Volatiles composition and timing of emissions in a moth-pollinated orchid in relation to hawkmoth (Lepidoptera: Sphingidae) activity

In the family Orchidaceae, many species have highly specialised floral structures and floral fragrances resulting from interactions with specific pollinators. Olfactory cues are important for the moths to locate orchids at a distance, whereas visual cues are important at a closer range. In this study, we combined a portable air entrainment kit with an automated video monitoring system for collecting volatiles and observing behaviour directly around-the-clock (24h) in the natural habitat of our target plant-arthropod system, the orchid Platanthera chlorantha and the hawkmoth Sphinx pinastri. We found that P. chlorantha was visited almost exclusively by S. pinastri. All the visits occurred after sunset, principally between sunset and midnight. Soon after midnight, visits dropped to levels recorded at sunset, then declined further towards sunrise. The period with most visits matched the peak production of the terpenoids (Z)-beta-ocimene and (E)-beta-ocimene. In contrast, linalool, (E)-cinnamyl alcohol and benzyl benzoate emission continued to increase beyond the period of peak visits up to sunrise. Methyl benzoate emissions declined throughout the night from a sunset peak. As temporal emission of the two volatile ocimenes from P. chlorantha flowers matches S. pinastri foraging visits to the flowers, we propose that they play a vital role in assisting hawkmoths locate their hosts. This is the first study to show correspondence in the timing of specific scent emissions in orchids and moth activity on the scale of hours.