Open Access

Jennifer Brodmann, Robert Twele, Wittko Francke, Manfred Ayasse

• The orchid genus Epipactis is represented by 25 species in Europe (Richards 1982). Epipactis helleborine (L.) Crantz is the most common and widely distributed species of the genus (Wiefelspütz 1970), and is a prime example for wasp-flowers, because it is mainly pollinated by social wasps (Hymenoptera: Vespidae), like Vespula vulgaris and V. germanica (Müller 1873). Darwin (1888) already noticed that E. helleborine is almost exclusively ignored by bees and bumblebees, an observation that was confirmed in recent investigations (Keppert 2001). The flowers of E. helleborine show morphological, physiological and phenological adaptations to the visit and the pollination by Vespidae (Keppert 2001). They possess a reddish-brown or dirty purple coloration of the inflorescence (Keppert 2001), have relatively small, mostly bulbous blossoms with a broad entrance and bulbous widened, nectar-rich juice holders (Müller 1873, 1881; Schremmer 1962). Although there is much reported about wasp-pollinated flowers there is little known about the signals that are responsible for the attraction of wasps. Wiefelspütz (1970) proclaimed the statement that only the visual stimulus is responsible for the wasp attraction. Recently studies, however, assumed that odour is involved in the wasp attraction (Keppert 2001). Hölzler (2003) showed that the main attraction of the wasp-flower Epipactis for pollinators is its olfactory stimulus. It remains an unanswered question why E. helleborine flowers almost exclusively attracts social wasps, as opposed to bees and bumblebees. In this study we analysed the role of floral volatiles which are responsible for the specific attraction of social wasps. We supposed a mimicry-system in E. helleborine for the specific attraction of pollinators for the following reasons. So-called “green leaf volatiles” (GLVs) are emitted by plants while herbivorous insects, for example caterpillars, feed on them. GLVs thereby attract predators or parasitoids of the herbivorous insects (Dicke & Sabelis 1988; Turlings & al. 1990, 1995; Dicke & Vet 1999). Among the GLVs so far identified in former studies there are aldehydes, compounds that were also found in flower extracts of E. helleborine (Hölzler 2003). Therefore, we postulated that E. helleborine flowers produce GLVs in order to attract prey hunting social wasps for pollination. We performed bioassays and analysed flower odour gained to headspace-sampling using gas chromatography (GC), mass spectrometry (GC-MS) and gas chromatography coupled with electrophysiological analysis (GC-EAD) to investigate the hypothesis that E. helleborine flowers mimic “green leaf volatiles” (GLVs) to attract their pollinators.
• Die Orchideenart Epipactis helleborine gilt als typische Wespenblume. Die Blüten weisen Anpassungen an den Besuch und die Bestäubung durch soziale Faltenwespen (Hymenoptera: Vespidae) auf und werden häufig durch Vespula vulgaris und V. germanica bestäubt. In früheren Untersuchungen konnte gezeigt werden, dass olfaktorische Reize bei der Bestäuberanlockung eine übergeordnete Bedeutung vor optischen Reizen haben (Hölzler 2003). Die Frage, warum E. helleborine fast ausschließlich ihren optimalen Bestäuber, die soziale Faltenwespe, zur Bestäubung anlockt, und nicht auch auf andere Blütenbesucher attraktiv wirkt, ist noch unbeantwortet. Wir untersuchten die Hypothese, dass E. helleborine Blüten GLVs, die von Herbivoren befallenen Pflanzen abgegeben werden, nachahmen, um Beute jagende Wespen zur Bestäubung anzulocken. Dazu sammelten und analysierten wir Duftstoffe von Epipactis Blüten und mit Pieris-Raupen befallenen Kohl und identifizierten vier gemeinsam vorkommende GLVs. In Y-Rohrtests konnte die wespenanlockende Wirkung dieser Verbindungen nachgewiesen werden.