Australasian Science: Australia's authority on science since 1938

Size Matters for Sexually Deceptive Orchids

The size and shape of orchids play a major part in an act of deception that entices male wasps to pollinate their flowers, with ecologists at the Australian National University discovering orchids that mimic how female insects look. Until now it had been thought that orchids only produced a scent to mimic the sex pheromones of female insects in order to deceive male insects.

“The orchids exploiting these wasps both have to look good and smell good to ensure that the transfer of pollen is established,” said Dr Marinus de Jager, who was a visiting postdoctoral fellow at ANU when he completed the study with Prof Rod Peakall.

Australia is home to most of the world’s species of sexually deceptive orchids. These include two different but closely related species of orchid, the broad-lipped bird orchid and the large bird orchid, which are pollinated by two different species of wasp. Despite their differences, they both produce the same chemical to attract their respective male wasps.

The research found that wasps of both species behave very differently towards each orchid, but in both cases males tried to mate more often and for longer with the orchid they normally pollinate.

Peakall said that shape and size has a dramatic effect on pollinating behaviour. “We can present the chemicals on little black beads and pollinators will come in and attempt to mate,” he said. “Without the right shape and size you will get the pollinator to the flower with just smell, but it won’t contact the pollen reproductive structures.”

de Jager said the research, which has been published in Functional Ecology (tinyurl.com/n9grd89), will lead to greater investigation into pollinator behaviour as a driver of floral trait evolution in a diverse range of orchids, both in Australia and internationally.