European regional assessment: Near Threatened (NT)
EU 27 regional assessment: Near Threatened (NT)

In the European region, the Dusky Large Blue occurs in central and eastern Europe. This species is listed as Near Threatened due to a decrease in population estimated to be over 20% but below 30% in the past 10 years. Although the decline measured by the European Butterfly Monitoring Scheme exceeds 50%, this is a non-significant decrease and therefore the Near Threatened category seems more appropriate.

Within the European region, this species occurs in central Europe: from eastern France, through northern Switzerland, southern and central Germany, southern Poland, Czechia, Slovakia, Austria, Hungary, Romania, northern Balkans and eastwards to Belarus, Ukraine and European Russia. Isolated populations occur in northern Spain and western Bulgaria. It is found at altitudes between 100 and 1,200 m.

In the EU 27 region, this species' area of occupancy is 10,676 km² and the extent of occurrence is 1,922,306 km².

The global range of this species extends to the Caucasus, the central and southern Urals and Central Asia. The global distribution area of the species is situated both within and outside Europe.

This is a local species, restricted to (semi-) natural habitats.

Declines in distribution or population have been reported in several countries. It is classed as Critically Endangered in Croatia, Netherlands, Romania; and Endangered in Bulgaria, Slovakia, Switzerland (Maes et al. 2019). It is also classed as Vulnerable in Austria, Belarus, France, Hungary, Poland, Slovenia, Ukraine; and Near Threatened in Czechia, Germany and Spain.

Article 17 reporting for this Habitats Directive butterfly (https://nature-art17.eionet.europa.eu/article17/species/summary/?period=5&group=Arthropods&subject=Phengaris+nausithous&region=) for the period 2013-18 indicates this species has suffered many declines and has an unfavourable conservation status in many countries.

The Dusky Large Blue is one of seventeen species included in the European Grassland Butterfly Indicator but there was insufficient data to calculate a trend for either Europe or the EU27 for the period 1990-2020 (van Swaay et al. 2022). However, during the last 10 years its population has declined by 51% in Europe and the EU 27, although this decrease was not statistically significant (BCE and UKCEH 2024). A population decline of at least 20% is thought to have occurred in the same time frame.

Regional experts report this species is declining in a part of its distribution (e.g. France, Switzerland, lowland Germany, western Poland, Romania, one metapopulation in Slovenia, Bulgaria), but in others it is stable (e.g. German Alps, Czechia, another metapopulation in Slovenia) or even increasing with new colonisations noted (e.g. eastern Poland). Population trends can be quite different between populations (e.g. Switzerland) and some appear to undergo wide fluctuations (e.g. Czechia, eastern Poland) (G. Dusej, Z. Fric, X. Houard, Z. Kolev, M. Musche, L. Rakosy, M. Sielezniew, R. Verovnik pers. comm.).

The Dusky Large Blue occurs on damp, moderately nutrient-rich grassland and rough vegetation. The butterflies are usually found on or near the foodplant Great Burnet (Sanguisorba officinalis). Having lived on the flowerheads of this plant for a few weeks, the small caterpillars go down to the ground, in order to be carried away usually by workers of the ant Myrmica rubra to an ant nest. There, they remain feeding on ant grubs, hibernating and pupating in the early summer. The newly-emerged butterflies leave the nest. The Dusky Large Blue is one of the most specialised of the 'ant blues' being most adapted to one species of host ant. Populations adopted by Myrmica scbrinodis as the main host ant are very rare. It has one generation a year. Habitats include humid grasslands and tall herb communities (36%), water-fringe vegetation (15%), blanket bogs (12%), fens, transition mires and springs (9%), dry siliceous grasslands (6%), mesophile grasslands (6%) (van Swaay et al. 2006).

The Dusky Large Blue is threatened by a lack of suitable grassland habitat management (both agricultural intensification, such as land drainage or overgrazing, and abandonment) in some parts of Europe and is very dependent on traditional land management practices. Habitat loss and changing habitat quality may also lead to increasing habitat fragmentation and isolation of remaining populations and subpopulations. As the species also frequently occurs along road verges, changes in road verge management roads can have a negative impact on this butterfly. In Bulgaria, the butterfly is threatened by urban development.

All butterflies are collected to some extent, but only for the extremely rare species can it be a problem and the trade in Europe is generally at a low level compared to other continents. There is no specific trade information for this species.

The species is listed on the Habitats Directive Annexes II and IV and Bern Convention Annex II. This species occurs in a number of protected areas across its range; and only in protected areas in Romania and Ukraine. It is unknown if all populations are in protected areas in France and Germany. Like other Phengaris species, the ecological demands of this species are relatively well known. Suitable habitats should be protected and appropriately managed to prevent overgrazing and/or scrub and tree invasion by maintaining or restoring traditional low intensity agricultural grazing practices. The effects of conservation actions should be monitored by butterfly monitoring schemes. The Dusky Large Blue was successfully reintroduced to the Netherlands in 1990.