Pest
Leaf Miners
Bladminérflue
How to recognise it
Pale, irregular tunnels or blotch-mines visible through the leaf surface: the larva of the fly Pegomya betae is feeding inside the leaf tissue itself. Affected areas turn brown and papery.
Prevention
Cover young plants with insect mesh from sowing onwards to exclude the egg-laying adult fly. Monitor regularly from May through August, when the fly is most active.
Treatment
No spray can reach larvae inside the leaf. Squeeze visible tunnels between your fingers to crush the larva, or remove and bin (never compost) heavily affected leaves promptly. Healthy, well-fed plants outgrow light infestations.