This photo was submitted by a grower in 2011.
Leafmining is formed by insects that produce“mines” in leaves by feeding. The larvae are in between the upper and lower leaf tissue where feeding occurs. Different species of leafminers feed upon different plants. Leafminers may be either a wasp, fly, moth or beetle species.
Usually the immature, or larval stage, of insects produces the “mines”. Most leafminers drop to the ground to pupate. Leafmining insects are typically classified as “serpentine leafminers” or “blotch leafminers”. Serpentine leafminers typically form leaf mines in a zigzag pattern. Blotch leafminers like in this photo generally make large mines without a particular pattern. Blotchminers may be confused with leaf spot diseases.
For more information on management see: New England Greenhouse Pest Management Guide