1. Results from four field studies show that communities of trap-nesting bees and wasps and their natural enemies are promising bioindicators for ecological change or habitat quality. These small and easy-to-handle communities can be analysed with respect to (i) species richness and related parameters, and (ii) ecological functions or interactions. The communities comprise Hymenoptera (Apidae, Sphecidae, Eumenidae, Pompilidae) and natural enemies belonging to many insect taxa. Traps consisted of 150–200, 15–20-cm long, reed internodes, put into tins or plastic tubes of 13–15 cm diameter; wooden posts with 2–10 of such reed-filled tins were exposed in the target habitat.
2. Species richness and abundance of bees (but not wasps) were closely related to plant species richness of the habitat, a measure of the bees’ food resource. However, availability of nest sites of above-ground nesting species was equally important: meadows with old trees supported greater populations than meadows without trees. A threefold increase in exposed traps resulted in a twofold increase in species.
3. The sensitivity of this bioindicator system profits from the fact that evaluations rely not only on presence/absence data, descriptive population attributes or diversity indices, but also on interactions or ecological functions. Monitoring ecological responses or multitrophic interactions, and their relationship to species diversity, is rarely done but much needed. Ecological functions include (i) the percentage mortality of trap-nesting bees and wasps due to parasitoids and predators, which was correlated with the species richness of these natural enemies; (ii) seed set of allogamous plants due to successful pollination by trap-nesting bees; and (iii) biological control by the predacious wasps.
4. With increasing isolation of fragmented habitats (when traps were exposed in a cleared agricultural landscape), both species richness of natural enemies and percentage mortality (parasitism and predation) declined significantly. In a comparison of habitat types (grasslands and field margins), species richness of the trap-nest community correlated with plant diversity, but percentage mortality, due to parasitism and predation, with field age only. The threshold distance to the nearest habitat was 106–530 m for a 10–50% decrease in mean mortality, and the mortality increased greatly in habitats that were older than 5 years. Accordingly, these studies emphasize the significance of a continuum of old habitat patches for the augmentation of natural enemies.
5. Exposure of standardized traps is an experimental approach with a small, interacting and reproducing community that can be easily characterized by simple parameters. Taxonomy and biology are well known, and quick evaluations can be done using the close correlation between the number of occupied traps and species richness. Species richness of trap-nesting bees and wasps was closely correlated with that sampled by sweep nets. Further criteria of indicator taxa that apply to this system are discussed in the text.