Gadenin日本語Get the appHow to recognize it
Citrus leafminer attacks only soft, expanding new leaves. The adult is a tiny silvery moth, less than a quarter inch (5 mm) across, active at dusk, and you will almost never see it. What you see is the damage: a silvery or pale trail winding across the leaf just under the surface, widening as it goes, often with a fine dark line of frass down its middle.
The leaf then curls tightly, usually rolling the edge over the mine, and the surface takes on a crumpled, blistered look. Badly hit shoots have every new leaf curled into a tube. The mines are between the leaf surfaces, not on them, which is why you cannot rub them off and why contact sprays do so little.
Do not confuse it with two other things. Slug or snail damage removes tissue completely and leaves holes; leafminer never breaks the surface. Cold or wind damage curls leaves too, but without any trail inside.
| Symptom | Cause | Distinguishing detail |
|---|---|---|
| Silver winding trail inside the leaf | Citrus leafminer | Trail widens along its length, dark frass line inside |
| Leaf tightly curled, no trail | Cold, wind or drought | Whole flush affected at once, older leaves too |
| Sticky leaves and black film | Scale, sooty mold | Sticky to the touch, insects at the veins |
| Holes right through the leaf | Slugs, snails, caterpillars | Tissue missing, silvery slime nearby |
| Leaf pale and stippled, fine webbing | Spider mite | Common on indoor trees in winter |
Why it matters less than it looks
On a mature lemon with a full canopy, leafminer is cosmetic. It cannot attack hardened leaves, so it only ever gets the newest flush, and the tree has plenty of older leaves still working. Fruit is not affected. Trees that look wrecked in late summer are usually carrying a normal crop.
It is genuinely damaging in two situations. A young tree in its first two years has very little leaf area to lose, and repeated attacks on every flush hold it back for a season or more. And the mines open the leaf surface, which gives citrus canker a way in where that disease is present.
The other reason to care is that leafminer is a signal. It follows soft, sappy growth, and soft growth is what you get from a heavy nitrogen feed or a hard prune. A tree that gets hammered every year is usually a tree being pushed too hard.
What actually works
The single most effective measure is to stop producing the food. Feed in the spring so the tree makes one strong flush early, while leafminer numbers are still low, and then let it harden. Avoid feeding and heavy pruning in late summer, which is when the moth population peaks and would find a fresh flush waiting.
- Do not spray the mines The larva is sealed between the leaf surfaces and contact insecticides cannot reach it. Spraying kills the parasitic wasps that do control it and makes next season worse.
- Use horticultural oil on new growth only A light petroleum or neem oil deters the moth from laying. It works only as a preventative, so start when a flush begins and repeat every seven to ten days while the leaves are still expanding. Stop once leaves harden. Never spray oil in temperatures above 90F (32C) or in direct sun.
- Do not pick off affected leaves A curled, mined leaf still photosynthesises. Removing it costs the tree more than the miner did. Only remove leaves if you are also fighting canker.
- Remove water shoots Long soft vertical shoots are the moth's preferred target and a reservoir on the tree. Taking them out is good pruning practice anyway and cuts the population.
- Protect a young tree properly For a tree under two years old, fine insect mesh over the canopy during the main flush is the only fully reliable answer, and it is practical on a small container tree.
Parasitic wasps take over within two or three seasons in a garden that is not sprayed. On unsprayed trees a large proportion of leafminer larvae are typically parasitised by late summer, which is why the problem usually shrinks by itself.
Keep a record of the flush dates
Leafminer is a timing problem, so the useful record is the timing. Each year write down the date each growth flush started, the date you first saw a silver trail, and how bad it got on a rough scale of one to five.
Two seasons of that tells you exactly how many weeks separate your spring flush from the arrival of the moth, and whether pulling your feed earlier moved that gap in your favor. It also shows you whether the parasitic wasps are getting on top of it, which is otherwise invisible year to year.