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Orchids are famous for their deceptions. Most of those with nothing of value to offer their pollinators lure them instead with the scents of more rewarding flowers or potential mates. Now, a report published online on August 6th in Current Biology, a Cell Press publication, reveals for the first time that a species of orchid, which lives on the Chinese island of Hainan, fools its hornet pollinator by issuing a chemical that honeybees use to send an alarm.
The discovery explains why the hornets, which capture honeybees to serve as food for their larvae, have been observed to literally pounce on the rewardless Dendrobium sinense flowers.
The compound the orchids produce, so-called Z-11-eicosen-1-ol, is a rarity even in the insect world, said researchers Manfred Ayasse and Jennifer Brodmann of the University of Ulm in Germany. It has never before been described in any plant.
"Of course, we are aware of the fascinating other examples of how orchids attract their pollinators," Ayasse said. "However, we did not expect to find such a new form of deception."
The researchers knew from earlier studies by their Chinese collaborators that there was something going on between D. sinense and the hornet Vespa bicolor. Hornets were the most frequent visitors of those orchids by far. And rather than landing and pausing on the flowers, as would be typical behavior for pollinators, the hornets instead pounced on the red center of the flower, much as though they were attacking prey.
In the current study, they found that hornets were more apt to tackle orchids with their natural scent or dummy honeybees impregnated with the floral scent than they were odorless bee dummies.
An examination of the floral extract turned up Z-11-eicosen-1-ol as one of few compounds that might be detected by the antennae of worker hornets. The chemical was known from other studies to be a major compound of honeybees' alarm pheromone and an essential component for prey recognition in hunting wasps. Behavioral experiments of hornets in the lab confirmed the predatory insect's attraction to the orchid flower's scent and to Z-11-eicosen-1-ol alone.
People might take a note from these orchids about how to manipulate Vespa hornets to their own ends, according to the researchers.
"Various species of Vespa are problems to beekeepers, because they plunder the hives," Ayasse said. "Besides this, their ravages of fruit crops make hornets a serious pest to man. Our results could be used to develop environmentally responsible traps for pest hornets."
The researchers include Jennifer Brodmann, University of Ulm, Ulm, Germany; Robert Twele, University of Hamburg, Hamburg, Germany; Wittko Francke, University of Hamburg, Hamburg, Germany; Luo Yi-bo, Institute of Botany, Chinese Academy of Sciences, Beijing, China; Song Xi-qiang, Hainan University, Haikou, China; and Manfred Ayasse, University of Ulm, Ulm, Germany.