Pollination by sexual deception of fungus gnats (Keroplatidae and Mycetophilidae) in two clades of Pterostylis (Orchidaceae)

Pterostylis is a diverse genus of terrestrial orchids with dull-coloured trap flowers, of which pollination has largely remained a mystery. Recently, one species has been confirmed to achieve pollination by the sexual deception of male fungus gnats. Here we assess if this pollination mechanism is used more widely in the genus. We 'baited' for pollinators using five representative Pterostylis spp. spanning the three main clades (A-C) of the genus: (A) P. concinna; (B) P. nutans and P. curta and (C) P. basaltica and P. boormanii. For each baiting trial the number of pollinators approaching, landing, attempting copulation and removing or depositing pollen was recorded. To determine experimentally if long-range attraction was via olfactory cues only, the number of pollinator responses were compared between visible (control) and concealed bait flowers (treatment). Four of the five Pterostylis spp., representing clades B and C, each attracted the males of a single species of fungus gnat of one of two families (Diptera: Keroplatidae or Mycetophilidae). At the flower, these males exhibited pre-mating behaviour, attempted copulation and successful pollination following entrapment. Experiments revealed strong attraction to concealed flowers. Although fungus gnats were unevenly distributed in space, when present, species from both families of gnats responded to bait flowers within four minutes. However, pollinator behaviour differed between Pterostylis clades in the part of the floral display they landed on, and type of sexual behaviour, irrespective of the family of pollinator. Our study confirms that sexual deception operates in two of the three major clades of Pterostylis and provides the first known examples of plant pollination involving keroplatid fungus gnats. We also show that pollinator baiting is an effective but under-utilized tool for understanding the pollination mechanisms of plants that use floral odours as a cue.