Orchids can be highly deceptive. About 10,000 nectarless orchid species deceive insects into visiting them by imitating either the scent of a nectar-bearing flower or, more dramatically, the shape and pheromones of an insect of the opposite sex. A new twist to this story has recently been described in a paper by a Sino-German research team, led by Jennifer Brodmann of the University of Ulm.
The Dendrobium sinense nectarless orchid is found only on the Chinese island of Hainan, and its flower looks vaguely like a white daffodil. During field observations in the moss forest of Hainan, Chinese researchers concluded that the orchid's sole pollinator is a hornet, Vespa bicolor. But instead of approaching the flower and landing on it gently, the hornets pounced on a red mark in the centre of the flower, as though they were attacking a prey. During this aggressive behaviour, the hornet gets D. sinense pollen on its thorax, ready to pass onto the next orchid that it pounces upon.
Laboratory studies using extracts of D. sinense flower scent confirmed that the hornets were attracted by a chemical cue released by the flower. To find out exactly what the substance was, the team isolated the compounds from orchid flower scent and injected it into a gas chromatograph to separate out the chemical components of the scent blend. As the flower odours were fractionated by the chromatograph, the researchers simultaneously recorded the electrophysiological activity of sensory neurons in the hornet's antenna, and identified which of the compounds the insects were able to detect. One of the most important responses was induced by a hitherto unknown flower volatile, (Z)-11-eicosen-1-ol.
In other orchid—insect deception systems, the substances that flowers mimic to attract male pollinators are often female insect sex pheromones. Here, however, the key compound — (Z)-11-eicosen-1-ol — is a component of the alarm pheromone of the Asian honey bee, Apis cerana. Behavioural tests confirmed that the hornets were highly attracted to this substance.
The explanation for this complex relation is that A. cerana is one of the hornet's main prey species. The orchid appears to be attracting the hornets by mimicking the alarm pheromone of their prey — hence the aggressive pouncing behaviour shown by the pollinating hornets. It is known that hornets use a mixture of chemical and visual cues to track down their bee prey — this study suggests that the bee alarm pheromone may be of decisive importance for them.
Hornets will raid Asian honey bee nests but can be repulsed by the bees, which wrap the predator up in a ball of their buzzing bodies, heating it to death. This elegant study reveals an extra dimension to the complex arms race between bees and hornets, by showing how a plant can ‘eavesdrop’ on their relationship in order to ensure pollination. It also demonstrates that chemical deception can be even richer than was previously imagined.
