Citizen science approaches are promising for raising awareness about the sensitivity of pollinators to environmental changes and simultaneously gathering data about their biology. Questions remain, however, about citizens’ ability to gather accurate data. Here, we present a citizen science monitoring approach of cavity-nesting wild bees and wasps in agricultural landscapes across Germany. By using nesting observation blocks (NOBs), which consist of individual wooden boards screwed together, volunteers record the colonisation and development of cavity-nesting Hymenoptera. To do this, they open the NOBs monthly and photograph each board individually. We tested if volunteers can be trained to accurately identify taxa from photographs by offering identification courses and an online quiz. For that purpose, twelve volunteers without taxonomic knowledge identified and counted larvae and cocoons of wild bees and wasps in 4,203 occupied cavities: 92.4% were correctly identified, 4.8% were incorrectly identified, and 2.8% were unidentifiable by experts. These results indicated that volunteers unfamiliar with these taxonomic groups successfully gained a high level of knowledge within one season supported by identification trainings. Using Wald chi-square tests, successful identification was mainly affected by the variability of the taxon. In view of increasing public demand for habitat restoration to halt and counteract declining pollinator populations, the proposed citizen science monitoring approach offers an opportunity for every interested citizen, regardless of their background knowledge, to engage with wild bees and wasps, and gain knowledge about their ecology.