Hawaii has a single group of native bees belonging to the genus Hylaeus (Hymenoptera: Colletidae) and known collectively as Hawaiian yellow-faced bees. The majority of the 63 species have experienced significant declines in range and population. In 2016, seven species received federal protection under the Endangered Species Act of 1973. Competitors and predators, such as invasive bees, wasps and ants, are thought to be important drivers of range reductions and population declines, especially at lower elevations where more non-native species occur. We evaluated the effects of invasive ants on nesting Hylaeus anthracinus using artificial nest blocks that allowed us to track nest construction and development. The blocks were placed in pairs at 22 points encompassing three sites on the north and east sides of Oahu. One block in each pair was treated with a sticky barrier to prevent access by ants, while the other block remained untreated. From December 2015 to December 2016, we monitored 961 individual nests in the blocks. Seventy percent of nests in control blocks were invaded by ants. Nests in treated blocks were more likely to produce at least one adult than nests in untreated blocks (38% vs. 14%, respectively). In untreated blocks, ants were the most common cause of nest mortality followed by lack of development, displacement (primarily by the competitor Pachodynerus nasidens) and presumed pathogens. The invasive ant, Ochetellus glaber was the only observed nest predator, although the big-headed ant, Pheidole megacephala was also present. Hylaeus anthracinus inhabits coastal strand habitat which occurs in a narrow band just above the high tide line. Nests at one site were destroyed due to a high wave event, highlighting this species’ vulnerability to sea level rise. Additionally, no adult bees or nests were observed at the points where yellow crazy ants, Anoplolepis gracilipes were established. An increased understanding of the factors limiting Hawaii’s yellow-faced bees will provide information for future conservation efforts that may include landscape-scale ant control, habitat restoration and translocations.
