Pollination of Wild Blueberries
When referring to the pollination of wild blueberries, there are two important terms: pollination and fruit set.
Pollination refers to the transfer of pollen from the stamens (the male part) of a flower to the pistil (the female part) of a flower of the same species, resulting in fer-tilization of the fruit (Figure 1).
This term refers to the proportion of flowers which produce fruit. The terms % fruit set and % pollination are some-times used inter-changeably.
To best understand the process by which a flower becomes a fruit, it helps to visualize the structure of a flower (Figure 1).
The parts of a flower are:
- The corolla: In some respects, this is the flower's coat. The corolla consists of the petals, which are fused together in the shape of a bell which protects the inner parts of the flower.
- The stamens: These are the "male" parts of the flower, and include the filament and the anthers.
- The anther: This is the part of the flower which produces pollen. Pollen is the small powdery material which fertilizes the flower's ovules. Ovules are the flower's minute eggs, which develop into seeds.
- The pistil: This is the "female" part of the flower, and consists of the ovary and the style.
- The ovary: This is the swelling at the base of the flower. It is green and bares a multitude of ovules, which produce seeds once they are fertilized.
- The style: This is a tube that is linked to the ovary. It is hollow and acts as an access route for pollen germination to the ovary.
- The stigma: The stigma is located at the tip of the style. This is the part which receives the pollen during its receptive period. It secretes a sticky substance which allows the pollen grain to adhere to its surface.
The onset of flowering varies from one clone to another. Within a clone, flowering begins simultaneously on the various stems. The first flowers to open are those that originate from the floral buds located closer to the base of the stem, and the other flowers open in an upward succession to the tip. There are generally 5 or 6 flower buds on each stem, and each flower bud generally contains at least five flowers.
Because there are successive waves of flowering within a clone, and because there are variations in flowering times between the clones, the flowering period may last as long as three weeks, which helps moderate the damage due to poor weather. A flower may remain receptive for up to 5 to 8 days, though this may be shorter when the weather is fine. The relatively short duration for any one given flower to be in bloom means that an adequate number of pollinating insects need to be present to ensure the transfer of a maximum number of pollen grains.
THE POLLINATION PROCESS
The process of pollination begins with the transfer of pollen. By way of insect transfer during their visits to flowers, pollen produced from the anthers is transferred incidently to another flower's stigma, which is located at the tip of the style. The pollen grain(s) germinate on the stigma, and form a pollen tube (Figure 2), which descends the style to the ovary and fertilizes the ovule(s). The fertilized ovule becomes a seed. A quality fruit requires the presence of several fertiized ovules, therefore the transfer of pollen needs to occur frequently
THE DEGREE OF POLLINATION
According to Aalders (1958) every blueberry fruit should contain at least 6 to 10 viable seeds. When there are less than 6 seeds, the fruit may be small and/or drop prematurely. He reported that every additional seed is responsable for a 5% increase in fruit weight and a half-day advance in fruit maturity.
Fruit development is therefore directly related to the number of viable seeds which are set. These seeds produce "hormones" which act as magnets for drawing on the plant's resources and making the fruit grow in size. The greater the number of viable seeds, the larger will be the size of the fruit and the greater will be the ability of the fruit to resist heat and water stress. Under poor growing conditions, fruits with few viable seeds will stay small and may abort or die. All attempts to increase the number of viable seeds within the fruit will result in a yield increase. In general, a little bluebery contains less than 8 viable seeds, a medium-sized blueberry contains 10 to 15, and a large blueberry contains 16 to 18.
GENETIC FACTORS AFFECTING WILD BLUEBERRY POLLINATION
Flowers on plants within a clone cannot normally pollinate each other. For fertilization to take place, pollen needs to be transferred from one clone to another. Very few clones are actually capable of setting fruit from their own pollen.
o Research has shown that 5% of wild blueberry plants are male-sterile, that is they do not possess stamens and therefore do not provide pollen.
- Pollen production capacity:
Nearly 45% of plants produce little or no pollen, thereby limiting the pollen available for pollination.
- Species incompatibility:
Up to five different species of blueberries can be present in a field (Factsheet A.2.0). Among these species, the low sweet (Vaccinium angustifolium) and the sour top (Vaccinium myrtilloides) are the most common. If sour top pollen fertilizes the low sweet, the fruit will abort within twelve days. When the two species are present in a field, this results in smaller fruit with fewer viable seeds.
Genetic factors can be quite important, and highlight the need for good pollination management.
OTHER FACTORS AFFECTING POLLINATION:
Temperature can affect pollination in several ways:
- If the weather is hot, the 5 to 8 day flowering period can be compressed, and the need for pollen transfer is more urgent.
- If the weather is cold, the pollinators may be working very little or not at all, and pollen tube growth may be very slow. This can result in poor pollination and fruit set.
- If the weather is rainy, the work of the pollinators will be hindered. In addition, the pollen grains may become removed from the stigma, or burst from taking in too much moisture.
- Wind affects pollination by slowing down the bees. Because of the flower shape and self-incompatibility, it is unlikely that wind plays a role in this crop's pollination, like it does in other crops.
