The pollination ecology of buzz-pollinated Rhexia virginica (Melastomataceae)

We examined the function of floral traits associated with buzz pollination through studies of Rhexia virginica (Melastomataceae) in the Muskoka region of Ontario, Canada. Controlled pollinations demonstrated that the species is self-compatible. but dependent on insects for pollen transfer Bumble bee s made 82 and 90% of observed insect visits to R. virginica in 1996 and 199 7, respectively, and effectively buzzed flowers. Buzz pollination did not a ppear to be highly "specialized" since various species of bumble bee were c apable of pollination, and pollen transfer efficiency appeared to be relati vely low. Experimental manipulations provided little support for the hypoth esis that the yellow color of melastome anthers mimics abundant pollen, the reby deceiving pollinators to visit regardless of whether most pollen has b een removed. Fruit set averaged 52.6% among populations, owing largely to i nfrequent pollinator visits and pollen limitation. Flowers of R. virginica were infertile after a single day of anthesis, but petals were subsequently maintained for 1-2 d and stamens underwent a color change from bright yell ow to red. Second-day flowers may function to increase floral display size and hence fertility, without a concomitant increase in pollen discounting. Studies of bumble bee foraging behavior and correlates of seed set provided indirect support for this hypothesis.