Wednesday, June 12, 2013

Pollination of Dutchman's Breeches ~ A Royal Affair

Dutchman's Breeches ~ Dicentra cucullaria
Dutchman’s breeches flowers very early in the spring typically at the end of April but this year closer to mid-May. The bright white 'pants' look like they are hanging on a clothes line to dry.

Small plants take a few years to establish and flower but it’s worth the wait; plant in small masses for better effect in a woodland garden. The foliage yellows and dies back by mid-June so it's a good idea to inter-plant with later flowering woodland species such as false solomon’s seal to cover the void left after dormancy.

The timing of flowering of dutchman’s breeches corresponds with the emergence of overwintering queen bumble bees. As queen bumble bees emerge from hibernation, they fly low to the ground in woodlands searching for appropriate nesting sites in abandoned rodent holes, leaf piles or other dry locations. The low growing, Dutchman’s breeches serves as an important nectar source to be used in the initial provisioning of their nests.

Queen bumble bees have the tongue length, size and strength to pry open the petals on the flowers. As the flower develops, the bottom of the outer petals reflexes revealing the yellow coloration near the opening. This coloration acts as a visual attractant to visiting queens. They land on the side of the flowers grasping onto the outer petals, they also, as pictured in this photo, grasp onto adjacent flowers with their rear legs.

Once they have a good hold, they rotate their bodies in order to access the flower opening, directing their head toward one of the nectar spurs. Pushing their tongues then heads in between the outer and inner petals opens the flowers and allows them to reach the nectar in the spurs. Pollen is brushed onto their head and thorax, as the inner petals are deflected revealing the anthers. Their front legs often grasp the inner petals which helps expose the anthers and stigma and ultimately transfers pollen onto their forelegs.

Pollen also accumulates on the bottom edges of the petals. The inner petals are hinged and when the bee removes its head from the flower, the petals return to their original position. Pollen that has dropped to the edges of the petals is transferred to the middle legs of the bee. The most important pollen placement for pollination of the next Dutchman's breeches flower visited is on the head and thorax.

Other smaller worker bumble bees chew small holes in the two nectar spurs to steal nectar because they are not strong enough to pry open the petals and have shorter tongues than queen bumble bees.

Even smaller bee species such as Cuckoo bees, Nomada sp. and mining bees, Andrena sp. fly around and occasionally land on the flowers investigating them for a potential reward, but none can be accessed.

Like many other spring-flowering woodland plants, the seeds of Dutchman's breeches are dispersed by ants. Ants are attracted to the protein-rich, fleshy elaiosome attached to the seeds. They carry them back to their nests consuming the elaiosome and discarding the fertile seed in their nest's trash pile.

In the absence of queen bumble bees, the flowers of dutchman’s breeches are self-compatible and often self-pollinate.

References: 
Macior, L. W. (1970). The pollination ecology of Dicentra cucullaria. American Journal of Botany, 6-11. Retrieved from: http://www.jstor.org/stable/2440374

Macior, L. W. (1978). Pollination interactions in sympatric Dicentra species. American Journal of Botany, 57-62. Retrieved from: http://www.jstor.org/stable/2442554