Salvia apiana — A carpenter bee flower?

Abstract

Salvia apiana has unique mask flowers restricting access to nectar by a bulged lower lip. Stamens and style protrude the flower tube. Most interesting, the staminal lever mechanism usually characterizing bee flowers in Salvia L. is lacking. In the present study, we aim to understand the peculiar pollination mechanism and to identify the pollinators and breeding system of the species. Field experiments were conducted at three natural localities in Southern California and the Rancho Santa Ana Botanical Garden, Claremont. Pollinator behavior was documented on video and interpreted considering frequency, handling time, percentage of successfully touching pollen and stigma, pollen deposition site on the pollinator's body and duration of stay per inflorescence. Hand-pollination and bagging experiments were conducted to identify breeding system and seed set. Salvia apiana is self-compatible. The pollen/ovule ratio of 11,000:1 points to xenogamy, but our experiments indicate that the plant suffers from pollinator limitation at the study sites. The most frequent pollinators were honeybees (Apis mellifera), which, however, are too small and light to handle the flower correctly. They only occasionally transfer pollen increasing geitonogamy. We identified Bombus vosnesenskii, Xylocopa tabaniformis and X. varipuncta as rare pollinators. These large bees were able to press the lower lip down thereby lowering the stamens and positioning the pollen sacs close to the bee's body. The largest bee, X. varipuncta, was found to fit best to the flower. Due to its foraging behavior, it particularly contributes to outcrossing. Though honeybees are the most frequent pollinators, we conclude that S. apiana is a carpenter bee flower. The species most likely suffers under the low number of Xylocopa bees which might result from the present dominance of honeybees, introduced to California three hundred years ago.