Despite their ecological importance for pollination of wild plants and crops and for natural pest control, long-term data on cavity-nesting wild bee and wasp populations remain limited. Nesting observation blocks (NOBs) are wooden boards with cavities of various sizes that serve as nesting sites. They represent a widely used method to study diversity, abundance and phenology of overground cavity-nesting wild bees and wasps as well as their antagonists. Thus, they serve as a suitable method to be used in citizen science monitoring programs. Due to the philopatric behaviour of many wild bee species, the females prefer to build their nests in the same cavity from which they emerged. This could cause an artificial increase in brood cell numbers within a NOB over time. To prevent the formation of artificial populations at monitoring sites, NOBs must be unavailable for recolonization after the emergence of overwintering bees. This also enables non-invasive sampling of residual materials such as faeces and pollen. These materials can provide high-resolution taxonomic data and insights into plant resource use through techniques like eDNA metabarcoding. In most ecological studies, NOBs are emptied using lethal methods such as freezing, which is unsuitable for citizen science-based monitoring programs. We therefore developed a non- lethal approach to prevent recolonization while meeting key criteria: (1) maintaining a stable microclimate within the cavities, (2) accommodating all cavity-nesting insects, and (3) ensuring low cost and maintenance for scalability. To meet these needs, we designed a fabric attachment for NOBs that is both water-repellent and breathable. Light is allowed to enter only through a single opening to leverage bees’ natural phototactic behaviour. Over a three-year period, we tested and refined various exit designs. Here, we present our prototypes, share practical challenges, and highlight key findings from the development process.
