Phengaris Alcon

Phengaris alcon, more commonly known as Alcon Blue or Alcon Large Blue, is a butterfly of the Lycaenidae family and is found in Europe and Northern Asia. It can be seen flying in mid to late summer. Like some other species of Lycaenidae, its larva (caterpillar) stage depends on support by certain ants; it is therefore known as a myrmecophile.

The butterfly lays its eggs onto the Marsh Gentian (Gentiana pneumonanthe); in the region of the Alps they are sometimes also found on the related Willow Gentian (Gentiana asclepiadea). The caterpillars eat no other plants.

Alcon larvae leave the food plant when they have grown sufficiently (4th instar) and wait on the ground below to be discovered by ants. The larvae emit surface chemicals (allomones) that closely match those of ant larvae, causing the ants to carry the Alcon larvae into their nests and place them in their brood chambers, where they are fed by worker ants and where they devour ant larvae.

When the Alcon larva is fully developed it pupates. Once the adult hatches it must run the gauntlet of escaping. The ants recognise the butterfly to be an intruder, but when they go to attack it with their jaws they can't grab anything substantial as the newly emerged adult butterfly is thickly clothed in loosely attached scales.

Over time, some ant colonies that are parasitized in this manner will slightly change their larva chemicals as a defense, leading to an evolutionary "arms race" between the two species.

Generally, Lycaenidae species which have a myrmecophilous relationship with the ant genus Myrmica are locked to primary host specificity. The Alcon Blue is unusual in this regard in that it uses different host species in different locations throughout Europe. It is known to use Myrmica scabrinodis, Myrmica ruginodis, and Myrmica rubra as the primary host within differing European zones.

The Phengaris alcon larvae are sought underground by the Ichneumon eumerus wasp. On detecting a P. alcon larva the wasp enters the nest and sprays a pheromone that causes the ants to attack each other. In the resulting confusion the wasp locates the butterfly larva and injects it with its eggs. On pupation, the wasp eggs hatch and consume the chrysalis from the inside.

Orachrysops niobe, another member of the Lycaenidae from South Africa, has a very similar life-cycle.