Use of reed stalk trap nests by insects within the reed beds and in nearby steppic habitats

Trap nests made from stalks or stems of eudominant plants, such as Phragmites australis (reed) and Solidago canadensis (goldenrod), have been frequently used in previous studies. Nevertheless, trap nest occupancy in large stands of source plants remains poorly understood. We deployed trap nests made from stems with and stalks without parenchyma to steppic grasslands and terrestrial reed bed margins (64 localities in Central Europe) and matched the data with those from Moericke traps and phytocenologic relevés. The reed stalk trap nests attracted a much more abundant assemblage of Aculeata than the goldenrod stem trap nests. However, a large part of the species nesting in goldenrod stem trap nests overlapped with the species that used (and preferred) reed stalk trap nests. Ten aculeate species, which were present in large amounts in trap nests, were completely absent from the Moericke traps, and others were present in limited quantities. In natural habitats, trap-nesting bees and wasps had lower dominance but similar Fisher alpha values as those in anthropogenic habitats. Cavities in stems with or stalks without parenchyma served as previously undervalued nesting resources and an important monitoring tool. Trap-nesting species readily adopted broken stems and stalks even within stands of the source plants, the common reed and the goldenrod. Therefore, the conservation of threatened bee and wasp species that are associated with these cavities and are limited to habitats of natural origin requires conservation management that allows the presence of broken stems and stalks of ≥1 year of age in the proximity of the potential feeding habitats. [Display omitted] •Plant stalk cavities are important nesting resources at sites of abundant food resources.•Reed stalk trap nests are adopted even where present naturally in high numbers.•Ten aculeate species were abundant in trap nests but absent from the Moericke traps.•Five aculeate species were present only in traps exposed in natural habitats.•Differences in quality of naturally available cavities need to be addressed.