Irregular or serpentine pale grey lines appear on both sides of the leaf blades as the larvae feed. These burrows are usually limited by the leaf veins and contain black fecal material visible as slim trace inside the tunnels. Entire leaves may be covered with mines. Damaged leaves may drop prematurely (defoliation). Defoliation can reduce yield and fruit size and expose fruit to sunburn. Should not be confused with Tuta absoluta (Tomato Leafminer) whose mines on leaves are wider and white or transparent.
Symptoms are caused by several flies belonging to the family of the Agromyzidae, with several thousand species worldwide. In spring, females puncture leaf tissues and lay their eggs, usually along the margins. The larvae feed between the upper and the lower leaf surface. They produce large white meandering tunnels with a trail of black frass left behind as they feed. Once they have reached maturity, larvae open a hole on the underside of the leaf and fall to the ground, where they pupate. Plant debris near the host are alternative pupating places. Leaf miner flies are attracted to the color yellow.
Spray neem oil products (Azadirachtin) against larvae onto leaves in the early morning or late evening. For example, spray Neem oil (15000 ppm) at a rate of 5 ml/l. Make sure to have good leaf coverage. Neem slightly enters the leaves and reaches some of the larvae inside the tunnel. Foliar applications of the entomophagous nematode, Steinernema carpocapsae, can reduce the leaf miner population. Other biological controls of leaf miners include parasitoids (e.g. Chrysonotomyia punctiventris and Ganaspidium hunteri) and nematodes (e.g. Steinernema carpocapsae).
Always consider an integrated approach with preventive measures together with biological treatments if available. Broad-spectrum insecticides of the organophosphates, carbamates and pyrethroids families prevent adults from laying eggs, but they do not kill the larvae. Moreover, they can lead to the decrease of natural enemies and the development of resistance in the fly, which in some cases can actually result in an increase in their numbers. Products such as abamectin, chlorantraniliprole, acetamiprid, spinetoram or spinosad can be used in a rotation to avoid the development of resistance.