Species of the week #113- Alcon Blue

For a long time, Alcon Blues have survived through ingenuity, but now they are threatened with extinction. The caterpillars of all Phengaris Butterflies resemble the larvae of the knotted ant in smell, voice and even skin surface. These carry the caterpillars into their nests and feed them, often even more intensively than their own larvae. Thus the caterpillars are well protected and receive first-class nourishment. Only when the butterfly hatches does the masquerade come undone and the butterfly has to leave the ants’ nest as quickly as possible.

Distribution Status in Rhineland-Palatinate extinct
Remaining deposits Alpine foothills, L√ľneburg Heath
Last sighting in Rhineland-Palatinate 1976 in Kindsbach
Habitat Damp meadows, heaths and spring bogs


Threat Drainage of the habitat, loss of the forage plant or host ant, mowing too early, intensive tillage and/or compaction of the soil.

In contrast to the better-known Dusky Large Blue, the Alcon Blue specialises in the beautiful marsh gentian. This grows mainly in wet places, which is why the Alcon Blue is also called the marsh blue in german.

The females lay several eggs loosely distributed on the buds of the marsh gentian, rarely also on leaves and stems. Occasionally, the swallow-wort gentian also serves as a host plant. After hatching, the caterpillars feed inside the plant and feed on the seeds or their appendages in the ovary. After two to three moults, they eat their way out and drop to the ground. Here they emit scents and are then picked up by two different species of node ants (Myrmica ruginodis and Myrmica rubra), carried into the nest and cared for until pupation.

At EU level, the Alcon Blue is on the forewarned list of species threatened with extinction. In Germany, the species is critically endangered and already extinct in Rhineland-Palatinate. The butterfly was last sighted in our area in the Landstuhler Bruch, a drained bog in the south-west Palatinate. Fortunately, the two species of knotted ants are still not endangered, but unfortunately the populations of the marsh gentian in Germany are in sharp decline. This plant is also classified in the Red List category of critically endangered. The main reason for the declining population is the destruction of its habitat. If the formerly extensively managed litter meadows are abandoned, drained, afforested or fertilised in the course of more intensive cultivation, then the gentians and the host ants, both of which are vital for the butterfly, inevitably disappear as well.

Politically necessary:

  • Rewetting of peatlands
  • Preservation of existing wetlands
  • Promotion of extensive management
  • Organisation of adapted mowing management

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Photo : By Svdmolen – Own work, CC BY-SA 3.0