Pollination by nectar-foraging thynnine wasps: Evidence of a new specialized pollination system for Australian orchids

Specialized pollination systems can contribute to plant diversification and species co-existence, but they also have important implications for plant conservation due to the potential risks of relying on a small number of pollinator species. Here, we studied pollination of Caladenia colorata (Orchidaceae), a rare species with floral traits intermediate between those that characterize Caladenia spp. pollinated by food foraging insects and those pollinated by sexual deception of thynnine wasps. We aimed to determine which species are floral visitors, how many species are involved in pollination and what is the pollination strategy. Using multiple potted plants as bait flowers, we found that thynnine wasps were the main insect group attracted, but only a single species exhibited the necessary size and behaviour to achieve pollination. Caladenia colorata produced meagre quantities of sucrose on the surface of the labellum on which the wasps appeared to feed. For the first time, we demonstrate pollination by food foraging thynnine wasps in Caladenia, meaning that transitions between this strategy and sexual deception could occur without a shift in pollinator group. Despite relying primarily on a single pollinator species, the pollination rate was comparable with other nectar-producing orchids. However, the specialization in this system means that the distribution of the pollinator needs to be taken into account prior to conservation translocations.