Leafminer damage on beet.
Though many different insects are known as leafminers the most common are the larvae of tiny black flies. They tunnel between the upper and lower layers of leaf tissue creating visible random trails, or mines, in the process.
Adult flies lay eggs on the undersides of leaves. After hatching, larvae tunnel into leaves to feed, gaining some protection from predators there. They are pests to beets, chard, lettuce, peppers, potatoes, and spinach.
Protect leafy vegetable crops with a lightweight row cover. Pick off and destroy infested leaves. Encourage natural parasites that attack leafminers by not spraying pesticides.
Photography by Ray Coleman, courtesy of David Liebman