Pollinator attraction in a sexually deceptive orchid by means of unconventional chemicals

Abstract

Ophrys flowers mimic virgin females of their pollinators, and thereby attract males for pollination. Stimulated by scent, the males attempt to copulate with flower labella and thereby ensure pollination. Here, we show for the first time, to our knowledge, that pollinator attraction in sexually deceptive orchids may be based on a few specific chemical compounds. Ophrys speculum flowers produce many volatiles, including trace amounts of (ω–1)–hydroxy and (ω–1)–oxo acids, especially 9–hydroxydecanoic acid. These compounds, which are novel in plants, prove to be the major components of the female sex pheromone in the scoliid wasp Campsoscolia ciliata, and stimulate male copulatory behaviour in this pollinator species. The specificity of the signal depends primarily on the structure and enantiomeric composition of the oxygenated acids, which is the same in wasps and in the orchids. The overall composition of the blend differs significantly between the orchid and its pollinator and is of secondary importance. 9–Hydroxydecanoic acid is a rarely occurring compound that until now has been identified only in honeybees. Contrary to the standard hypothesis that Ophrys flowers produce only ‘second–class attractivity compounds’ and are neglected once the pollinator females are present, we show that flowers are more attractive to the males than are their own females.