Technically, raspberries are self-fruitful, meaning that they do not require cross-pollination to fruit. However, for best berry production, raspberry plants require the presence of bees. Bees transfer pollen from anthers to the pistil within each individual flower. Fortunately, raspberry flowers produce a lot of nectar and pollen, which attracts bees. Crop yields may drop as much as 70 to 80 percent if bees are absent during flowering.
Raspberry plants flower for one to three weeks. Flowers are approximately 1 inch in diameter with five white petals. Each hermaphroditic flower has many stamens, which are inserted on the calyx, as well as ovaries, which typically remain on berry drupelets. Flowers begin shedding their petals about a day after they open. An abundant amount of rich nectar is secreted inside the ring of stamens, which attracts insect pollinators. Studies have shown that best fruiting occurs in cases where each flower is repeatedly visited by bees for a minimum of four consecutive days.
Honeybees are the main raspberry pollinators, representing more than 90 percent of insect flower visits, on average. In fact, research has shown that raspberries can be sufficiently pollinated by honeybees alone, in the absence of all other insects. Wind, even when blowing consistently strong, does not provide any source of pollination. Most bee foraging takes place between 10 a.m. and 4 p.m. Insecticide should not be applied during this time frame.
Self-Pollination Versus Bee Pollination
The presence of honeybees significantly improves the rate of raspberry production. It also results in larger, heavier berries. However, if cross-pollination is not brought about by bees, some self-pollination will occur. The degree of success with self-pollination will vary by cultivar. There has been a significant amount of research over the decades comparing insect-pollinated raspberry production with caged fruit production, where insects are purposely excluded. During one such study, 16 to 70 percent of flowers set fruit when insects were excluded compared to 64 to 98 percent fruit-set rate when insect visits were allowed.
Sufficient numbers of bees must be present during flowering to assist raspberry pollination in any large growing operation. At least one colony for every 2 acres is generally recommended. Native bee populations have steadily declined over the past several decades. Consequently, managed honeybees, which are available for rent, are becoming an increasingly important part of commercial raspberry growing. Ensure good distribution of hives. Bees will most actively pollinate within 100 yards of their hive.