Four parameters were used to determine pollinator effectiveness of floral visitors to two sympatric species of Symphyotrichum (S. lanceolatum and S. lateriflorum): (1) pollinator efficiency; (2) relative abundance of visitors; (3) amount of pollen carried on a visitor's body; and (4) visitor foraging time per capitulum. The most effective pollinators of S. lanceolatum were Apis mellifera (Apidae), Eristalis spp. (Syrphidae), Dolichovespula arenaria, and Polistes fuscatus (both Vespidae). Halictidae (sweat bees) and Tachinidae (Diptera) were also potentially effective pollinators. For S. lateriflorum, effective pollinators included D. arenaria, Vespula maculifrons (Vespidae), Lasioglossum spp. (Halictidae), and A. mellifera, with large syrphid flies also having moderate importance. The Halictidae was the only group of visitors that demonstrated a significant difference in the amount of time spent foraging the floral heads of these two aster species. Despite spending more time foraging the floral heads of S. lateriflorum, halictid bees carried the same average pollen load regardless of the species of Symphyotrichum from which they were collected. Halictid bees, however, are potentially more effective pollinators of S. lateriflorum than of S. lanceolatum.
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Vol. 113 • No. 953