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Identifying the Pest
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 the larvae. When numerous larvae are feeding in a single leaf, their tunnels may merge, creating large blotches.
Leafminer Control
Because for most of their lives they’re protected inside the leaf, it’s difficult to control leafminers with insecticidal sprays. 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 the tunneling larvae. Carbaryl, neem oil, or pyrethrin are 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.
Host Plants
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: Apples, Beets, Citrus, Garlic, Onions, Spinach, Swiss chard, Tomatoes.
