Opportunistic habitat use by Osmoderma barnabita (Coleoptera: Scarabaeidae), a saproxylic beetle dependent on tree cavities

1. Genus Osmoderma, a flagship taxon for invertebrate conservation in Europe, consists of beetles specialised in tree hollows. 2. We established the range and habitat requirements of O. barnabita in Finland. Taking a hierarchical approach, we used volunteer entomologists to map the national distribution of the species. Within the range defined, we established its habitat requirements at a hierarchy of scales, from the local microhabitat (i. e. tree species, amount and type of wood mould), to the host tree (its size and status as being dead or alive), and the insolation properties and accessibility of the site. 3. In Finland, O. barnabita was found to occur within a single area, focused on the country's largest oak-containing woodland. Within this restricted range, the pedunculate oak (Quercus robur) was proved to be the most preferred host, with additional occurrences on trees such as lime (Tilia) and alder (Alnus). Large tree individuals were more frequently occupied than the smaller ones, with an incidence of 90% on the largest oaks. Against this background, factors describing other aspects of the environment added no independent power to explain the incidence. 4. Based on the current results, O. barnabita requires a large network of tree hollows. Within such a network, it will occur with a high probability in almost any large, hollow tree, regardless of the finer characteristics of the tree or its cavities. Thus, the conservation management of O. barnabita should be focused on securing a good supply of suitable veteran trees rather than managing individual trees with established occurrences of the species.