Pollination of Wild White Indigo ~ Baptisia lactea

Wild White Indigo ~ Baptisia lactea (B. alba)

The tall racemes of wild white indigo project above the other prairie forbs and grasses in early spring before these competing plants start to grow. It is statuesque in form, with the blue-gray foliage and light gray, sturdy stems. If you purchase or grow wild white indigo, small seedlings can take a few years to establish before flowering.

Wild white indigo plants grow in open, mesic prairies in full sun.

Worker and queen bumble bees are the primary pollinators of wild white indigo. Their size and strength allows them to pry open the flowers accessing the nectar inside. In a cool spring where the flowers open later than average, more queen visits occur. In an average season, the flowers are visited by more worker bumble bees than queens.

The flowers of wild white indigo are protandrous - the male reproductive organs develop before the female reproductive organs. Flowers open and mature from the bottom upwards on the raceme.

Bumble bees fly towards the upright flower raceme landing on one of the lower flowers in the pistillate (female) phase which typically produce more nectar than those at the top in the staminate (male) phase.

After bumble bees seek out the higher nectar rewards on the pistillate flowers, they work their way up the flower raceme to the upper flowers in the staminate (male) phase. Pollen is therefore transferred during the last stage of their visit on staminate (male) flowers. The bumble bees move on to a new flower raceme carrying pollen with them to pollinate the next staminate (female) flower visited.

A smaller worker bumble bee pries open a staminate (male) flower. Remaining in this position for close to a minute, it rubs its rear legs over the exposed stamens collecting pollen.

Once the flowers are pollinated, an inflated seed pod develops housing the seeds, turning from light green to dark gray over the summer.

The seeds of wild white indigo are heavily predated on by a small weevil, Trichapion rostrum (Apion rostrum). This weevil can be very destructive, consuming most or all of the developing seeds inside a seed pod. Female weevils seek out the pods in June and drill a small hole at the base. Eggs are laid on the pod and using her snout, the weevil pushes them through the hole into the pod.

To save energy and resources, wild white indigo plants often abort pods during ripening, especially those with weevil populations inside. The pods decay and the developing weevils inside die.

References:
Haddock, R. C., & Chaplin, S. J. (1982). Pollination and seed production in two phenologically divergent prairie legumes (Baptisia leucophaea and B. leucantha). American Midland Naturalist, 175-186. Retrieved from http://www.jstor.org/stable/2425307

Horn, S., & Hanula, J. L. (2004). Impact of seed predators on the herb Baptisia lanceolata (Fabales: Fabaceae). Florida Entomologist, 87(3), 398-400.

Petersen, C. E., Petersen, R. E., & Meek, R. (2006). Comparison of common factors affecting seed yield in the congeners, Baptisia alba and Baptisia bracteata. Transactions of the Illinois Academy of Science99, 31-36.

Petersen, C. E., & Sleboda, J. A. (1994). Selective pod abortion by Baptisia leucantha (Fabaceae) as affected by a curculionid seed predator, Apion rostrum (Coleoptera). Great Lakes Entomologist, 27, 143-143. Retrieved from http://insects.ummz.lsa.umich.edu/mes/gle-pdfs/vol27no3. pdf#page=18