Real-time dynamics of aculeate hymenopteran reed gall inquilines in oligotrophic reed beds of anthropogenic and natural origin.

This is the first study providing long-term data on the dynamics of bees and wasps and their parasitoids for the evidence-based management of reed beds. Ten years ago, we identified Lipara (Chloropidae) - induced galls on common reed (Phragmites australis, Poaceae) as a critically important resource for specialized bees and wasps (Hymenoptera: Aculeata). We found that they were surprisingly common in relatively newly formed anthropogenic habitats, which elicited questions about the dynamics of bees and wasps and their parasitoids in newly formed reed beds of anthropogenic origin. Therefore, in the winter and spring of 2022/23, we sampled reed galls from the same set of reed beds of anthropogenic and natural origin as those in 2012/13. At 10 sites, the number of sampled galls was similar in both time periods (80-122% of the value from 2012/13); 12 sites experienced a moderate decline (30-79% of the value from 2012/13), and the number of galls at six sampling sites was only 3-23% of their abundance in 2012/13. Spontaneous development was associated with increasing populations. After 10 years of spontaneous development, the populations of bees and wasps (including their parasitoids) bound to Lipara-induced reed galls increased in abundance and species richness or remained at their previous levels, which was dependent on the sampling site. The only identified threat consisted of reclamation efforts. The effects of habitat age were limited, and the assemblages in habitats of near-natural and anthropogenic origin largely overlapped. However, several species were consistently present at lower abundances in the anthropogenic habitats and vice versa. In conclusion, we provided evidence-based support for the establishment of oligotrophic reed beds of anthropogenic origin as management tools providing sustainable habitats for specialized reed gall-associated aculeate hymenopteran inquilines, including the threatened species.