Pollination by deceit in Paphiopedilum barbigerum (Orchidaceae): a staminode exploits the innate colour preferences of hoverflies (Syrphidae)

Abstract

Paphiopedilum barbigerum T. Tang et F. T. Wang, a slipper orchid native to southwest China and northern Vietnam, produces deceptive flowers that are self-compatible but incapable of mechanical self-pollination (autogamy). The flowers are visited by females of Allograpta javana and Episyrphus balteatus (Syrphidae) that disperse the orchid's massulate pollen onto the receptive stigmas. Measurements of insect bodies and floral architecture show that the physical dimensions of these two fly species correlate with the relative positions of the receptive stigma and dehiscent anthers of P. barbigerum. These hoverflies land on the slippery centralised wart located on the shiny yellow staminode and then fall backwards through the labellum entrance. They are temporarily trapped in the inflated chamber composed of the interconnected labellum and column. The attractive staminode of P. barbigerum strongly reflects the colour yellow (500-560 nm), a colour preferred innately by most pollen-eating members of the Syrphidae. No scent molecules were detected using GC mass spectrometry analysis, showing that the primary attractant in this system is visual, not olfactory. Pollination-by-deceit in P. barbigerum is contrasted with its congener, P. dianthum, a brood site mimic that is pollinated by ovipositing females of E. balteatus. As the natural rate of fruit set in P. barbigerum (mean 26.3% pooled over three seasons) is lower than that of P. dianthum (mean 58.5% over two seasons), the evolution of false brood sites in some Paphiopedilum spp. should be selectively advantageous as they may provide an increase in the attention and return rates of dependable pollinators to flowers that always lack a reward.