Orchid–pollinator interactions and potential vulnerability to biological invasion

Mutualistic relationships between plants and their pollinators have played a major role in the evolution of biodiversity. While the vulnerability of these relationships to environmental change is a major concern, studies often lack a framework for predicting impacts from emerging threats (e.g. biological invasions). The objective of this study was to determine the reliance of Platanthera ciliaris (orange-fringed orchid) on Papilio palamedes (Palamedes swallowtail butterfly) for pollination and the relative availability of alternative pollinators. Recent declines of P. palamedes larval host plants due to laurel wilt disease (LWD) could endanger P. ciliaris populations that rely heavily on this butterfly for pollination. We monitored pollinator visitation and fruit set and measured nectar spur lengths of P. ciliaris flowers and proboscis lengths of its floral visitors in Jackson County, MS, USA. Papilio palamedes was the primary visitor with minimal visitation by Phoebis sennae (cloudless sulfur butterfly). Lengths of P. ciliaris nectar spurs were similar to proboscis lengths of both pollinator species. Fruit set was moderate with access to pollinators (55 ± 10.8 %), yet failed (0 %) when pollinators were excluded. Visitation increased with inflorescence size, but there was no such pattern in fruit set, indicating that fruit set was not limited by pollinator visitation within the range of visitation rates we observed. Our results are supported by historical data that suggest P. palamedes and P. sennae are important pollinators of P. ciliaris. Although P. sennae may provide supplemental pollination service, this is likely constrained by habitat preferences that do not always overlap with those of P. cilaris. Observed declines of P. palamedes due to LWD could severely limit the reproductive success and persistence of P. ciliaris and similar orchid species populations. This empirical-based prediction is among the first to document exotic forest pests and pathogens as an indirect threat to plant-pollinator interactions.