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