EN
Nests of red mason bees (Osmia bicornis L.) are inhabited by characteristic accompanying fauna, which to a varying extent restricts the development of the bee population. The aim of this research was to conduct a comparative analysis of nest material from four sites. At three sites artificial nests of common reed and bee cocoons were placed for the first time (M1 – Kłoda, Masovian Voivodeship, M2 – Sąpłaty, Warmian-Masurian Voivodeship, M3 – Kanie, Masovian Voivodeship). At the fourth one (M4 – an experimental apiary of the Warsaw University of Life Sciences) mason bee nests and cocoons had been placed every year for 10 years. The number of species accompanying mason bees, the number of dead larvae, fully formed cocoons and parasites/ cleptoparasites at the newly populated sites was determined and compared with the perennial mason bee nesting site. In nests M1, M2 and M3, 5-7 species of accompanying fauna were found, whereasin M4 as many as 14 species were identified. The parasites that restricted the bee population most significantly at all sites were Cacoxenus indagator, Monodontomerus obscurus and Chaetodactylus osmiae. Not all of these species caused most damage at the perennial bee nesting site. C. indagator occupied most cellsin M4, whereas C. osmiae in M3. M. obscurus parasitized the same number of cells in M3 and M4. Other species identified mainly in M4 were Tribolium castaneum, Dermesteslardarius, Ptinusfur,Trichodes apiarius,Auplopus carbonarius, Chrysisignita,Anthrax anthrax, Graphopsocus cruciatus, Lepsima saccharina, Fornicula auricularia, Bombus terrestris.