1a stem borer, rarely penetrating into petiole and midrib, with a isolated excursions into the leaf blade: Psylliodes napi

1b real leafminer => 2

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

2a mine full depth; larva with chitinised head => 3

2b mine upper- or lower-surface (sometimes a few full depth patches); larva a maggot => 7

3a small blotch mine, < 1 cm; larva with toracal, and abdominal feet; elder larvae living free => 4

3b mine > 1cm, may be a corridor; larva remains in the mine till pupation => 6

4a larva: prothoracic plate largely blackisch brown; older larve live among spun leaveas => 5

4b prothoracic plate bone colourd, a few spots; older larvae live free under a leaf: Plutella xylostella

5a larva: pinacula colourless (though the bases of the setae themselves are black): Cnephasia incertana

5b pinacula black: Cnephasia stephensiana

6a larva without feet; mine an untidy corridor: Ceutorhynchus minutus

6b larva with thoracic feet; mine eventually widening into a blotch: Phyllotreta nemorum

7a corridor from start to end => 8

7b irregular blotch => 11

8a pinnately branched broad corridor overlying the midrib: Liriomyza strigata

8b corridor little branched, not associated with the midrib => 9

8a puparium in the mine in a, usually lower-surface, pupal chamber; frass in discrete grains: Chromatomyia horticola

8b larva leaves the mine before pupation, through a semicircular exit slit; frass in thread fragments => 10

10a mine usually forms a secondary blotch: Liriomyza bryoniae

10b usually no secondary blotch: Liriomyza brassicae

11a frass in thread fragments; mine in May-June: Liriomyza xanthocera

11b frass powdery, in blackish-green clouds; mine in May-October: Scaptomyza flava