Flowers of sexually deceptive orchids mimic the sex pheromones of their pollinator species. Pollination occurs when the male pseudocopulates with the flowers. We investigated the “pseudopheromones” of orchids as well as the sex pheromones and mating behavior of their respective pollinator species in the Australian orchid genus Chiloglottis, which is pollinated by thynnine wasps (Hymenoptera: Tiphiidae), and the European genus Ophrys, pollinated by solitary bees (Hymenoptera: Apoidea). We examined semiochemicals and their detection by the males using gas chromatography with electroantennographic detection (GC-EAD). Gas chromatography – mass spectrometry was used to identify odor compounds and behavioral tests to investigate reactions of males to natural and synthetic odor compounds. Behavioral tests with flowers of Chiloglottis trapeziformis showed that males of its pollinator species, Neozeleboria cryptoides, are attracted over long distances, which relates to their mating behavior. Female thynnines are flightless and call for males by releasing sex pheromones from a low perch. They are rapidly located by the males and transported to a feeding site. In GC-EAD experiments with C. trapeziformis volatiles, only one peak elicited electroantennographic reactions in male antennae. This peak was found in the labella as well as in the sepals of the flowers. C. valida, which occasionally share the same pollinator species, produces the same compound(s), in labella and sepals. Although identifications and field tests remain to be done, this suggests that the mimicry of the sex pheromone requires only a few compounds. In Ophrys sphegodes, on the contrary, 14 alkanes and alkenes have been found to elicit electroantennographic responses and were shown to elicit copulation behavior in the pollinator bee species, Andrena nigroaenea (Schiestl et al. 1999, Nature 399: 421-422). These compounds also constitute the sex pheromone. The low volatility of these compounds suggest them to act primarily at close distance, which is in accordance to the mating behavior of the bee species. Andrena nigroaenea males search for females in aggregations on odor marked trails. Females are thought to fly in an aggregation where mating occurs. Differences between the mating behavior in thynnine wasps and solitary bees appears to have led to the evolution of different pheromone communication systems. Thus Australian and European sexually deceptive orchids, have recruited male pollinators with contrasting chemical communication systems.

Index terms: Neozeleboria cryptoides, Chiloglottis, Andrena nigroaenea, Ophrys sphegodes, orchid pollination

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