D3116: Pollinators, pollinator dependence, and reproductive biology of six North American Rhododendron (Ericaceae) with varying conservation status

Pollination and mating systems are critical components in plant and pollinator conservation. Much of the charismatic plant genus, Rhododendron, are threatened or endangered or occur in threatened or endangered habitats. Despite this, limited research has been done in North American species. We examined the reproductive and pollination biology of six species endemic to the southeastern United States that also vary in conservation status. The species included are the federally endangered R. chapmanii, the state endangered R. prunifolium, R. austrinum, R. alabamense, and the common R. canescens, and R. viscosum. We performed single visit efficacy of floral visitors to determine effective pollinators for each species, as well as hand pollination treatments to determine pollinator dependence, mating systems. and if populations were receiving optimal pollination or were pollen limited. Results indicate that species differ in degree of self-fertility, ranging from highly fertile in R. chapmanii to no compatibility in R. alabamense. Effective pollinators are different for each species and range from bumble bees (Bombus sp.) to swallowtail butterflies (Papilio sp.). A diverse suite of pollinators visited flowers including bees in the genera Bombus and Xylocopa, moths in the family Sphingidae, hummingbirds, and a variety of butterflies dominated by Papilio species. Most plant populations were pollen limited and all species were dependent on animal pollinators for reproduction. Our study highlights the importance of animal pollinators for North American Rhododendron and the need to expand conservation efforts to pollinators, particularly in the case of threatened and endangered species.