Are Pollinators Exerting Selection Pressure on the Azalea Hybrids on Gregory Bald?

Gregory Bald is renowned for an azalea hybrid swarm which contains a very diverse array of flower colors and forms. This report involves the study of the flower constancy of the pollinators to the different azalea flower types. The flower type classification is based on the floral morphology, color and presence of odor. The azalea species involved in the hybrid swarm are Rhododendron bakeri (Lemon and McKay) Hume, R. aborescens (Pursh) Torrey and R. viscosum (L.) Torrey. Pollinators' constancy to flower type and pollinators' preference for flower type were observed on square plots that contained two or more flower types.

The following aspects of the floral biology of the plants were examined to determine if there were any differences among the flower types: protandry, wind pollination, nectar amount, percent nectar sugar, the presence of nectar amino acids, nectar constituent sugars and the effects of temperature and time on pollen and nectar availability. Pollinator morphology and activity were examined to determine the relative contribution of each species group toward the azalea seed set. The factors examined were bumblebee tongue length, pollinator foraging preference (pollen or nectar) and the effect of time and temperature on pollinator activity.

The rare flower types attracted more pollinators than the common flower types. The large number of insect visits to the rare flower types is tenuously correlated with an increased amount of nectar sugar production of these plants. Since the rare flower types are too rare to promote flower constancy, the pollinators also visit the common flower types. Bumblebees are the most important flower visitors and cross-pollinators of the azaleas. Butterflies, small bees and vespid wasps are common flower visitors but not important cross-pollinators.

Tests for apomictic reproduction and self pollination indicate that these two forms of reproduction are not very important in this azalea population. This result combined with the observations of the flower constancy of the bumblebees indicates that outcrossing is probably the major form of reproduction of these hybrids.