Does the orchid Luisia teres attract its male chafer pollinators (Scarabaeidae: Protaetia pryeri pryeri) by sexual deception?

Abstract

The epiphytic orchid Luisia teres (Asparagales: Orchidaceae) releases floral scent that attracts males of the cupreous polished chafer Protaetia pryeri pryeri (Coleoptera: Scarabaeidae) for pollination. We analyzed this floral scent to identify the attractant(s). When various flower parts were extracted with diethyl ether and assayed, male chafers were attracted to the extract of petals but not to that of sepals or lips. The petal extracts were chromatographed on silica gel column and the activity was found in ether and methanol fractions. Gas chromatography/mass spectrometry (GC/MS) analyses and subsequent NMR analyses revealed that both active fractions contained 2,3-dihydroxypropyl isovalerate (2,3-DHPiV) as a major compound. Natural 2,3-DHPiV showed identical retention time to that of synthetic (R)-enantiomer in subsequent HPLC analyses using a chiral column, while (S)-enantiomer was not detected. The amount of this compound was ca. 5 µg/flower. In field tests performed in Okinawa, Japan, males of P. pryeri pryeri were observed to fly around and land on a cotton ball impregnated with synthetic (R)-2,3-DHPiV, but no male was attracted to the (S)-enantiomer or the racemic mixture. Thus, the chemical attractant released from the flower was identified to be (R)-2,3-DHPiV. 2,3-DHPiV was also found in a volatile sample collected from the virgin females by GC/MS analyses using achiral column, while this compound did not occur in males. Because of the difficulty in HPLC detection of a small amount of 2,3-DHPiV, the chirality remains to be determined. Nevertheless, these results strongly suggest that the flowers of L. teres release the same compound as females of its pollinator P. pryeri pryeri, indicating that the flower of this orchid chemically mimics the female sex pheromone.