewel
Black Amber	Yes		Queen Rosa, Durado.
Stirling	Yes	Mid Sept	Laroda, Wickson, Santa Rosa
Friar	Yes, self-sterile	Mid Sept	Casselman, Laroda, Santa Rosa, Black Amber
Ruby Blood	Yes	Aug-Sept	Santa Rosa, Mariposa, Narrabeen, Wickson.
Simka	No, can be selfed		Friar, Black Amber.
Tegan Blue
Narrabeen	Yes (early flowering)	Mid August	Mariposa, Santa Rosa, Ruby Blood
Autumn Giant
Radiance
Kelsey	Yes	Late August	Wickson, Santa Rosa, Beauty
Gulf Ruby
Mariposa	Yes (early flowering)	Late August	Beauty, Laroda, Santa Rosa, Narrabeen
Queen Anne	Yes	Late August	Laroda, Santa Rosa, Casselman
Casselman	No, but preferable, can be selfed
Satsuma	Yes - self-sterile	Early Sept	Santa Rosa
Wilson	Yes		Santa Rosa, Queen Rosa, Friar
Queen Rosa	Yes (early flowering)	Mid Sept	Casselman, Friar, Laroda, Santa Rosa, Simka, Black Amber.
Jackson	(late flowering)	Late Sept
Durado	Yes
Salad	Yes (early flowering)	Mid August	Narrabean, Mariposa, Santa Rosa
Beauty	No, but preferable, can be selfed		Laroda, Santa Rosa, Wickson
Wickson	Yes	Aug-Sept	Santa Rosa, Ruby Blood, Beauty

Flowering:

August to October (see table above). Duration ranges from 4 to 15 days depending upon year and genotype (cultivar) (Ref. 4).

One year old branches have 12 to 117 flowers/m², whilst those of two years or more have 96 to 153 flowers/m² - Acta Horticulturae 478 (1998). The average number of flowers emerging from each bud was 2.7 in cv Satsuma (Ref. 11).

In a examination of 63 plum cultivars, 18% had early flowering, 20.6% had medium - early flowering, 17.5% had medium flowering, 19% had medium to late flowering and 23.9% had late flowering (Ref. 4). The authors believe that because of the effect of temperature and a difference between years of the flowering period (which can be 3 or 4 days) there should be at least two pollinisers for self-sterile plum cultivars. One polliniser beginning to flower 2 days before and the other 2 days after the self-sterile cultivar.

The effective pollination period (EPP):

For most plum cultivars the period after flowering (anthesis) suitable for pollination is only short from 3 to 7 days (Ref. 4).

Nectar production:

Sucrose content of nectar is 21%. The concentration is low and as a result pollination of plum flowers by honey bees is affected, because the sugar level is below the optimal that attracts honey bees to flowers.

Pollen:

Anthers shedding pollen open slowly below temperatures of 14°C, whilst above 20°C they open quickly releasing their pollen. A temperature below 10°C also results in the viability of the stigma (the organ that receives the pollen) being from 3 to 5 days long whilst at above 13°C it is severely reduced to 1 or 2 days (Ref. 4).

At temperatures below 10°C, pollen germinates slowly, pollen tube growth is slow, the stigma has low receptivity and fertilisation will not occur or the embryo will abort very early. Calcium influences pollen tube growth (Acta Horticulturae 478 (1998).

Plum trees have a high nutritional requirement for Boron. Boron plays an important role in pollen production, its germination and pollen tube growth. Boron is applied to the ground at 2 kg/ha as Boric Acid and foliar sprays at 0.67 kg/ha two weeks before flowering.

Prune pollen is heavy and sticky (Ref. 10). Airborne pollen was about 24 grains/m³ (range: 1 to 176 m³) - Ref. 11.

Honey bees can harvest 1.22 kg pollen per hive over the period August 29 to September 10 (13 days) for cv Satsuma (Ref. 11).

Temperature
After Szabo and Nyeki (1996) (see Ref 4)

Anthers	Under 14°C	Between 14° and 20°C	Above 20°C
Rate of pollen release (dehiscence)/hour (%)	1 to 5	2 to 10	4 to 16

Pollen sprays:

Can be used, but it is more cost effective to use honey bees.

Thinning fruit:

Etephon (Ethrel) used at 0.015-0.03% concentration plus 0.01% NAAM (Amidthin), 30 days after flowering - Acta Horticulturae 478 (1998).

Hives per hectare:

Prunes: 2, 2 to 3 hives/ha; 3 hives/ha (Agriculture Western Australia), 1 Beetube per 25 metres of trellis row; 2.5 hives/ha (Ref. 10), 2 to 4 hives/ha (in Ref. 11). These figures need adjusting (higher) if orchard plantings are high density.

