Adult leafminers are minute larvae of black flies and some wasps and moths that damage a number of flowering plants and vegetables. They lay eggs on the undersides of leaves. Upon emerging, they tunnel into the leaves between the upper and lower surfaces to feed on the inner part. Affected foliage will look discolored or diseased. Leaves will show light green to brown serpentine mines or tunnels between the lower and upper leaf surfaces, in which dark specks of excrement and/or tiny maggots may also be visible. A few weeks after the infestation begins, affected leaves may turn completely brown or yellow, look blistered or curled, or collapse. Besides damaging leaves, leafminers can damage stems below the soil, and also carry black leg and soft rot diseases. Several generations develop each summer.
Flowering plants susceptible to damage by leafminers include chrysanthemum, columbine, delphinium, hollyhock, larkspur, nasturtium and verbena. Vulnerable vegetables include beans (lima and snap), beets, cabbage, chard, lettuce, peppers, radishes, spinach and turnips. To prevent damage, check the undersides of the leaves of vulnerable plants for telltale rows of small, chalky white eggs, then scrape them off and destroy them. Repeat weekly for three to four weeks. If leafminer damage occurs, pick off and destroy all infested leaves, pruning back stems until only uninfested growth remains. Repel larvae by spraying plants with several weekly applications of insecticidal soap in late June and early July. If leafminers are a recurring problem, consider covering vulnerable plants in the spring with floating row covers, like Reemay.
Leafminers overwinter in the soil and emerge in early spring, and their chances of survival are reduced with a thorough fall cleanup. Remove weeds, especially lamb's-quarters. Cultivate soil in the late fall to expose these insects to birds and other predators. Wait to lay down a winter mulch until after the ground has frozen solid. Do not compost soil or plant material that may be infested.
The larvae of a number of sawfly species also mine the leaves and stems of a variety of plants. For more information on these pests, see Sawflies - Borers and Miners.