The American cranberry (V. macrocarpon) is a perennial trailing woody vine that is native to acidic, sandy bogs, swamps, shorelines and streambanks of northeastern North America. Along with blueberries, it belongs to the heath family (Ericaceae). An estimated 20 million flowers bloom per cultivated acre. Bees always probe cranberry flowers for nectar. Simultaneously, some foragers will vibrate the staminal column for pollen as they hang from the pendant flower. Diverse bee species work and pollinate cranberry flowers; our recent collaborative research shows that honey bees are satsifactory cranberry flowers so long as the weather during bloom is favorable for their foraging flights.
Cranberry blooms during June and July, bearing ranks of solitary flowers along short upright shoots that grow above the mat of vines. An estimated 20 million flowers bloom per cultivated acre. Varieties of V. macrocarponare largely self-fertile. The tart red berries contain up to 35 tiny black seeds. Vigorous vegetative multiplication has facilitated cranberry’s cultivation in the past 150 years. The berry is a rich source of vitamin C; fresh berries were packed in barrels aboard whaling ships to prevent scurvy among the crew.
Cranberry flowers are hermaphroditic, and possess basal nectaries and a tight beak-like column of stamens. The pendant white flowers possess four strap-like petals that are reflexed at floral maturity. Flowers contain meager nectar rewards, but the pale pollen is rich in protein. The stigma of the single pistil extends beyond the anther tips, thus limiting opportunities self-pollination. Anthers shed their pollen grains through paired terminal pores at the anther tips. Unvisited flowers rarely produce fruit, even when mechanically shaken as by wind.
Our recently published collaborative research reveals through experimental manual pollinations that there exists a dose-response relationship between pollination and fruiting for cranberry. Just a few pollen grains are needed to produce fully-sized berries; additional pollen appears to be superfluous for crop production (see Cane publications).
Bees always probe cranberry flowers for nectar. Simultaneously, some foragers will vibrate the staminal column for pollen as they hang from the pendant flower. To release pollen, they either audibly buzz the flower (e.g. bumblebees) or drum the staminal column using their mid- or hind legs (e.g. Megachile and Osmia). For unknown reasons, honey bees use their fore legs to drum the staminal column for pollen, however, which precludes simultaneous probing for nectar. Honey bees may probe cranberry flowers for nectar either legitimately, parting the staminal tips with their inserted proboscis, or on occasion illegitimately, by inserting their proboscis between the bases of two staminal filaments. Illegitimate foragers make no stigmatic contact.
Honeybees have been disparaged by some as being ineffective cranberry pollinators. While it is true that only a minority of foragers are seen to be collecting pollen (and thus contacting the stigmas), single visits by pollen foraging honeybees regularly deposit sufficient pollen to elicit a full set of large berries (see Cane publications). Hence, honey bees can be satsifactory and affordable pollinators of commercial cranberries where weather during bloom is warm; where weather during bloom is too cool for honey bee flight, supplemental pollinators will be beneficial.