The seldom-seen adult is a 1/4-inch brownish fly that lays eggs on the leaf. The eggs hatch into larvae which tunnel into the leaf, feeding in serpentine trails between the top and bottom layers of leaf tissue. You could prevent leaf miner damage by using an insecticide, targeting the adult every seven days, but it hardly seems worthwhile. They usually attack and destroy the old foliage on the seed stalks, which die naturally after seed ripens. New growth comes from the base, which doesn't seem to be damaged as badly. Just cut off the top, dead seed stems and be rid of the unsightly leaf miner damage as well. The seed will have matured and been harvested by then anyway. Affected plants will usually thrive in spite of damage. Remove and destroy attacked leaves early in the season to reduce later infestations.