Better Than Real: Males prefer flower’s scent to female wasp’s

In an extreme case of sex fakery, an orchid produces oddball chemicals that mimic a female wasp’s allure so well that males prefer the floral scents to the real thing, scientists say. This plant’s come-on is different from that of a related orchid that flirts with bees.

OOPS! A male wasp is discovering something not quite right about his newest mate, despite the plant’s red hair and lovely scent. Ayasse

The Mediterranean orchid Ophrys speculum manufactures whiffs of the same scent that the female wasp Campsoscolia ciliata does. The flower misleads male wasps into mating attempts that benefit the plant by spreading pollen, explains Manfred Ayasse of the University of Ulm in Germany. He and his colleagues have now identified the attractants as chemicals not previously known in plants. The orchids produce them more abundantly than female wasps do, and males prefer the stronger bouquet, the researchers say in an upcoming Proceedings of the Royal Society of London B.

This finding adds to the growing respect for the powers of deceptive orchids, according to Ayasse. Biologists had previously concluded that the plants make only mild attractants that males neglect once females appear. Last year, however, scientists found that an Australian orchid releases a scent that attracts inexperienced male bees as well as the actual female scent does (SN: 7/27/02, p. 56: Making Scents of Flowers).

Several hundred orchids have been identified worldwide that use sexual deception to attract pollinators, says coauthor Florian P. Schiestl of the Geobotanical Institute ETH in Zurich.

With brushy red hairs, the O. speculum blooms look vaguely like the wasps that pollinate them. The flower produces 100 to 150 volatile compounds, but tests by several scientists had failed to figure out which ones matter, says Ayasse. He and his colleagues hitched a gas chromatograph, which separates scent components, to equipment that measures nerve impulses in insect antennae. With this setup, the researchers identified the 10 compounds that the male antennae detect.

When the researchers offered whiffs of these substances to the wasps, only a few related compounds inspired males to start attempting to mate. One chemical, 9-hydroxydecanoic acid, had been previously described only in honeybees.

The scientists next set out pairs of dead females, one with the scent from a flower blown over it and the other with the scent from a real female wasp. Males responded to the floral scent with more than twice as many attempts to mate as they did to the true sex lure.

This orchid’s chemical seduction takes the opposite strategy from that of a species in the same genus that the team had analyzed earlier (SN: 7/3/99, p. 11). That study had found that Ophrys spegodes relies on a specific blend of more than a dozen common chemicals rather than a mix of a few rare compounds. Finding sister species that take such different approaches indicates that “the [orchid] system is very flexible,” Schiestl says.

Says pollination biologist Elizabeth Elle of Simon Fraser University in Burnaby, British Columbia: “Think of it as an arms race. At the moment, the plants are ahead.”

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Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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