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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 specialized of the “ant blues” being most adapted to one species of host ant. Populations using Myrmica scabrinodis as the main host ant are extremely rare and probably confined to the edge of the range or to east Europe. The Romanian populations belong to the subspecies kijevensis and prefer Myrmica scabrinodis as host ant. The Dusky Large Blue has one generation a year. Habitats: 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%). In Germany, the Rhone valley and parts of Poland, it supports a beautiful, highly specialized ichneumonid parasitoid Neotypus ‘pusillus’, which possibly consists of two cryptic species centred on Germany-Poland and the Rhone valley, each host-specific to Phengaris nausithous.