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Leaf Miners

Garden pest

Description​

Leaf miners are the larvae of several kinds of flies, moths, beetles, and sawflies, rather than a single species with one scientific name. They feed between the upper and lower surfaces of a leaf, leaving narrow, winding tunnels or larger, irregular blotches. The damaged tissue often looks pale or translucent at first, then turns brown as it dries. Leaf miners affect vegetables, ornamentals, trees, and shrubs; the host plant and shape of the mine vary by species. A healthy, established plant usually tolerates modest damage because much of the leaf remains functional. Heavy infestations can reduce leaf area, especially on seedlings or crops grown for their leaves, and may cause badly damaged leaves to drop. Most feeding occurs out of sight inside the leaf, where the surrounding tissue protects the larvae from many sprays and predators. Adults do not make the tunnels; they lay the eggs from which the mining larvae hatch.

Symptoms​

  • Winding tunnels in leaves
  • Spots on leaves
  • Leaves dropping

How to Identify​

Hold an affected leaf up to the light. A leaf-miner tunnel lies within the leaf tissue, so its surface is usually intact. Look for a pale, winding trail or an expanding blotch, sometimes with a dark line or specks of droppings inside. A live larva may be visible near the growing end. Older mines dry to tan or brown and may appear spot-like. Unlike chewing insects, miners usually leave no open hole when damage first appears. Fungal leaf spots tend to have more uniform lesions and lack a tunnel or internal droppings. Trails made by slugs are on the leaf surface and can be wiped away.

Prevention​

Inspect plants regularly, particularly young leaves, so you can catch new mines before damage becomes extensive. Check transplants before bringing them into the garden, and remove heavily infested leaves when doing so will not weaken the plant. Clear out badly infested crop debris and manage nearby weeds that can host leaf miners. For susceptible vegetables, install fine insect netting or a floating row cover before adults arrive; seal the edges and avoid trapping an existing infestation underneath. Remove the cover when crops need insect pollination. Keep plants growing steadily with appropriate water and nutrition, but avoid excessive nitrogen, which can promote tender growth attractive to some species. Preserve parasitic wasps and other natural enemies by avoiding routine broad-spectrum insecticide applications.

Treatment​

First, decide whether treatment is needed: a few mines on an otherwise vigorous plant are mainly cosmetic. For a small infestation, pinch the leaf around a visible larva to crush it, or remove affected leaves and put them in the trash. Do not strip a plant of so many leaves that you cause more harm than the miners. Check new growth over the following weeks, because larvae already inside leaves and newly arriving adults can keep the damage going. Natural enemies, especially tiny parasitic wasps, often bring populations down if they are not disrupted by insecticides. If damage remains serious, identify the host plant and, if possible, the miner before choosing a pesticide. A product containing spinosad may help against some species when applied as young mines appear, but older larvae are difficult to reach. Follow the label for the crop and pest, and never apply it to flowers visited by bees.

Plants Affected​

Amaranth, American Basswood, Arborvitae, Aspen, Basswood, Beet, Birch, Black Locust, Boxwood, Buxus, Cork Arborvitae, Holly, Honey Locust, Hornbeam, Horse Chestnut, Ilex, Kaffir Lime Leaf, Lambsquarters, Leek, Lilac, Locust, Lovage, Orange, Paper Birch, Quaking Aspen, Quinoa, Sorrel, Spinach, Swiss Chard, Syringa, Thuja, Trembling Aspen, Water Lily, White Birch

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