Handling Leafminers

I've been noticing some squiggly white lines on my tomato leaves since last week. It started with one or two leaves, but then today, I realized it had expanded to more leaves, even to my younger seedlings! At first, I thought it was the watermark caused by careless watering, but I couldn't assuage my curiosity and decided to do some scrupulous research and was quite surprised with my findings. It's not trivial at all! 

Leafminers are actually the larval (maggot) stage of many insects such as flies, moths, sawflies and beetles. The female adult flies lay their eggs on the surface or undersides of the leaves. 

Life Cycle: Mature larvae overwinter in the soil under host plants. As temperatures warm in the spring larvae pass to the pupal stage and appear as young adults in late April. Mated females use their needle-like ovipositor to lay up to 250 eggs just under the surface of the leaf epidermis. Deposited eggs may appear as small raised spots on the leaf. Within 10 days hatching larvae tunnel through the mid-leaf tissue, feeding as they go and leaving tell-tale wavy lines that are visible on the surface. Larvae mature in 2-3 weeks, and when ready to pupate, leave the leaf and drop to the soil. Once on the ground, they dig 1-2 inches into the soil and pupate. Adults emerge within 15 days as adult flies. (snippet copied from PlanetNatural)

(The snippets below are taken from Better Homes & Gardens)
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 larvae. When numerous larvae are feeding in a single leaf, their tunnels may merge, creating large blotches.

Leafminer Control

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.

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 Apple

Beets
Citrus
Garlic
Onion
Spinach
Swiss chard
Tomato