Dusky Large Blue
Phengaris nausithous
The Dusky Large Blue is a butterfly in the family Lycaenidae known for its extraordinary life cycle. The species is obligately dependent on the Great Burnet plant for egg-laying and the ant Myrmica rubra as a host. Adults have dark brown upper wings, while the underside displays a characteristic row of dark spots on a light brown background. It primarily inhabits damp, extensively managed meadows and is protected across Europe.
Details
Identification
Upper wing surface dark brown, underside uniform light brown with a row of black, white-ringed eyespots; no orange spots present.
Social behavior
Solitary as adults; however, the larvae live as social parasites within ant colonies.
Diet
Young caterpillars feed on burnet plant tissue; later stages prey on ant larvae. Adults feed on nectar.
Hunting strategy
Chemical mimicry: The caterpillar mimics the scent of ant larvae to be fed in the nest or to prey upon them.
Spawning substrate
Flower heads of the Great Burnet (Sanguisorba officinalis).
Overwintering
Overwintering as a larva (caterpillar) inside the ant nest.
Ecology
Ecological role
Specialized parasitoid of Myrmica ants and an important pollinator of its host plant.
Natural predators
Insectivorous birds, spiders, and parasitoid wasps.
Competitor species
Phengaris teleius (Scarce Large Blue) often utilizes the same resources.
Ecosystem service
Pollination of Sanguisorba officinalis and contribution to the biodiversity of wet meadows.
Threats
Loss of wet meadows due to drainage, intensive agriculture, and inappropriate mowing schedules.
Scientific profile
Profile
Distinguishing features
Absence of orange submarginal spots on the underside; dark, nearly uniform upper side; strict dependency on Great Burnet (Sanguisorba officinalis) as the sole host plant for egg-laying; underside with only a single row of ocelli.
Habitat
Wet meadows, purple moor-grass meadows, fens, and ditch edges with high populations of Great Burnet. Requires the simultaneous presence of the host ant Myrmica rubra.
Protection & threats
Main threats
Agricultural intensification (early mowing before September, fertilization), drainage of wetlands, habitat fragmentation, and scrub encroachment after abandonment of land use.
Population trend
Decreasing in large parts of Central Europe; classified as vulnerable in Germany (Red List 3).
Conservation measures
Implementation of adapted mowing regimes (mowing only from September onwards or rotational mowing), rewetting of sites, protection of host ant habitats, and connectivity of isolated populations.