1a mine begins or ends in the stem, petiole or midrib => 2

1b mine at most on the midrib, otherwise free in the leaf => 8

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

2a larva a maggot => 3

2b larva with chitinised head => 5

3a a corridor that ends upon a thick vein: Phytomyza rufipes

3b a corridor that begins on the midrib: Delia platura & D. florilega

3c narrow corridor, usually upper-surface, may be overrun later, that widens into an irregular blotch over, and partly within, the midrib => 4

4a frass in thread fragments; mine in May-June: Liriomyza xanthocera

4b frass powdery, in blackish-green clouds; mine in May-October: Scaptomyza flava

5a larva with thoracic feet; in autumn: Psylliodes napi

5b larva without thoracic feet; in spring => 6

6a mine begins in a part of the stem that is galled and swollen => 7

6b stem not swollen: Ceutorhynchus pallidactylus

7a swelling strong, containing ≥ 3 larvae: Ceutorhynchus napi

7b swelling weak, usually 1 larva: Ceutorhynchus rapae

8a mine full depth; larva with chitinised head => 9

8b mine upper- or lower-surface (some full depth patches may be present); larva a maggot => 14

9a small blotch, < 1 cm; older larve live free => 10

9b mine > 1 cm, corridor or blotch => 13

10a mine not contracted; older lavae live free at the underside of the leaf: Plutella xylostella

10b mine lightly contracted; older larvae among spun leaves => 11

11a larva: pinacula colourless (the bases of the setae themselves are black) : Cnephasia incertana

11b pinacula black => 12

12a larva: behind/below the anus a chitinous comb: Cnephasia asseclana

12b anal comb absent: Cnephasia stephensiana

13a larva without feet; mine an untidy corridor: Ceutorhynchus minutus

13b larva with thoracic feet; a corridor that widens in the end into a blotch: Phyllotreta nemorum

14a corridor from start to end => 15

14b irregular blotch, preceded by a corridor (that may be overrun) => 4

15a broad, pinnately branched corridor, its main branch overlying the midrib: Liriomyza strigata

15b corridor little branched, not pinnate, not associated with the midrib => 16

16a puparium in the mine in a, usually lower-surface, pupal chamber; frass in discrete grains: Chromatomyia horticola

16b before pupation the mine is vacated through a semicircular exit slit => 17

17a first part of the corridor in dense loops, suggesting a secondary blotch: Aulagromyza mamonowi

17b first part of the corridor more or less straight: Liriomyza brassicae