Figure 1
(a) Standardised, nesting observation block (NOB) made of 25 wooden boards and used in the monitoring module of cavity-nesting wild bees in agricultural landscapes. The cavity diameters are arranged in a fixed ratio: seven boards with holes of 3.2 mm diameters, seven boards with 4.8 mm, five boards with 6.4 mm, five boards with 8 mm and one board with 9.5 mm. The boards are16 × 16 cm in size. To make sure that the brood cells are not disturbed during the data collection, transparent plastic films are ironed onto the boards. An aluminium roof protects against rain. The NOBs are set up at a height of about 1.50 m and are orientated towards the south. (b) Schematic workflow of the tasks and education offers for volunteers throughout a monitoring year.
Figure 2
(a) Red points show all sites of nesting observation blocks of the monitoring testing phase in Germany 2021. Volunteers set up nesting observation blocks (NOBs) and photographed them monthly, the identification was done by experts. Blue points show sites of NOBs both photographed and identified by volunteers. The volunteers participated from different regions of Germany. (b) The photo of a colonised nesting board illustrates exemplarily the task and challenges of volunteers by identifying the cavity-nesting wild bee and wasp species: Although some larvae and cocoons are easily recognisable (triangles), others are more difficult due to infestation of parasitoids (star), not clearly separable brood cells (oval), or undefinable cells like the empty space between nest closure and first brood cell (= atrium) (square).
Figure 3
(a) Rate of successful identification per taxon by volunteers. (b) Rate of successful cell counting per taxon by volunteers. Data is given on average across volunteers.
Figure 4
Rate of successful identification of cavity-nesting taxa in the online quiz over time. Over eight weeks, a new quiz task had to be answered every week. The dots show the success rate of identification per quiz participant. With time, successful identification by volunteers increased on average per weekly task.