The Osmoderma species complex is here treated as five separate species (barnabita, eremita, cristinae, italica and lassallei), following Audisio et al. (2007, 2008). Distribution limits of these different forms remain poorly resolved, but for the purpose of these assessments we follow the approximate distribution limits outlined in Audisio et al. (2007, 2008). There is ongoing debate as to whether or not these forms constitute valid species, but for the purpose of this assessment we are assessing each form separately.

Global and European regional assessment: Near Threatened (NT)
EU27 regional assessment: Near Threatened (NT)

Osmoderma eremitais an obligate saproxylic species of scarab beetles endemic to Europe. The species is widespread from eastern northern Spain towards Germany, northern distribution limits are in southern Sweden. The species is dependent on old trees with specific conditions of hollows and cavities and suitable amount of wood mould. The European and EU27 extent of occurrence (EOO) and area of occupancy (AOO) are large, both exceeding the thresholds for threatened species. The habitat - old, hollow trees - is in continuous decline and the known sites are severely fragmented at national and local level considering the low dispersal abilities of this species. The rate of loss of old and veteran trees has not been quantified, but it is significant and continuous, and it may potentially exceed 10% in the next twelve years (= three generations). Moreover, there is very little regeneration of suitable habitat across the species' range and a distinct age-gap developed between the available old trees and the next generation of potentially suitable trees. Once the existing inhabited old trees have died, there will be no replacements in many areas for several decades. Even if immediate efforts are made now to re-plant appropriate tree species, there may still be a 'gap' during which time there would be very little suitable habitat available. Action is urgently needed to protect and appropriately manage existing veteran trees, as well as to ensure the habitat continuity. Previously, some experts considered that a category of Data Deficient is more appropriate considering the past taxonomic uncertainty and limited amount of distinguishing morphological features. The species is assessed as Near Threatened as the AOO is estimated to be between 2,000 - 2,500 km², the population is considered to be severely fragmented and the area of occupancy, habitat and number of mature individuals is continually declining so it is close to qualifying under B2ab(ii,iii,v).

This central and western European species is found in western Europe, from northern Spain towards eastern Germany and from southern Sweden to northern Italy (Audisio et al. 2007, Bezděk 2016, Maurizi et al. 2017). In Denmark the species is registered from ten localities in 2008, all from southern and eastern part of the country. In France the species is reported from more than 300 localities and it is widespread distributed in the country (Tauzin 2000). In Switzerland, fewer than ten localities are known for Osmoderma eremita (Y. Chittaro, pers. comm.). Only one site is known in Norway in the south of the country. It is widespread in southern Sweden but localised, following the available habitat. The known European EOO and AOO are large, both exceeding the thresholds for threatened species.

The European and EU27 population sizes have not been quantitatively estimated. The population is likely large but severely fragmented on the European or EU27 scale, considering the low dispersal abilities of this species. The population is in strong decline in south-eastern France (B. Dodelin pers. comm. 2024). In Spain the species only occurs in the north in the mountainous areas: there was a single recent breeding site identified in Cantabria (Alexander 2005) and several other are known including a few sites in Navarra (San Martin et al. 2001) and the Basque Country. It is widespread but localized in France (mainly in the south) and Germany (not in the east and south). In Switzerland, the species is in a sharp decline and assessed Critically Endangered (Monnerat et al. 2016). It occurs in northern part of Italy, where it is rare. Despite a high number of recently discovered localities, the population is likely decreasing as the habitat does.

This is an obligate saproxylic species of temperate forests and anthropogenic habitats (Ranius et al. 2005) preferring xerothermic habitats. Larvae and adults are bound to old, hollow trees and develop in wood mould accumulated in cavities and hollows. Tree species is not that important as are the conditions of mould and cavity (red or white decay). It has been reported from various deciduous tree species. Oak ( Quercus spp.) appears the most important tree species for Osmoderma eremita, followed by lime (Tilia spp.), beech (Fagus sylvatica), chestnut in Switzerland (Chittaro and Morin 2013), and other trees. Mulberry trees (Morus spp.), plane (Platanus spp.), walnut tree (Juglans regia) and hornbeam (Carpinus betulus) are other species Hermit Beetle has been documented to develop in.

Suitable trees occur in a wide variety of situations - old wood pastures, historic parklands, hedgerow trees, old avenues, orchards, etc., all places where trees have been retained into maturity and old age, for a variety of reasons (K. Alexander pers. comm. 2009). The conditions of inhabited trees vary across the European range, with open-grown trees important in the cooler and damper west, but with shading more important under more continental conditions.

The development cycle takes two to four years depending on the latitude and local conditions. This species has poor dispersal capacity (usually not leaving host tree; if leaving, in most cases not travelling further than 250-300 m from the host tree), and consequently linear habitat features (e.g. avenues of ancient trees) are critically important for retaining connectivity and hold viable populations. The habitat is in continuous decline and severely fragmented in most parts of the species' range.

Osmoderma eremita is dependent on availability of suitable old trees with cavities and hollows, with a suitable amount and condition of wood mould under specific microclimatic conditions. Availability of this habitat is naturally restricted and there is a distinct age-gap developed between the present generation of old, suitable trees and the new generation of potential trees. Old and veteran trees are considered an irreplaceable natural resource since many of these trees are a hundred years old or older. The species clearly prefers trees in open conditions and, therefore, suffer from abandoning common traditional practices such as grazing by large herbivores. Fragmentation and increasing isolation of beetle populations are likely another key factors considering the low dispersal abilities of Osmoderma eremita. Encroachment of open forest and park habitats by development is also a severe threat to this species (P. Thomsen pers. comm. 2009).

It is used for scientific research and monitoring. It also appears in commercial trade for collecting purposes.

It is included in the Annexes II and IV of the European Habitats Directive (Trizzino et al. 2013) and Annex II of the Bern Convention, under the Osmoderma eremita complex. Numerous known sites are within nature conservation areas (e.g., in France, Germany, Sweden). It has been assessed as Critically Endangered in Switzerland (Monnerat et al. 2016) and Vulnerable in Italy (Audisio et al. 2014) and Sweden (SLU ArtDatabanken 2023).