Scent and nectar rhythms of an African orchid are timed for its pollinator

Pollinators play a key role in shaping angiosperm floral diversity. Relationships between animal pollinators and floral size, shape, colour, and nectar chemistry have been characterized. Less attention, however, has been paid to the temporal dimension of plant-pollinator interactions. These include the timing of anthesis, nectar production, and scent emission. Part of the problem has been the difficulty of making observations over 24 hour periods due to observer fatigue, low visitation rates, and human presence disturbing pollinators. Recently, motion-activated cameras with close-up lenses and infrared illumination have come into greater use to effectively document nocturnal pollinators.

In a new article published in Annals of Botany, lead author Marco G. Balducci and colleagues used direct observation and motion-activated cameras to study pollinator activity around the African woodland orchid Bonatea polypodantha in relation to the plant’s daily nectar and scent production rhythms. The group aimed to establish the orchid’s pollinator species and whether floral opening, scent, and rewards were synchronized to that pollinator’s behaviour patterns.

Observations established that B. polypodantha is pollinated exclusively by short-tongued hawkmoths, Nephele comma, but are also subject to nectar thievery by long-tongued hawkmoths. These latter moths are unable to act as pollinators due to the length of their tongues. Hawkmoth activity occurs only at night, with the short-tongued species engaging in a crepuscular foraging pattern peaking at dusk and dawn. The long-tongued nectar thieves were active throughout the night.

Despite pollinator activity peaks at both dusk and dawn, the flowers displayed peaks of anthesis, scent emission, and nectar availability only around dusk. No similar peaks were detected in the morning, suggesting the plants are adapted mainly for dusk foraging. Some scent was present during the dawn activity period, but the quick visits the moths made to the flowers at that time suggested that visual clues may have been the primary method of locating flowers once light levels rose.

“This study highlights the utility of using motion-activated cameras to explore aspects of pollinator behaviour, such as diel activity patterns,” write the authors, noting that insect species, pollen transfer, and pollinator behaviour patterns were all discernible from the video footage. “These cameras can thus accelerate the rate of discovery of new pollination systems, particularly in remote localities and for plants with flowers that are visited during the night.”