Common reed (Phragmites australis) gall as the limiting nesting resource of rare wetland bees and wasps (Hymenoptera: Aculeata & Evanioidea) in Central Europe

Abstract Common reed ( Phragmites australis ) is often subject to eradication by conservation management and is also commercially harvested. However, numerous Palaearctic cavity-nesting aculeate Hymenoptera use old reed galls induced by Lipara flies (destroyed by such human activities) as an important nesting resource. We performed a manipulative field experiment involving the exposure of trap nests made of Lipara -induced galls, Moericke traps and phytocenologic examinations at 66 paired sampling sites in the Czech Republic, where common reed is considered an indigenous plant. Lipara -induced reed galls comprised a limiting nesting resource for obligate reed specialists within both mowed and unmowed reed stands. Reed mowing prescribed for nature conservation purposes affected species richness and the abundance of aculeate hymenopterans. The sites with mowed reed stands attracted more species to the trap nests, and the sites were also associated with a higher abundance of aculeates in the trap nests and Moericke traps. However, when the trap nests were exposed at sites distant from any reed stands, the species richness and abundance of aculeates nesting in the trap nests dropped dramatically. The study species did not form any micro-territories when occupying the trap nests containing multiple galls suitable for nesting. The trap nests did not equally support the nesting of all species known to use Lipara -induced reed galls, and the analyzed environmental factors displayed species-specific responses. It remains to be investigated, whether and which species that are known to nest in Lipara -induced reed galls can use also other alternative nest resources, such as reed stems and stems of other wetland-associated plants.