1a fleck mine: Apterona helicoidella

1b corridor or blotch => 2

1c galls, etc => Tables for all parasites per species

2a blotch, extending over several leaf segments; at the start of the mine, at the underside, an elliptic egg shell: Chirosia histricina

2b mine usually limited to one leaf segment; no egg shell visible => 3

3a frass in two rows of pearl chains (occurrence on this host very doubtful): Phytoliriomyza hilarella

3b frass in one, almost uninterrepted thread along the sides of the corridor: Chromatomyia scolopendri