Adult miners are minute black flies, about 1/10" long, usually with yellow stripes. They lay eggs on the undersides of leaves. Yellowish larvae hatch and tunnel inside the leaves and feed between the upper and lower surfaces. Leafminers emerge from the soil in early spring and egg laying continues throughout the summer.
Larvae cause white to brown tunnels or blotches. There can be stem damage below the soil. Leaves can turn yellow and look blistered or curled. Leafminers carry black leg and soft rot diseases. Several generations develop each summer.
Preventive steps include weed control to reduce egg laying sites. Get rid of thistles, plantains, and dandelions because these are the top egglaying habitats. Compared with cultivated orchards, cover cropped orchards did much better in having fewer insect populatons and fruit damage due to greater numbers of natural predators inhabiting the ground cover.
Vulnerable plants include beans, beets, cabbage, chard, lettuce, peppers, radishes, spinach, and turnips.
Other control strategies include screening out the fly with agricultural fleece. Also removing infested leaves before the larvae mature.
Planting a trap crop such as radishes or lamb's quarters will notify you of an infestation. Place them within 15' of the crop threatened. Destroy them as soon as they start working on the trap crop.
If you want to spray them, use light horticultural oil. Use 2-3% solution and spray the pests directly because it smothers them. This can be used throughout the growing season.
Handpicking and fall cleanup are other good habits to have. Also, planting crops after mid-June passes the main infestation period for the pest. Attract birds who eat miners.
Another good method is to use NEEM. It works at small percentages of solution.
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