Pollinator attraction of the wasp-flower Scrophularia umbrosa (Scrophulariaceae)

Certain species of Scrophularia (Scrophulariaceae), such as S. nodosa and S. umbrosa, are mainly pollinated by social wasps and are consequently described as wasp-flowers. Because plants attract their pollinators with the help of various floral cues, such as floral odour and/or optical cues, we have investigated the role of olfactory and visual floral signals responsible for wasp attraction in S. umbrosa. Using a combination of chemical (GC, GC-MS) and electrophysiological analyses (GC-EAD), we identified ten compounds in the complex floral odour bouquet that are detectable by the wasps' antennae. As in the wasp-flower Epipactis helleborine, we found so-called 'green leaf volatiles' (GLVs) in the floral odour; these GLVs are highly attractive to the wasps. GLVs, mostly six-carbon aldehydes, alcohols and acetates, and other volatile organic compounds (VOCs), are emitted by many plants infested with herbivores, e.g. caterpillars. In contrast to other investigated wasp-flowers, behavioural experiments have demonstrated that, in addition to the floral odour of S. umbrosa, visual cues are involved in pollinator attraction.