Phew! Orchid perfume turns revolting

Orchids that can smell so alluring that bees try to mate with them can also smell repulsive to the insects.

At first, a blossom of the European orchid Ophrys sphegodes gives off odors mimicking the scent of a female bee (SN: 7/3/99, p. 11). After the bloom has fooled a male into pollinating it, the odor changes, explains Florian P. Schiestl of the Australian National University in Canberra.

Overall scent production dwindles a bit, but the flower increases what had been a minor constituent of its aphrodisiac, report Schiestl and Manfred Ayasse of the Institute of Zoology in Vienna, Austria. Female Andrena nigroaenea bees secrete the same substance, farnesyl hexanoate, and line their nurseries with it.

Large whiffs of the stuff tend to repel male bees. Female bees reek of it once they’ve mated, and researchers point out that it helps males zero in on unmated female bees.

The orchid’s perfumery works in a similar way, the researchers suggest in the Feb. 22 online Oecologia. After pollination, a flower’s additional farnesyl hexanoate is still only one-thousandth the amount produced by a mated bee. Yet when researchers dabbed even that trace amount on unpollinated flowers, males visited them significantly less often than they did flowers dabbed only with solvent. The flowers’ switch-from wildly alluring to mildly appalling-directs the bee toward blossoms still in need of a visit, say the researchers.