Quantifying electivity in interaction partners to assess the competitiveness of an introduced solitary wool carder bee

Non-native pollinators confound conservationists with their potential to both contribute to an ecosystem service and compete with native pollinators. We first documented Anthidium manicatum (Hymenoptera: Megachilidae), a solitary wool carder bee, at our research site in 2016. Male A. manicatum, sometimes nicknamed “bully bees,” are territorial and engage in competitive aggression towards both intraspecific and interspecific competitors. Our objective was to measure the electivity in interactions of male A. manicatum by quantifying interactions with the relative abundances of different interaction partners. From 2020-2022, we observed floral visitors on ornamental Salvia farinacea, which is grown annually on Gonzaga University’s campus in Spokane, Washington. We observed interactions involving male A. manicatum and conspecific males, conspecific females, and other insects to quantify how frequently they interacted with different partners. To measure the relative abundances of different species, we used video recording, live timed counts, and mark-resight methods. We calculated electivity indices and found that male A. manicatum elected to interact most with conspecific females, followed by conspecific males, and then heterospecific individuals. We documented switches between weeks with abundant A. manicatum followed by weeks with high Apis mellifera. While A. manicatum engages in interspecific aggression, we suggest that its competitive effects are limited by strong intraspecific competition and a low proportion of male-female interactions that lead to copulation. We posit that planting diverse flower species has mitigated the competitive impacts of A. manicatum at our site. We will discuss ongoing work to assemble an interaction web from observations made by the public.