Orchid Mimics Honey Bee Alarm Pheromone in Order to Attract Hornets for Pollination

Approximately one-third of the world's estimated 30,000 orchid species are deceptive and do not reward their pollinators with nectar or pollen. Most of these deceptive orchids imitate the scent of rewarding flowers or potential mates. In this study, we investigated the floral scent involved in pollinator attraction to the rewardless orchid Dendrobium sinense, a species endemic to the Chinese island Hainan that is pollinated by the hornet Vespa bicolor. Via chemical analyses and electrophysiological methods, we demonstrate that the flowers of D. sinense produce (Z)-11-eicosen-1-ol and that the pollinator can smell this compound. This is a major compound in the alarm pheromones of both Asian (Apis cerana) and European (Apis mellifera) honey bees and is also exploited by the European beewolf (Philanthus triangulum) to locate its prey. This is the first time that (Z)-11-eicosen-1-ol has been identified as a floral volatile. In behavioral experiments, we demonstrate that the floral scent of D. sinense and synthetic (Z)-11-eicosen-1-ol are both attractive to hornets. Because hornets frequently capture honey bees to feed to their larvae, we suggest that the flowers of D. sinense mimic the alarm pheromone of honey bees in order to attract prey-hunting hornets for pollination.