POLLINATION


 

A large, healthy blueberry plant produces thousands of "flower" buds every year. Up to 16 individual flowers may develop from each bud and every flower is a potential berry. But in order to set fruit, pollen that is produced by the flower's anthers must reach the stigma so it can "fertilize" an ovule that will develop into a seed inside the flower's ovary. There are dozens of these developing seeds inside each berry and nearly all of them must be "fertilized" in order for the fruit to develop normally and reach its full size (Brewer and Dobson 1969).

Blueberry pollen is sticky and relatively heavy. It cannot move on its own and it is not easily blown around by the wind like pine pollen or corn pollen. Furthermore, the shape and position of blueberry flower parts effectively prevent the pollen from falling onto a receptive stigma -- even in cultivars that are self-fertile. Therefore, in order to set fruit, the flowers of a blueberry plant must be pollinated by insects.

Numerous species of bees (bumble bees and solitary bees) are indigenous pollinators of blueberry plants in North America. In addition, the honey bee (Apis mellifera), a species introduced from Europe in the mid 1600's, is used extensively by growers to augment populations of native pollinators. Bees are attracted to the flowers by odors and sweet nectar that is produced by glands near the base of the stigma. Both pollen and nectar serve as food for the bees and their offspring.

As insects visit blueberry flowers, pollen adheres to their bodies and is carried with them as they move from flower to flower. When bees probe for nectar inside a flower, they brush against the stigma and unwittingly leave behind some of the pollen they are carrying. Certain species of bumble bees vibrate each flower with their flight muscles as they collect pollen. This buzzing activity (known as sonication) shakes pollen from the anthers so it is easy to collect, and also tends to increase the likelihood that pollination will occur.

Illustrations:

  • Southeastern blueberry bee, (Hapropoda laboriosa).
  • Honey bee (Apis mellifera).
  • Comparison between bumble bees and carpenter bees.
  • Punctures for nectar-robbing made by carpenter bees.
  •  

    Southeastern Blueberry Bees

    The southeastern blueberry bee (Habropoda laboriosa) is a solitary, ground-nesting bee (family Anthophoridae) that has proven to be an especially effective pollinator of both highbush and rabbiteye blueberries (Cane and Payne 1988). It is a native species whose range extends along the eastern seaboard from southern Pennsylvania to northern Florida. There is only one generation per year. Populations are most active near the peak of highbush bloom and forage for only a few weeks in the spring. Little is known about the nesting habits of these bees, but care should be taken to preserve wild habitat around the edges of commercial blueberry fields.

     

    Bumble Bees

    There are several species of native bumble bees (Bombus spp.) that nest in woodland habitats and actively forage in nearby blueberry fields. These bees are generally regarded as good blueberry pollinators because they begin foraging very early in the morning and work under cooler and cloudier conditions than most other pollinators. Since bumble bees have relatively long mouthparts, they will visit blueberry cultivars with deep nectaries that other bees ignore. Unfortunately, bumble bee populations tend to fluctuate greatly from year to year depending on the severity of winter temperatures and other ecological factors. They are not always present in sufficient numbers to provide good pollination for commercial growers (Free 1993).

     

    Honey Bees

    Honey bees (Apis mellifera) are less efficient as blueberry pollinators than many species of wild bees, but they are the only ones that are easy to manage and transport. They do not usually forage when air temperature is below 60°F or wind velocity is above 16 miles per hour. Compared to other bees, they have relatively short mouthparts which may not allow them to reach the nectaries in blueberry cultivars with deep flowers.

    Despite these disadvantages, honey bees are widely recognized as a valuable resource for blueberry growers (Dorr and Martin 1966). As a general rule, if there are fewer than 4-8 native bees foraging on each blueberry plant during the warmest part of the day, then additional pollination effort is probably needed in the form of rental hives. One hive is usually recommended per acre of blueberries. Hives should be in the field by the time 5-10% of the flowers are open and should remain undisturbed until most of the blossoms have withered. To maximize foraging, place each hive in a sheltered location where the morning sun will reach the entrance and stimulate early activity. Eliminate competing nectar sources by mowing surrounding weeds and flowers. In large blueberry fields, groups of hives should not be separated by more than 300 yards.

     

    Carpenter Bees

    Carpenter bees (family Xylocopidae) often resemble large bumble bees. These insects use their strong mandibles to excavate nesting holes in wood. They are often seen flying around old packing sheds or other wooden structures. Carpenter bees visit blueberry flowers, but they cut a hole near the base of the corolla and "steal" nectar. Since they never make contact with the stigma, pollination is less likely to occur. Honey bees quickly learn to use these holes and also become "nectar robbers". The southeastern blueberry bee and other native bumble bees have not been seen using the robber holes. They continue to pollinate flowers that have been damaged by carpenter bees.

    A carpenter bee can be distinguished from a true bumble bee by looking carefully at its abdomen. Bumble bees always have a dense coat of hair over the entire abdomen, giving them a very fuzzy appearance. Carpenter bees, in contrast, have little or no hair on their big black bottoms!


    References

  • Brewer, J. W. and R. C. Dobson. 1969. Seed count and berry size in relation to pollinator level and harvest date for the highbush blueberry, Vaccinium corymbosum. J. Econ. Entomol. 62(6): 1353-1356.
  • Cane, J. H. and J. A. Payne. 1988. Foraging ecology of the bee Hapropoda laboriosa (Hymenoptera: Anthophoridae), an oligolege of blueberries (Ericaceae: Vaccinium in the southeastern United States. Ann. Entomol. Soc. Amer. 81: 419-427.
  • Dorr, J. E. and E. C. Martin. 1966. Pollination studies on the highbush blueberry, Vaccinium corymbosum L.    Mich. Agr. Exp. Sta. Quart. Bull. 48: 437-448.
  • Free, J. B. 1993. Insect Pollination of Crops. 2nd edition: Chapt. 24. Ericaceae. Academic Press. pp. 212-220.

  • Last updated: 29 May 1997
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