Notes from rusty patched bumble bee (Bombus affinis Cresson) nest observations

Abstract

Abstract The rusty patched bumble bee, Bombus affinis, is federally endangered in the United States and Canada. Information on natural history is crucial to conservation planning, but little is known about B. affinis nesting biology, habitat, overwintering, and mating behaviours. We report observations on three B. affinis nests in Minnesota and Wisconsin found in 2020. Previous to this report, there were only four published descriptions of B. affinis nests, with the last nest recorded nearly 30 years ago. Here, we share findings on nesting habitat, nest size, mating behaviours, and nest parasites. The nests were located in between the foundation and a layer of insulation in an urban residence, in an urban backyard near a house foundation, and in a degraded semi‐wooded, riparian natural area. Colony size estimates ranged from 251 to 1341 bees. We observed males chasing gynes near a nest entrance and one mating event approximately 5 m from the nest. We documented the chalcid wasp, Melittobia acasta, as a new host association. These observations increased the known colony size range, documented a new host association, and indicate the potential importance of urban and degraded habitats for recovery of the endangered rusty patched bumble bee.