Grand challenges can potentially be solved by changing the institutional context in which they are embedded. However, these social and environmental problems are so large and complex that they call for institutional work across multiple fields. Previous research has tended to focus on institutional change within single fields or between pairs of fields, but has paid only limited attention to the more complex architectures of fields and the ways in which interested actors can engage in institutional work across multiple fields. This study examines the institutional work of Apisuisse, the Swiss umbrella organization of beekeeping associations, and shows that this actor navigated across fields of different levels and types in an attempt to alter the institutional context outside its focal field and tackle the grand challenge of bees. The findings also reveal mediated institutional work as attempts to have an influence on the target field by working in another, intermediate field, which included relying on vertical, i.e., hierarchical, and horizontal relationships between fields, as well as drawing on the composite nature of fields. This study advances research on fields, institutional work, and grand challenges.