Orchids mimic insect pheromones to lure pollinators

The world’s tens of thousands of orchid species use various strategies to get themselves pollinated, German researcher Manfred Ayasse told the people who attended his March 9 presentation at the Smithsonian’s Tupper Auditorium. About 40 percent of these species use some sort of deceptions to attract animal visitors, generally from the insect class.

A few hundred of these use sexual deception, appearing to male bees, wasps or beetles like attractive mates. These males are fooled into attempting to copulate with the flowers, which cover them in pollen to be carried to the next faux foxy flower. (These guys never seem to learn.)

Sexually deceptive orchids were first noticed by scientists back in 1917, but it wasn’t until decades later that it was discovered that the phenomenon isn’t merely a matter of looks. In 1961, it was discovered that orchids also use chemical cues to get male insects all hot and bothered.

In the past decade, Ayasse and his fellow scientists have been getting into the specifics, comparing the chemical bouquets of sexually deceptive orchids and the pheromones by which female insects attract males.

First, they localized the sex pheromones of the females and the male attracting scents in the orchids, and found that they were dealing with very complicated chemical mixes, typically with more than 100 chemical compounds in the flower scents.

Then they took odorless females, swabbed them with various of these plant scents, and observed how males reacted to them. Thus they discovered which perfumes drive the males wild.

The results allowed them to narrow the search, and spectrographic analysis isolated 14 active chemicals that the female pheromones and flower bouquets have in common. Further experimentation showed that these ingredients don’t work all by themselves --- only the right mixture will excite a male.

It gets even more complicated. It seems that different flowers on the same orchid put out slightly different scents, as do flowers that have been visited by male insects and those that haven’t. It turns out that male bees that fall for orchids are a pretty lecherous lot, who don’t care to visit the same flower twice. The differences in scent mixtures serve as these insects’ guides, and thus prevent the orchids from self-pollinating. It also seems that there is a mechanical backup to this process, as the stigmata of flowers that have been visited bend and that’s taken by the male insects as a cue to visit someone else.

Moreover, once a flower has been pollinated, its chemical bouquet changes --- diminishing to mimic the smell of a female insect who has already mated and thus become unattractive to males.

So how effective is this orchid pollinating strategy? “Most of the plants to not get a visit by a pollinator,” Dr. Ayasse said. But enough of them do confuse enough male insects so that the species survive.

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Sexually deceptive orchids