Why do my leaves have pale winding tunnels in them?

Leaf mines are one of the easiest things in the garden to identify with certainty and one of the hardest to do anything about. The larva is inside a sandwich, protected by the leaf itself, which is why every contact treatment fails and why removal is the whole of the practical answer.

Step by step

1. Hold the leaf up to the light.A mine is translucent because both leaf surfaces are still there. You will usually see a dark line of frass along the tunnel and, at the widest end, a small dark speck — the larva. That confirms the mine is active rather than old.
2. Note the shape of the mine.A serpentine mine is a wandering line that starts as a thread and widens steadily as the larva grows, never crossing itself. A blotch mine is an irregular pale patch, sometimes beginning as a short thread that opens out. Both are leaf miners; the shape identifies the species rather than changing what you do.
3. Recognise the common ones.Holly leaves with pale blotches and a yellow tinge; celery, parsnip and lovage with large brown blotches; leek and onion with white streaks; chrysanthemum and cineraria with wandering lines; beet, chard and spinach with big pale blotches; and horse chestnut trees going brown and papery from midsummer, which is horse chestnut leaf miner.
4. Pick off and bin mined leaves.This is the only reliable control and it works best if done early and repeatedly, because it removes the larvae before they pupate. Bin them or put them in council green waste rather than composting, since some pupate in the fallen leaf.
5. Squash the larva if the leaf is worth keeping.On a plant you cannot afford to defoliate, press the end of the mine firmly between finger and thumb. It is crude and it works.
6. Use fine mesh on the crops where it matters.Leek, celery and beet leaf miner are all laid by adult flies, so insect-proof mesh over the crop from planting excludes them entirely. That is far more effective than anything done afterwards.
7. Clear fallen leaves under affected plants in autumn.Several species overwinter as pupae in the leaf litter. Raking and removing under a badly mined holly or horse chestnut reduces next year's population, though not on a scale that saves a street tree.
8. Decide whether it matters at all.On an ornamental shrub or a mature tree, mines are cosmetic — a horse chestnut browned by leaf miner every August is not being killed by it. They matter on leaf crops, where the mined leaf is the thing you eat, and on young or small plants losing a large share of their leaf area.

Tips

• Mines are one of the few plant problems where the culprit is definitely still there. It is worth the ten seconds with the light behind the leaf.
• Do not confuse a mine with the pale wandering trail left by a slug grazing the surface, which is open and slightly rough rather than translucent.
• Beet leaf miner on chard is worth controlling by picking, because the plant produces new leaves continuously and you are eating them.
• Horse chestnut leaf miner looks devastating and does not kill trees. Bleeding canker on the same trees is the problem that does.

Common mistakes

• Spraying a contact insecticide at leaf mines — The larva is between two intact leaf surfaces. Nothing sprayed on the outside reaches it, and you have treated the plant for no benefit.
• Composting mined leaves in a garden heap — Several species pupate in the leaf and emerge next season. Bin them or use council green waste.
• Removing every mined leaf from a plant that is mostly mined — You defoliate the plant to remove a cosmetic problem. Take the worst, leave the rest, and mesh the crop next year.

Questions people ask