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

Alcon blue

Phengaris alcon

Alcon blue is rare in Denmark. Wing span: 28-38 mm. Short flight period, from early July to early August. In scattered, very local and often solitary colonies in bog mires, damp heaths, and similar locations with numerous bell campanula and large populations of host ant.

Status in this countryRare in DenmarkEN (Endangered)
Alcon blue

Flight period

Short flight duration, from the beginning of July to the beginning of August.

Habitat

In scattered, very local, and often individually sized colonies found in heath moors, damp heaths, and similar locations with abundant Gentianella and large populations of the host ant.

Identifying features

Wingspan: 28-38 mm. The male's upperside is shining violet-blue with a narrow, black border.

Similar species

The male can primarily be confused with worn males of Amanda’s blue. However, the orange markings on Amanda’s blue are usually sufficient for reliable differentiation.

Distribution

Alcon blue has disappeared from approximately 1/7 of its known locations over the past 40-50 years. Currently, it can be found on Rømø, Fanø, and Læsø, in some areas along the Jutland west coast, and in Thy, in the plantations near Frøslev Mose, close to the Danish-German border, and finally in Grene Sande near Billund, which is one of the last 3-4 Danish populations.

Biology

The larvae of the alcon blue feed in the seed heads of marsh gentian, possibly also other gentians. After a few moults, a larva drops to the ground, where it is adopted by one of two Myrmica ant species (Myrmica rubra and Myrmica ruginodis). It then lives in the ants’ nest and is fed by the ants in the same way as their own larvae.