Bumble Bees and Baptisia: A Pollination Story

Plants use many “tricks” to entice insects into the work of pollination. The shape, color, and bloom period of a plant’s flowers can greatly influence who their potential pollinator mate may be. Such is the case when considering the relationship between bumble bees and flowers in the genus Baptisia.

There are two plants of the genus Baptisia common to North America, blue false indigo (B. australis) and wild white indigo (B. alba). They are members of the pea or legume family (Fabaceae) and feature flowers typical of the family. Think of sweet peas climbing a trellis in your grandmother’s cottage garden, or the often ornamental flowers of some pole beans. These unique, 5-petaled flowers feature a broad upper petal (known as a banner), two lower petals spread out to the side (wings) and two more petals fused together and curled around the flower’s reproductive parts (the keel).

Due to their size and strength, relative to other pollinators, bumble bees are well adapted to access the nectar and pollen within these uniquely shaped flowers. Bumble bees grip the keel with their mid and hind legs, using the leverage produced to propel them forward into the heart of the flower where they can access nectar. This has the benefit (from the flowers’ perspective) of lowering the keel and exposing the bumble bees’ fuzzy abdomen to the pollen-covered anthers.

A bumble bee works its way up the blooms of Blue False Indigo (Baptisia australis). Pollen attached to her rear legs from the previous flower stalk will be deposited onto the next set of blooms. Photo: Justin Wheeler, the Xerces Society

A bumble bee works its way up the blooms of Blue False Indigo (Baptisia australis). Photo: Justin Wheeler, the Xerces Society

These plants produce their blooms on long stalks (known as racemes). Blooms mature from the bottom up. As the blooms mature, older flowers are more pistillate (female), producing more nectar than pollen. Bumble bees will approach the stalk and land on the lower flowers first — seeking higher nectar rewards. They will then work their way up, ending with the more pollen-rich staminate (male) flowers at the top. As they move to the next stalk, pollen attached from the staminate flowers of the previous stalk is then transferred to the pistillate flowers of the current stalk.

You can see from this example how interconnected plants are to their potential pollinators. It’s almost as if they’re made for each other!

by Justin Wheeler, Web and Communications Specialist

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