Leafminers are immature insects that feed between the upper and lower surfaces of leaves. The adults may be flies, moths, sawflies, or beetles. The female adult lays eggs on the leaf surface. When the larvae hatch they tunnel into the leaf and begin feeding. Leafminers attack all kinds of plants, from vegetables to fruits, flowers, trees, or shrubs, although each species of leafminer usually feeds on only one or two types of plants. In pines and other conifers, the pests are called needleminers.

Leafminer larvae are tiny, and somewhat flattened to fit inside a leaf. As the larvae feed, they eat the green tissue inside the leaf, leaving a thin, winding trail covered by a papery sheath. The trail may contain small brownish black pellets of insect excrement, and if you look closely you may be able to see larvae. When numerous larvae are feeding in a single leaf, their tunnels may merge, creating large blotches.

Because they're protected inside the leaf for most of their lives, it's difficult to control leafminers with insecticidal sprays. However, damage caused by the pest is seldom severe enough to justify spraying except to make the plant look better. For ornamental plants, you can spray a systemic insecticide such as acephate to kill tunneling larvae. Carbaryl, neem, or pyrethrin is effective if sprayed just as the larvae are hatching. If you see large, merged tunnels, the larvae may have already completed their life cycles, making insecticidal sprays pointless.

Control is more important for leafy vegetable crops because feeding by the leafminers damages the edible portion of the plant. Protect vegetables from egg-laying adults by covering the plants with a floating row cover. Secure the edges of the row cover to the ground so that no adults can enter. Remove and destroy affected leaves.

Leafminers may attack many ornamental plants. Some of the preferred hosts are:

Arborvitae
Aspen
Azalea
Birch
Bougainvillea
Boxwood
Butterfly weed
Chrysanthemum
Columbine
Cottonwood
Delphinium
Elm
Holly
Impatiens
Juniper
Lantana
Lilac
Locust
Magnolia
Oak
Pine
Verbena
Water lily

Commonly affected food crops are:

Apple
Beets
Citrus
Garlic
Onion
Spinach
Swiss chard
Tomato