Floral visitation patterns of two invasive ant species and their effects on other hymenopteran visitors

1. Floral nectar of the native Hawaiian 'ohi'a tree, Metrosideros polymorpha, is an important food source for several native honeycreepers and yellow-faced bees, Hylaeus spp., but is also attractive to invasive ants. 2. I undertook this study to compare floral visitation patterns of two widespread invasive ants, the Argentine ant, Linepithema humile, and the big-headed ant, Pheidole megacephala, and to determine their effects on nectar volume and floral hymenopteran visitors. 3. In the first year of the study, Argentine ants visited inflorescences more frequently than big-headed ants at mid-day and in the afternoon, but did not occur in higher densities than big-headed ants at any time of day. In the following year, Argentine ants visited inflorescences both more frequently and in higher densities than big-headed ants. Argentine ant density had a stronger association with nectar concentration than big-headed ant density. 4. Nectar volume did not differ between ant-excluded and ant-visited inflorescences for either ant species. However, ant density was negatively associated with nectar volume for both species. 5. Hylaeus spp. never visited inflorescences with big-headed ants, while non-native honeybees visited inflorescences with and without ants of either species in equal frequency. 6. Most studies of the effects of invasive ants on native arthropods have focused on interactions on the ground. Flowers should not be overlooked as microhabitats from which native arthropods may be displaced by invasive ants.