Leaf miners
Also known as Leafminers, Liriomyza, Agromyzid flies
In short
Leaf miners are insect larvae that live and feed between the upper and lower surfaces of a leaf, leaving pale winding tunnels or blotches. The damage is often cosmetic, but heavy mining can make leaves drop and slow seedlings and leafy crops.
How it looks
The larvae of several insect groups mine leaves: flies (especially the family Agromyzidae), moths, sawflies and beetles. The common vegetable leafminers (Liriomyza species) are small, active black flies, often with a prominent yellow triangle on the back between the wings; their larvae are small yellow cylindrical maggots visible inside the mine when a leaf is held up to the light, and the pupae are small, brown and seed-like, usually in the soil or on the leaf. The mine itself is the easiest thing to see: a pale, narrow serpentine track that widens as the larva grows, or a broad blotch, often with a dark line or dots of droppings (frass) inside.
Signs
Pale, twisting lines or blotches on leaves; a dark frass trail inside the mine; fine white speckling (stippling) where adult flies have punctured the leaf to feed and lay eggs; with heavy attack, yellowing, leaf drop and slower growth. The mine shape and host plant are often enough to identify the species.
Life cycle
Females insert eggs into the leaf; the larvae mine between the leaf surfaces, then pupate inside the mine, on the leaf or, in the common vegetable leafminers, after dropping to the soil. The Liriomyza life cycle can take only about two weeks, with many generations a year.
What favours it
Warm weather; seedlings and leafy vegetables. Broad-spectrum insecticides applied against other pests kill the parasitic wasps that normally keep leafminers in check and can trigger outbreaks.
Prevention
- Cover seedlings and leafy crops with fine protective cloth or mesh.
- Keep plants well watered and growing strongly so they tolerate some mining.
- Check new plants for mines before bringing them in.
- Avoid broad-spectrum sprays so parasitic wasps survive.
Treatment
Start with the first step and move down only if the problem persists.
- Tolerate light mining; it usually has little effect on growth.
- Pick off and destroy mined leaves, or squeeze the mine to crush the larva inside.
- Rely on parasitic wasps such as Diglyphus, which usually keep leafminers below damaging numbers.
- Insecticides are often not very effective because the larvae are protected inside the leaf; if used, spinosad is an option but can also harm beneficial insects — follow the product label.
Pros and cons
Cons
- Unsightly mines on ornamental and edible leaves.
- Heavy attacks cause leaf drop and slow seedlings and leafy greens.
Look-alikes
- Thrips
- Silvery patches and flecking on the leaf surface, not tunnels with a frass line.
- Spider mites
- Fine speckling spread over the leaf with webbing; no tunnels.
- Fungal leaf spot
- Round or angular spots, often with dark margins, rather than winding tracks.
Affected plants
Many vegetables (spinach, lettuce, celery, tomato, peppers, cucurbits, brassicas) and ornamentals; specialised species mine particular trees and shrubs.
Related
Sources
Written in our own words from these sources. Check a product label before using any treatment.