- Frost during flowering can damage both the style and the ovary, thereby hindering the transfer of pollen. Poor yields are often blamed on frost, though poor yields are most often due to low activity by pollinating insects. When there really is a frost, the pistil (that is, the style and the ovary) turns black within a few hours.
Plants that are not vigourous, whether due to genetic factors or poor soil fertility, are more likely to be poorly pollinated.
The distribution of clones:
When clones cover large areas, it is more difficult to get a uniform transfer of outside pollen from the edges to the center of the clone. Transplanting other clones into the center of large clones can help to remediate this.
The population of pollinating insects:
Pollination is performed by insects. The quantity of insects will therefore have an important impact on the pollination rate. This situation is even more true in large fields. Native pollinators tend to be more abundant on the edges of fields than in the centres. The work of native pollinators seems to be inadequate for all but the smallest of fields. According to our observations, native pollinators appear to contribute from 10 to 20% of pollination, depending on the field size and the weather conditions present during flowering. In most situations, the introduction of managed pollinators is essential.
THE POLLINATING FORCE
Pollination is the most important component of wild blueberry production. The greater is the transfer of pollen, the greater is the likelihood that an adequate number of compatible pollen grains will be present on the stigma to ensure fertilization of the ovules.
Fact sheet B.2.0 deals with determining % fruit set in wild blueberry fields. It may not be realistic to seek to obtain a % fruit set of 75 to 100%, since there are too many genetic and environmental factors which can enter the picture. Pollination rates of 40 to 50% are considered very good, while 50 to 60% would be considered excellent. If the rate is below 30%, this is an indication that the pollinating force is too low.
As we know, wind is of little use for pollen transfer, since the pollen is too heavy and too sticky. This leaves only insects, unless we can one day find a mechanical means to transfer pollen from one clone to another.
There are four major groups of pollinators that we can look to for pollination:
- Bumble bees. These solitary bees are excellent pollinators of wild blueberries. They forage a few flowers of one clone then move on to another. Unfortunately, their numbers are generally low, and commercialization of appropriate species of these insects is not sufficiently advanced to provide adequate numbers at a suitable price.
- Solitary bees. This group represents a large variety of bees, which are present in all fields. Their numbers vary greatly from one year to another. They are thought to be present in greater amounts in small fields, since the nature of these fields ensure their survival. There are as many as 50 species of these bees, most of which are andrenids (miner bees) and halictids (sweat bees). Research has yet to determine the contribution of these insects to wild blueberry pollination.
- Honey bees. These bees are good pollinators of wild blueberries. Recent work by de Oliveira (1994) has demonstrated direct relationships between yield and bee densities in the large tracts of blueberry land in the Lac St. Jean region of Quebec. In these studies, the higher the concentratin of bees, the greater was the yield. When bees numbers were lower, yield dropped as well. Since the wild blueberry flower is not an excellent source of nectar, the bees work best if there are few alternate food sources. For best results, the hives should be strong. The presence of windbreaks can improve the effectiveness of honeybees, by improving the micro-cimate. A stocking rate of at least 2.5 hives/ha. (1 hive/acre) is desirable, and these should be well managed. For more information on this topic, please refer to factsheet B.3.0.
- Alfalfa Leafcutter Bees. The use of this bee in wild blueberries is very recent. With an adequate stocking rate, fruit set has been shown to be increased by this bee. These insects seek pollen primarily, so they are not adversely affected by the relatively poor nectar production of wild blueberries. They are also solitary bees, and their range of action is generally less than 100 metres (300 feet). When they are placed in wild blueberry fields, they are not prone to looking for alternative pollen sources. For more information on alfalfa leafcutter bees, please refer to fact sheet B.7.0.
Because of their nature, wild blueberry flowers require cross-pollination. A desirable fruit set can rarely be met without the introduction of managed bees. The quantity of bees is important in order to counter the effects of genetic and climatic factors which can influence pollination. A good pollination force will ensure adequate pollination in a short period of time.
In order to achieve this, it is important to do things which favour the effectiveness of pollinators or improve their numbers (e.g., the installation of windbreaks and a rational use of pesticides).
Aras, P. D. de Oliveira, D, and L. Savoie. 1995. Effect of a Honey Bee (Hymenoptera: Apidae) Gradient on the Pollination and Yield of Lowbush Blueberry. Université du Québec à Montreal.
Eck, P. and N.F. Childers. 1966. Blueberry Culture. Rutgers University Press. 378 pp.
Ismail, A.A. Honey Bees and Blueberry Pollination. Bulletin 629, Cooperative Extension Services, University of Maine.
Karmo, E. A. 1974. Blueberry Pollination - Problems, Possibilities. Bulletin No. 109. N. S. Dept. Of Agriculture.
Vickery, V.R. 1991. The Honey Bee: A Guide for Beekeepers. Particle Press, Pincourt, Québec, 250pp.Prepared by: Gaétan Chiasson, P.Ag. Horticulturlal Specialist and John Argall, P.Ag., Blueberry Specialist, N.B. Department of Agriculture & Rural Development.