Osmoderma

Abstract: Nutrient richness of wood mould in tree hollows with the Scarabaeid beetle Osmoderma eremita.— Trunk hollows with wood mould harbour a rich invertebrate fauna with many threatened species, and it has been suggested that the beetle Osmoderma eremita (Coleoptera, Scarabaeidae) is a keystone species in this community. We estimated the amount of nitrogen and phosphorus in wood mould and compared the coarse fraction which constitutes frass of O. eremita with the finer fraction of wood mould, and found that the nutrient richness was higher in frass. O. eremita larvae have a fermentation chamber that harbours nitrogen fixing bacteria. As the levels of absorbable nitrogen are a limiting factor in insect growth, an increase in nutrient richness is one of several possible explanations why the species richness of saproxylic beetles is higher in hollow oaks where O. eremita is present in relation to similar trees where the beetle is absent.

Abstract: Beetles living in tree hollows can feed on a wealth of substrates-e.g. the rotten wood surrounding the cavity, leaf humus falling into the hole, and larval frass accumulating in the cavity. In this paper, we examine the role of these main substrates in Finnish tree hollows in the female preference and larval growth of the hermit beetle Osmoderma barnabita. We rear larvae on diets consisting of wood material (as affected by brown-rot), leaf humus, and larval frass, in varying proportions. To pinpoint the effects of microbes, we contrast larval growth on sterilized versus unsterilized larval frass, and on pure mycelia of the cavity-creating fungus Laetiporus sulphureus. Finally, to relate larval performance to female preference, we examine female choice among the three main substrates used in the larval rearings. We found that the presence of one substrate modifies the influence of another, with larval growth and survival being highest on pure leaf humus. Microbes came with both positive and negative impacts on larval performance, as larvae grew quicker on unsterilized than on sterilized larval frass, but were also struck by higher mortality. On pure fungal mycelia, larvae neither grew nor survived. Female preference reflected larval performance, with leaf humus being preferred over other resources. Overall, our study suggests that organisms inhabiting tree holes may be dependent on subsidies entering the cavity from outside, and that ovipositing females may specifically respond to the presence of such subsidies. Thus, the quality of a microhabitat may depend on what enters it from outside.

Abstract: The hermit beetle Osmoderma eremita (Coleoptera: Scarabaeidae) is a flagship species for invertebrate conservation efforts by the European Union. This taxon has recently been revealed as a species complex likely encompassing five cryptic species. The northernmost population of Osmoderma is found on the island of Ruissalo in Turku, Finland. This population has been protected as species O. eremita, but its true species affinity has never been established. To resolve its identity,we sequenced themitochondrial COI gene from seven specimens sampled in Ruissalo. Based on a phylogenetic hypothesis generated from the sequences combined with previously published data, the Finnish hermit beetle was identified as Osmoderma barnabita. Information regarding the ecology and life cycle of O. eremita should then not uncritically be assumed to apply to the Finnish population. Rather, the Finnish population should be treated as a separate entity in conservation and management of European Osmoderma.