"Leaf miner" is a catchall name for certain moth, beetle, and fly larvae that tunnel within plant leaves, leaving twisting trails on the surface. As larvae mature, they drop to the soil to pupate. The tiny adults are rarely seen; they lay eggs on leaf undersides. Leaf miners cause primarily cosmetic damage (a particular problem on hollies in the landscape), but they may also ruin leafy vegetables such as chard, lettuce, and spinach.
Cover soil with black plastic mulch so that larvae can't burrow underground; this is an effective control under ornamental plants. In vegetable gardens, till soil between rows to expose pupae; use floating row covers over new garden areas to prevent infestation by adults flying in from nearby locations.
Because larvae are protected by leaf membranes, chemical controls are not effective.