Butterfly ID
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Field guide · Denmark
Blues, coppers and hairstreaks · Denmark

Large blue

Phengaris arion

Large blue is rare in Denmark. Wingspan: 32-45 mm. From mid-June to early August. Warm, flowering habitats grasslands, often along the coast, with abundant thyme and possibly marjoram, as well as mounds of the host ant.

Status in this countryRare in DenmarkCR (Critically endangered)
Large blue

Flight period

Mid-June to early August.

Habitat

Warm, flower-rich habitats grasslands, often along the coast, with abundant thyme and possibly marjoram, as well as patches of host ant colonies.

Identifying features

Wingspan: 32-45 mm. Both sexes have a greyish-blue upperside with characteristic black spots on the forewing.

Similar species

It can be confused with females of Alcon blue, and specimens without spots may potentially be classified with Amanda’s blue, but usually, the size alone, together with the large black spots, is sufficient for a reliable species distinction. A certain characteristic of Denmark is that the species only occurs at one location, Høvblege on Møn, where Alcon blue is not found and Amanda’s blue is rarely seen.

Distribution

Large blue is a species that – from thriving in many Danish locations during the mid-1900s – has been very close to extinction in Denmark. The last Danish location is Høvblege on Møn, where the population is stable, but very vulnerable.

Biology

The larvae feed on thyme, mainly broad-leaved thyme but formerly also narrow-leaved thyme, as well as marjoram. After several moults, a larva drops to the ground. If all goes well, it is adopted by an ant of the species Myrmica sabuleti and carried into the nest.