Reproductive success and pollination of the Tuncurry Midge Orchid (Genoplesium littorale) (Orchidaceae) by Chloropid Flies

Abstract

The Midge Orchids (Genoplesium R.Br.) (Orchidaceae) are thought to attract pollinators by nectar reward. All verified records of Genoplesium pollinators are small flies of the families Chloropidae and Milichiidae, suggesting pollinator specificity. We investigated pollination of the Critically Endangered Tuncurry Midge Orchid, Genoplesium littorale D.L.Jones. In common with other Genoplesium species, G. littorale is pollinated exclusively by chloropid flies. Although there is specificity at the pollinator family level, G. littorale is oligophilous, being pollinated by five putative chloropid species in two genera, Conioscinella and Cadrema. Most visitors were female with females greatly predominating among flies bearing pollinaria. Examination of flowers on ten inflorescences showed G. littorale is outcrossing with high pollen vector activity; pollinaria had been removed from 71% of post anthesis flowers. A set of criteria for distinguishing outcrossed, autogamous and apomictic flowers based on observations of pollinaria removal, pollination of stigmas and fruit set on individual flowers ruled out the occurrence of autogamy and apomixis in G. littorale. Fruit set on inflorescences averaged 44% prior to seed dispersal and varied significantly among sub-populations. Nectar is produced in the groove of the labellum callus, although flowers emitted no odour detectable by humans. Detailed examination of 29 flowers revealed no chloropid eggs, indicating the pollination syndrome is not brood site mimicry. The absence of strong dung or carrion-like odours also makes sapromyophily unlikely. The Genoplesium pollination syndrome is nectar reward, but may also represent an example of 'kleptomyiophily', recently described in Aristolochia rotunda. Herbivory reduced reproductive capacity by half overall, varied significantly among sub-populations and may be a significant threatening process for G. littorale. Strategies to reduce herbivory in this critically endangered species should be investigated.