Different suites of floral traits are associated with historical selection by particular functional groups of pollinators, but contemporary floral phenotypes are not necessarily good predictors of a plant’s effective pollinators. To determine the extent to which plant species specialize on particular functional groups of pollinators, it is important to quantify visitation rates for the full spectrum of flower visitors as well as to experimentally assess the contributions of each functional group to plant reproduction. We assessed whether attracting both diurnal and nocturnal flower visitors corresponded to pollination generalization or specialization in the Pacific coastal dune endemic Abronia umbellata var. umbellata. In multiple populations over two years, we observed flower visitors during the day and at night to assess visitation rates by different insect groups and conducted pollinator exclusion experiments to assess the contributions of diurnal and nocturnal visitors to seed production. Flower visitation rates were 8.67 times higher during the day than at night, but nocturnal visitation resulted in significantly higher seed set, suggesting that nocturnal noctuid and sphingid moths are the chief pollinators. Most diurnal visitors were honey bees, with tongues too short to reach A. umbellata nectar or contact stigmas and effect pollination. The prevalence of honey bees, combined with the lack of successful seed production resulting from diurnal pollination, suggests that honey bees are pollen thieves that collect pollen but do not deposit it on stigmas. Our results underscore the need to experimentally assess the contributions of different groups of flower visitors to plant reproduction.
