A hierarchical approach for finding undiscovered populations of an endangered bumble bee

Understanding the distributions of rare species is necessary to guide monitoring and inform species recovery efforts. The rusty patched bumble bee (RPBB; Bombus affinis, Cresson) is an endangered species with an extant, known distribution centered around urban areas of the Midwestern United States. We tested a novel approach for finding undocumented RPBBs outside of urban centers and estimated the species occurrence at two scales that are relevant to management. We confirmed presence of RPBBs at 54% of the sampled 100 km² grid cells where the species was previously undocumented, expanding the species’ known distribution by 5700 km². After accounting for imperfect detection, our occupancy model estimated the number of occupied grid cells was 67 of 105 sampled grids, suggesting our methods were effective for finding undiscovered RPBB sites. Occupancy within 100 km² grids was positively related to the number of occupied neighboring units but was not related to the area of developed land within 100km² grid cells or smaller subunits (i.e. 3.14 ha patches or roadside transects). We highlight the utility of our approach for guiding future survey efforts by identifying an additional 145 grid cells where the occupancy status of RPBB is unknown but we predict a relatively high likelihood of RPBB occurrence. Our approach can be extended to find undiscovered RPBB sites in other areas and applied to other bee species where occurrence information is lacking outside of their core distribution.