Two-story colonies with at least 8 frames of brood and bees had a minimum of 52 bees/minute exiting the hive in a plum orchard, reaching a maximum of 267 bees/minute. Brood area expanded to 10 to 12 frames after being in pollination (Ref. 11).

Because of the early flowering (late winter - early spring), and the fact that most of the bees visiting flowers soon after opening are pollen gatherers, stimulation of the hives with sugar syrup is probably required, together with hives being used with plenty of unsealed brood.

Honey production:

26 to 50 kg/ha.

Honey bee foraging:

Prunus salicina - Honey bees formed 86.5% of flower visitors (Ref. 1). In Victoria, 88.5% of insects were honey bees (Ref. 11).

An average of 8.8 bees/tree were foraging from 4 two-story colonies located 60 metres from the trees. The range was 1.9 to 27.5 bees/tree. Of the total foragers, 66% were pollen gatherers and 44% were nectar gatherers (Ref. 11).

Effect of distance from apiary on pollination:

Between 125 and 300 m from an apiary, fruit set declined by 242% (Ref. 1).

Foraging activity of honey bees decreased at a linear rate with increasing distance from apiary (Ref. 2).

Other pollinators:

Osmia cornuta - Mason bees

Review of bee pollination benefits:

• 153.3 per cent increase in yield of the Japanese plum variety Satsuma in open and bee-excluded plots.

• When honey bees were used, the yield of plums increased by 150%, from 8 t/ha to 20 t/ha .

• French prunes: when trees were caged, fruit set was less than 1.3% compared with 3.6 to 21.8% for open pollination (with honey bees), and 15.5 to 19.4% when trees were caged with honey bees (Ref. 10).

• 44 per cent of honeybee foragers are nectar gatherers.
• 3.7 times increase in fruit (prunes).

References:

1. TITLE: Pollination of Japanese plum.
   AUTHOR: Casilda, A.; Munoz, F. and Gomez, P.
   JOURNAL: Vida Apicola (1994) 65: 30-37.

2. TITLE: Insect controlled pollination in Japanese plum (Prunus salicina Lindl.)
   AUTHOR: Calzoni, G. L. and Speranza, A.
   JOURNAL: Scientia Horticulturae (1998) 72: 227-237.

3. TITLE: Role of bee pollination in the fruit set and yield of self-fertile and self-sterile apple, sour cherry and plum cultivars
   AUTHOR: Benedek, P. and Nyeki, J.
   JOURNAL: Horticultural Science (1995) 27(3/4): 34-37.

4. TITLE: Flowering phenology of plum cultivars under Hungarian ecological conditions
   AUTHOR: Szabo, Z. and Nyeki, J.
   JOURNAL: Proceedings of the 2^nd International workshop on pollination. Acta Hort (1996) 423: 23-29.

5. TITLE: Fruit set of selected self-sterile and self-fertile fruit cultivars as affected by the duration of insect pollination
   AUTHOR: Benedek, P. and Nyeki, J.
   JOURNAL: Proceedings of the 2^nd International workshop on pollination. Acta Hort (1996) 423: 57-63.

6. TITLE: Fruit set of plum cultivars under Hungarian ecological conditions
   AUTHOR: Nyeki, J. and Szabo, Z.
   JOURNAL: Proceedings of the 2^nd International workshop on pollination. Acta Hort (1996) 423: 185-191.

7. TITLE: Fruit characteristics of some plum varieties under the ecological conditions at Adana, Turkey
   AUTHOR: Ozguven, A. I., Kuden, A. and Tatli, H.
   JOURNAL: Proceedings of the 5^th International symposium on temperate zone fruits. Acta Hort (1997) 441: 341-343.

8. TITLE: Control of tree growth in low-chill stonefruit using Paclobutrazol: A synopsis of research findings over the past five years
   AUTHOR: George, A. P., Campbell, J. A. and Hieke, S.
   JOURNAL: Proceedings of the 5^th International symposium on temperate zone fruits. Acta Hort (1997) 441: 39-.

9. TITLE: Bee management for prune pollination
   AUTHOR: Robbin W. Thorp
   JOURNAL: unknown source (page 53-55).

10. TITLE: Honey bee pollination of Japanese plums (Prunus salicina Lindl. cv Satsuma in the Goulburn Valley, Victoria
    AUTHOR: Langridge, D. F. and Goodman, R. D
    JOURNAL: Aust. J. Exp. Agric. (1985) 25: 227-230.

Page reviewed: March 2006