Pollinator‐mediated isolation in sympatric milkweeds ( Asclepias ): do floral morphology and insect behavior influence species boundaries?

Summary• We explored whether mechanical or ethological differences provide pollinator‐mediated floral isolation capable of reinforcing existing species barriers among sympatric Asclepias with divergent floral morphologies: A. incarnata , A. verticiallata and A. syriaca . • In a common garden, we quantified pollinator visitation and flight patterns, differences in corporal attachment of pollinia to insects, and the potential outcome of putative floral barriers for interspecific pollination and fruit set. • We detected significant variation in the importance, constancy, and behavior of major pollinators on sympatric asclepiads, including Bombus , Xylocopa and large sphecid wasps. Pollinia attach differentially to the arolium on insect legs for A. syriaca , but to the tarsal hairs in other asclepiads. Fruit‐set was lower in mixed than unispecific patches of Asclepias . • We detected mechanical isolation between A. syriaca and its congeners and a tendency toward wasp pollination in A. verticillata . All three species appear to show some specialization for long‐tongued hymenoptera and lepidopterans. Pre–mating barriers provide a potentially effective means of reducing interspecific pollination, but more study is needed in species visited by generalists.