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, following Bezděk (2016).

Global and European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

Osmoderma lassallei is an obligate saproxylic species that is found in Greece and European Türkiye. The lifecycle of this species is bound with old-growth deciduous trees of southern European temperate forests. Larvae develop in wood mould in hollow trunks of live old trees, and the development cycle takes two years or longer. The known extent of occurrence (EOO) is small and declining, and likely not exceeding the threshold of 10,000 km² and therefore meets the threshold for Vulnerable. The known EU27 EOO is unlikely to exceed 4,000 km² and meets the threshold for Endangered. Not enough information is available to calculate the European and EU27 AOO but it is very unlikely to exceed the threshold of 500 km² for each area. Based on current knowledge, the number of locations is 10 in Europe and eight in the EU 27 region. The population is unlikely to exceed 10,000 mature individuals. The population is likely fragmented, but this requires further confirmation and cannot be certainly stated. The main threat considered decline in the extent and quality of the habitat - old, hollow deciduous trees - and progressing age gap between the present old generation and the younger generation of potentially suitable trees. The rate of loss of veteran trees has not been quantified, but it is significant, and it may potentially exceed 20% in the next ten years (= five generations). Moreover, there is very little regeneration of suitable habitat across the species' range. Once the existing veteran trees have died, there will be no replacements in many areas. Even if 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. However, considering the lack of data the effect of the main threat on the population of O. lassallei remains unknown. The species is therefore precautionarily considered Vulnerable for both Europe and the EU27 Member States. This may change in the future if more locations are identified.

Some experts considered that a category of Data Deficient is more appropriate considering the taxonomic uncertainty and lack of distinguishing morphological features. Action is likely required to protect and appropriately manage existing veteran trees, as well as to ensure habitat continuity for the future.

The distribution of this species is restricted to north-eastern Greece and the European part of Türkiye (Ranius et al. 2005, Audisio et al. 2007, Bezděk 2016). There are unconfirmed records available from Albania and North Macedonia (Kovács and Mesaroš 2020). The number of known locations considered for this assessment is 10 (eight in the EU27 region), but considering the potential sites it is likely that slightly over 10 locations may be identified in a future assessment.

The European EOO is about 10,000 km² and the known EU27 EOO is about 4,000 km². Not enough information is available to calculate the European and EU27 AOO but it is very unlikely to exceed the threshold of 500 km² for each area.

No information is available on the population size and trend for this species. The population is suspected small and decreasing considering the general trend of the habitat. The number of mature individuals is estimated to be 900 – 1,000 individuals taking into account the average number of individuals per tree, and the maximum estimate of the potential global population size is 10,000. The fragmentation status of the population is unknown.

This is an obligate saproxylic species of southern temperate forests. Larvae develop in wood mould in cavities and hollows at base of trunks of old, live deciduous trees containing large volumes of wood mould derived from decaying heartwood. Suitable trees may occur in a wide variety of situations. The development cycle lasts two years or longer under suboptimal conditions. The habitat - old, hollow deciduous trees - is in continuous decline across Europe.

The main threat for this species, bound to old, hollow trees, is considered the continuous decline in the extent and quality of the habitat and the progressing age gap between the present generation of old deciduous trees and the next one, which is not developed and not mature. The effect of the main threat on the European population of Osmoderma lassallei is unknown due to the lack of data.

It is used for scientific research.

The species 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. It is unknown if the species occurs in any nature conservation area.

Actions are likely needed to protect and appropriately manage existing old deciduous trees with cavities and hollows, as well as to ensure habitat continuity for the future.