A large, healthy blueberry plant produces thousands of flower buds every year. With up to 16 individual flowers developing from each bud and every flower a potential berry, pollination needs in blueberries are great. 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.
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. Therefore, in order to set fruit, the flowers of a blueberry plant must be pollinated by insects.
Numerous native bees (including bumble bees and solitary bees) are indigenous pollinators of blueberry plants in North America. In addition, Honey bees are 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. Some species of 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 pollination will occur.
A single visit by a honey bee to a blueberry flower can result in relatively few seeds developing: many bees are required to meet the pollination needs of the blueberry plant. In addition to foraging for blueberry nectar and pollen through the opening at the apex of blueberry flowers, — often referred to as “legitimate” foraging — honey bees may also take advantage of slits made by carpenter bees to feed on nectar. This type of foraging — termed “robbing” or “poaching” — does not contribute as much pollination to flowers as legitimate foraging, but it does result in some measurable pollination. Observations in blueberry fields also suggest that “robbing” honey bees may visit flowers more quickly that those foraging legitimately allowing honey bees to visit more flowers when robbing.
Honey bees are stocked in commercial blueberry fields at rates of one to four hives per acre and are typically the most abundant bee species in blueberry fields for this reason. It is best to plant different varieties of blueberries within 100 feet of each other, so bees can travel and cross pollinate. Blueberries cannot be fertilized by their own pollen.