At first a rather short, straight gallery, soon widening into an elongated blotch. Unlike the other Cosmopterix species which mine into Phragmites it does not construct a silken tube for shelter and hence, the larva is visible through the mine. Several larvae sometimes mine the same leaf.
See also bladmineerders.be
The mine and caterpillar of Cosmopterix lienigiella differ from those of Cosmopterix scribaiella in the following points: - the lower part of the mine contains a collection of feces and does not end in a narrow passage - the caterpillar has darker segmental incisions which gives it a pattern with transverse bands - since the caterpillar does not produce a pupal web, both the caterpillar and the pupa are sometimes quite clearly visible in transmitted light - pupation occurs at the upper end of the mine pointing towards the tip of the leaf - the pupa lies upside down in the mine

The species hibernates in the larval stage, inside the mine.
Pupation in the mine in springtime.
The adults fly around the food plant at dusk and later come to light.

Monophagous on Phragmites australis.

Most types of habitat where the foodplant grows, with a preference for large reed beds in both fresh and brackish water.