Promiscuous flowers attract high numbers of bees and even higher numbers of non-bee flower visitors

Flowering plants are visited by a variety of flower visiting insects, which range from obligate to opportunistic flower foragers, and efficient to inefficient pollinators. Plants vary in their promiscuity: some are visited by few species of insects while others are visited by many. The promiscuity of flowers in a community is ecologically important because more promiscuous flowers might aid the dispersal of flower-dwelling microbes (e.g. insect pathogens, viruses, and nectar inhabiting microbes). However, we know very little about how overall plant promiscuity (including all flower visitors, not just pollinators) varies for different plant species in a community, and which insect groups dominate on more or less promiscuous plants. To explore the patterns of plant promiscuity in a plant community, and how it differs for pollen-gathering-specialists (non-parasitic bees) and other orders of flower visitors (other Hymenoptera, Coleoptera, Diptera, and Lepidoptera), I analyzed Charles Robertson’s flower visitor dataset, which consists of over 15,000 visits to 441 plant species observed from 1887 to 1916. I found great variation in overall promiscuity across plant species. Considering each insect group separately, there are plant species with more flower visitor species than expected by chance. Further, there tend to be more super-promiscuous plants for other visitors than for non-parasitic bees. Finally, as plant promiscuity increases, the proportion of visitors that are bees decreases; plant promiscuity seems to be driven by other flower visitors. Since other visitors are over-represented on super-promiscuous plants, they should not be ignored in floral ecology, as their effects on the movement of floral microbes could be great.