The flowers of many kinds of orchid look and smell like female insects. This fools males into trying to mate with them, transferring pollen as they go along. But males only mount flowers that have not yet been pollinated--a selectivity that has puzzled researchers. Now, a study in the 1 March issue of Oecologia provides the answer. The paper shows that orchids' sexual deception of insects goes farther than scientists had ever imagined.
To find out why male pollinators prefer virgin flowers, Florian Schiestl and Manfred Ayasse from the University of Vienna, Austria, studied the European orchid Ophrys sphegodes. That plant's flowers have roughly the same shape, color, hairiness, and scent as females of the bee Andrena nigroaenea, its only pollinator. The researchers chemically analyzed the odors of more than 100 flowers, half of them unpollinated, the other half 1 to 4 days after pollination.
They found that in pollinated flowers, there's a two-fold increase in a substance called farnesyl hexanoate, an insect pheromone normally released by female bees to signal that they have already been inseminated. And indeed, when Schiestl and Ayasse applied a droplet of this compound to unpollinated flowers, the male bees lost all interest in them. Andrena nigroaenea visits the orchid infrequently, says Schiestl, so the plant benefits from making sure any visiting males do not waste time on flowers that have already been taken care of.
Orchid boffins are baffled by the new data. "This shows that the interaction between orchids and pollinating males is even more fine-tuned than previously known," says Andreas Erhardt, who studies orchid pollination at the University of Basel in Switzerland.