Visible Signs of Damage:

Mines in Leaves: Larvae create wavy lines, tunnels, trails, or blotches just under the surface of the leaves, often appearing white, gray, or paler than healthy leaf tissue. These are the "mines" created by the larvae feeding. 

Drooping and Withering: Leaves may start to wilt and droop, especially in areas where the larvae are feeding. 

Stunted Growth: Young plants may show stunted growth, and older plants may become less vigorous. 

Reduced Fruit Bearing Capacity: The ability of the plant to produce fruit is significantly reduced, and some fruits may be inedible. 

Leaf Fall: In severe cases, affected leaves may fall off prematurely. 

Target crops:

leafminer has a wide host range that includes bean, beet, carrot, celery, chard, cucumber, eggplant, lettuce, melon, onion, pea, pepper, potato, squash, tomato as well as ornamental flowers such as chrysanthemum, gerbera daisy, and marigold. Pea leafminer has an equally broad host range that includes broccoli, cauliflower, phlox, petunia, and viola in addition to many of the same hosts as the American serpentine leafminer. Vegetable leafminer feeds on many of the same plants as the other species, especially those in the cucurbit, legume, and solanaceous families. Allium leafminer [Phytomyza gymnostoma (Loew)], native to Europe, is a recent invasive pest to the northeastern U.S. that targets allium hosts both wild and cultivated (such as onion, leek, garlic, chive and shallot)