XVI International Botanical Congess

Pollinator effectiveness is seldom quantified for co-occurring plants sharing a common pool of generalist" foragers. Milkweeds are very amenable to such studies since pollen is packaged en masse in paired pollinia. An adjoining corpusculum persists on insects providing evidence of pollen load, pollinium removals and insertions are counted directly in flowers. We measured pollination efficiency and pollinator foraging in sympatric populations and experimental arrays of A. incarnata, A. syriaca, and A. verticillata. In video clips, the most common visitor, Bombus griseocollis, varied in its effectiveness on milkweeds with differing floral morphologies. Pollination efficiencies were highest for large-flowered A. syriaca which had the most specialized pollinator fauna, but fruits of small-flowered asclepiads were more likely to mature after pollination.