A: Once you brought a clipping of the tomato leaves into the office, the culprit was easy to identify. The cause of the white lines is probably the American serpentine leafminer, Liriomyza trifolii. The American serpentine leafminer is the larval stage of a very tiny fly.The eastern part of the United States has had this insect chewing on our vegetables for many years but it has just been introduced to California which means it will not be long before it is found throughout the southern states.

The good news is leafminers do not live very long and their life cycles require 3 – 4 weeks. Damage done by the females feeding and puncturing holes in the leaf tissue causes a yellow spotting. Once the egg is placed in between the leaf tissue it develops into a larva which feeds on the leaf tissue. This causes the distinctive white lines along the upper leaf tissue. It only takes about 3-4 days for the egg to develop into a larva. The feeding habit of this insect does reduce the ability of the leaves to produce carbohydrates and heavy feeding may cause the leaves to drop but it does not cause the death of the plant. The holes produced by the female have the potential for allowing in fungi, bacteria or viruses which can in turn cause greater damage to the plant.

Parasitic wasps are important natural enemies of this fly and therefore using broad spectrum pesticides on a regular basis is not advisable. It is also important to reduce the weed population around garden at they are often the source of many insect pests.

At the end of the season it may be advisable to bury the crop as leaf miners have difficulty surviving in deep soil. One study by Price and Poe in 1976 indicated natural enemies to the leaf miner (predators and parasitoids) were less likely to be found around tomatoes grown in plastic mulch and tied to stakes. As an alternative, consider using organic mulches such as pine straw, shredded oak leaves or newspaper.

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