Egg-laying Females of the Alcon blue lay their eggs on the flowers of the Marsh Gentian (Gentiana pneumonanthe).

The Marsh Gentian is now quite rare throughout Europe, and is considered endangered in several countries. It grows on wet heathlands. Large blue butterflies are unusual in that their caterpillars hatch through the base of the egg and burrow straight into the flower head. Inside the flower head the caterpillar will develop through the first three instars in about two weeks. The caterpillar feeds on the flower tissue and developing seeds during this time.
		The picture to the left shows a third-instar caterpillar. The caterpillar grows very little during its time inside the Gentian plant. When it moults for the third time and becomes a fourth-instar caterpillar it is only 3-4 mm long, and weighs only 2-3 mg. At this stage the caterpillar will chew a hole in the plant from the inside and crawl out from the flower head.