The hermit beetle (Osmoderma eremita sensu lato) is a flagship species for invertebrate conservation in Europe. Protecting this species will also aid in the preservation of other saproxylic communities on veteran trees. However, the European hermit beetle is not a single species but comprises at least four cryptic species, the distribution, morphology, and genetics of which have diverged from each other. In the current thesis, I focus on the hermit beetle in Finland. My work had five objectives: First, to establish the species identity of the Finnish population, which was previously unclear. Second, to retrace the origin and demographic history of this population with the aid of a genetic marker. Third, to establish the current range of the species in Finland. Fourth, to study its habitat requirements within its Finnish range. Fifth, within its habitat, to dissect the species’ larval substrate requirements. Overall, I hoped to derive clear-cut recommendations for the conservation of this threatened species in its northernmost range. 

With respect to its taxonomic affinity, the Finnish population of the hermit beetle turned out to belong to the eastern clade of O. eremita s.l., more specifically to Osmoderma barnabita Motschulsky, 1845 (syn. O. coriarium). In terms of variation within the mitochondrial COI gene, the Finnish population proved to be characterised by very low genetic variation. Overall, the diversity of the COI marker was found to decrease northwards of Central and Eastern Europe, a pattern which may be attributed to post-glacial migration. In Finland, the occurrence of the species was confirmed to be restricted to the Turku region, with Ruissalo Island being the core of its distribution. The eastern clade hermit beetle species has not been recorded in any other part of Fennoscandia. 

Within its Finnish range, the hermit beetle occurred most frequently within large-sized hollow oaks (Quercus robur), of which the species occupied ca 90 %. Nevertheless, the beetle was encountered in hollows of multiple different tree species (Tilia sp., Alnus glutiosa, Acer platanoides, Sorbus aucuparia), and consequently, conservation should not be focused on particular tree species. Neither the specific location of the tree (e.g. open area, dense forest) nor the characteristics of the cavity (e.g. the size of the entrance hole, the volume of wood mould) had any clear-cut effect on the presence of the species. Thus, the species seems fairly liberal in terms of its habitat requirements. 

Within their habitat, the larvae of the hermit beetle develop on a substrate consisting of decaying organic material. Here, a high content of nutrient-rich leaf humus was observed to promote both larval growth and female oviposition, whereas the material of the cavity walls, including brown-rot wood and mycelium of the sulphur polypore (Laetiporus sulphureus), were of inferior quality as regards larval development. A link between female preference and larval performance indicates that female beetles are able to detect the quality of their oviposition site for their larvae. 

Overall, my results suggest that the regional populations should be managed as separate subunits, and that the northernmost, Finnish population of O. barnabita may be marked by low total genetic diversity. The conservation management of this population should be focused on securing a continuous supply of veteran trees, and large-sized broadleaf trees should be retained regardless of the tree species. As hermit beetle presence on host trees is often difficult to establish, more effort should be invested in the strategic, long-term maintenance of a good supply of hollow trees than in the ad hoc rescue of single tree-level occurrences. Furthermore, study of the hollow substrates should continue as the results can be beneficial in improving the future habitat quality of the hermit beetle.