Leaf Miners

Leaf miners are insect larvae small enough to live and feed between the upper and lower surfaces of a leaf. As they eat, they leave pale tunnels (mines) that show through the leaf surface. The larvae belong to several groups of insects: mostly small flies, but also some moths, sawflies and beetles. Damage is usually cosmetic on ornamentals, but on spinach, chard and beet greens the mined leaves are the crop, so a heavy infestation can ruin a harvest.

Recognizing the damage

Mines come in two basic shapes, and the shape is a good clue to the culprit:

• Blotch mines: broad, irregular, tan to whitish patches that later turn brown and papery. On spinach, chard and beets, these are made by the larvae of the spinach leafminer and beet leafminer (Pegomya species), small gray flies. Hold a mined leaf up to the light and you can often see the white, legless maggots inside.
• Serpentine mines: thin, winding trails that widen as the larva grows. Typical of Liriomyza leafminer flies on tomatoes, peppers, beans, cucumbers, melons, lettuce and many flowers, and of the columbine leafminer.

Trees have their own leafminers, such as the birch leafminer (a sawfly), the citrus leafminer (a tiny moth) and several on aspen.

Life cycle

The adults are mostly small and easy to overlook. Female flies lay eggs on or into the leaf, often on the underside; spinach and beet leafminer eggs are tiny, white and laid in small groups. The hatching larvae tunnel into the leaf and feed for one to a few weeks, then usually drop to the soil to pupate. They spend the winter as pupae in the soil, often in the bed where the host crop grew, and there are several generations a year. Damage on spinach and chard is usually worst in spring and early summer.

Prevention

Row covers. For spinach, chard and beets, a floating row cover or fine insect netting is the most effective control. Put it on at sowing time and seal the edges, before the flies arrive. These crops do not need pollinators, so the cover can stay on until harvest. See protecting greens from leaf-eating pests.

Rotation. Because the pupae overwinter in the soil, plant spinach, chard and beets in a bed that did not grow any of them last year. Otherwise flies can emerge from the soil underneath your new row cover.

Clean up and cultivate. Remove and destroy mined leaves as soon as you see them, before the larvae finish feeding and drop to the soil. After harvest, pull spent plants and turn the soil to expose pupae. Control lamb's quarters and other weedy relatives of spinach and beets, which host the same leafminers.

Treatment

Pick and squash. On small plantings, the simplest approach is to pick off mined leaves, or pinch the leaf where the larva is visible to crush it, and check leaf undersides for eggs.

Let natural enemies work. Tiny parasitic wasps attack leafminer larvae inside their mines and keep Liriomyza in particular under control. Broad-spectrum insecticides kill these wasps and can make leafminer problems worse.

Sprays. Because the larvae are protected inside the leaf, most contact sprays do not reach them. Spinosad and neem (azadirachtin) products labeled for leafminers on vegetables can reduce larvae when applied early, as mines first appear. Do not use systemic insecticides on vegetables or herbs unless the product is specifically labeled for that crop, and always observe the waiting period before harvest. Remember too that Bt, the usual organic caterpillar spray, has no effect on fly larvae.

On ornamentals and trees, a few mined leaves rarely affect the health of the plant and usually need no treatment.