Osmia species (Megachilidae) pollinate Cypripedium parviflorum (Orchidaceae) and Packera paupercula (Asteraceae): a localized case of Batesian mimicry?

Abstract

The pollinator-non-rewarding Large Yellow Lady's-slipper (Cypripedium parviflorum var. pubescens [Willdenow] Knight) and the rewarding Balsam Ragwort (Packera paupercula [Michaux] Á. Löve & D. Löve) exist together in some eastern Ontario alvars where they share a group of medium-sized, dark blue metallic pollinators in the bee genus Osmia (Mason Bees, Megachilidae). I provide evidence of floral mimicry of the ragwort by the orchid based on several observations: (1) Osmia visiting P. paupercula also visit C. parviflorum var. pubescens; (2) Osmia are more frequent visitors to P. paupercula than to other co-blooming plants; (3) Osmia are the primary pollinators of C. parviflorum var. pubescens; (4) the behaviour of Osmia on the flower of C. parviflorum var. pubescens involving landing on the staminode suggests mimicry; and (5) the largest populations of C. parviflorum var. pubescens in Ontario are in alvar landscapes where P. paupercula is abundant. Pollination in Large Yellow Lady's-slipper may vary geographically from non-model to species-specific Batesian mimicry, which is rare in orchids. This latter pollination mechanism may have evolved in ice-front environments during the Pleistocene, but then became isolated to unusual partial analogues of ice-front habitat during the Holocene with pollination in most of the current range appearing to be a generalist strategy.