Nesting Habits of Cavity Nesting Bees and Wasps in Portland, Oregon

Native bees are threatened by habitat loss through urbanization, however, there is increasing interest in creating bee nesting habitat in urban areas. Few studies have determined what species are present in the region, or examined the role of nest height or cavity size in attracting the approximate one-third of native bees that nest in cavities. To determine what species were present, and whether they showed preferences for nesting at a certain height or cavity width, we set up artificial wooden cavity nest blocks across fourteen locations in the greater Portland, OR area. Wooden posts were erected with nest blocks at three heights (0.5, 1.5, and 2.3 m), and to accommodate a diversity of bee species, cavity widths of 3.0, 5.0, 6.0, 8.0, and 10.0 mm were provided. The nests were retrieved at the end of the season and the bees and wasps reared in the lab. We found that bees occupied approximately 15% of the total available cavity nests, but when bees and wasps were combined, occupancy rates reached approximately 30%. Sixteen species of bees used the nest blocks, including six nest building Megachilidae genera, two cleptoparasitoid Megachilidae genera, and nest building Colletidae genus. Bees built the greatest number of nests (60%) in 5.0 mm cavity widths. Additionally, 5.0 mm cavity widths accommodated the greatest diversity of bee species (eight nest building species and one cleptoparasitoid species). Nest building wasps were also common in the nest blocks and occupied nearly 17% of the total available cavities. At least twelve species of wasps in four families built nests in cavities. Wasps most often used the 3.0 mm wide cavities (55% of their nests). In addition to the nest building wasp taxa, at least six species of wasp in five families and one fly species parasitized cavity nests. Overall, all nest heights were used by at least some species, although heights at 1.5 m were the least used. Human constructed nest cavities provide an excellent opportunity to learn more about the various hymenopterans that use them. Nest descriptions and photos are included to be used as a pictural guide of cavity nesting species in the Portland area. Novel nest descriptions are provided for Megachile fidelis and a new record of introduced bee species Pseudoanthidium nanum is provided. Additionally, Megachile angelarum was documented as a new host species for the parasitoid bee fly, Anthrax irroratus. Nesting preference data will be used to better inform residents of greater Portland how best to provide nesting habitat for cavity nesting bees, and the solitary wasps that use similar nesting sites. Although this was not a comprehensive study of all of the cavity nesting species in Portland, we now have a much better understanding of the cavity nesting species and their nesting habits to promote conservation efforts