Plant-pollinator interactions constitute one of the most widely recognized mutualisms, yet most investigations focus on single species or specialized pollinators. We used multiple measures to evaluate the efficiency and effectiveness of a diverse assemblage of pollinators on three co-flowering species: Asclepias syriaca L., A. incarnata L., and A. verticillata L. Hymenopterans exceeded other insects in their prevalence as major pollinators, but most did not vary significantly among plant species in the number of flowers visited or in visit duration per plant, except for native Bombus griseocollis. Significant variation in visit duration was also uncommon among insects foraging on a single taxon, except for Apis mellifera, which foraged longer on plants of A. syriaca than native hymenopterans and lepidopterans. Among the hymenoptera, bumblebees, carpenter bees, and wasps visited more plants per foraging bout, respectively, on A. syriaca, A. incarnata, and A. verticillata. Insect fidelity also varied seasonally and among plant species. Thus, honeybees, lepidopterans, and sphecid wasps foraged with greater relative constancy on A. syriaca, A. incarnata, and A. verticillata, respectively. All bees and wasps carried more pollinaria of A. incarnata and A. verticillata than of the larger-flowered A. syriaca, but most insects had higher inferred pollinium transfer rates on A. syriaca than on its congeners, especially sphecid wasps. Overall, the actions of pollinators were individualistic and indices of pollinator effectiveness based on pre-contact foraging versus vector pollinium loads were not strongly congruent. Our results highlight the need for new and creative approaches to studying the role of generalized pollination systems in the origin and maintenance of sympatrically flowering species.
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Vol. 94 • No. 2