Most research on wild bees has focused on their role as pollinators, while their importance as soil ecosystem engineers has been largely overlooked, despite the fact that most species nest in the soil. There is limited quantitative knowledge regarding the architecture of nests created by wild bees and the temporal evolution of bee burrows. The aim of this study was to evaluate the feasibility of repeatedly scanning ground-nesting bee nests using X-ray computed tomography to quantify the morphology and temporal evolution of burrow systems created by both solitary and social species. We installed eleven large cylinders at locations with ongoing nesting activity of ground-nesting bees, and repeatedly scanned these soil columns with a medical X-ray scanner over a period of 16 months. From the X-ray images, we extracted bee burrows to visualize and quantitatively characterize their morphology and temporal evolution. The architecture and temporal evolution of burrows strongly differed between the studied social and solitary ground-nesting bee species. Burrows created by the solitary species were simple, linear and unbranched burrows, which were not reused and decayed with time. The burrow systems created by the social species were more complex, with highly branched networks of horizontal and vertical burrows, which increased in complexity and size over time during the bee activity period. The persistence of burrows created by ground-nesting bees varied greatly, with some decaying within a few weeks and others remaining mostly intact for the entire 16-month study period. This study demonstrates the potential of X-ray imaging to provide new insights into the underground life of ground-nesting bees, and highlights the locally important role of ground-nesting bees as soil ecosystem engineers.
