My tomato leaves have white lines on them. What causes this, and what should I do about it? -Bill

Those white squiggly lines are a trademark of leaf miners. Leaf miners are insect larvae (Liriomyza munda) which hatch from eggs deposited between the upper and lower surface of plant leaves. The hungry larvae munch their way around the leaf, leaving a telltale white trail or tunnel.

Leaf miners affect a host of edible and ornamental plants and are very difficult to control because they hide inside the leaf. Insecticidal sprays not only have a hard time reaching them, but such products kill off the beneficial wasps that would be happy to have your leaf miners for breakfast. Luckily, leaf miner damage is mostly cosmetic unless the leaves of the plant are going to be harvested and eaten (such as basil or chard). Extensive infestation with leaf miners can reduce the overall health and vigor of the plant, and the tunnels can provide an entry point for fungus or bacteria if the leaves are not removed.

Here are some tips for controlling the leaf miners on your tomatoes:

White trail left by a leaf miner.

• Pick off and destroy any affected leaves and leaf miner larvae.
• Grow your veggies organically, without the use of sprays or chemicals that could upset the balance of beneficial predator insects, since leaf miners can be largely controlled by natural predators.
• Keep your plants healthy and well watered, so they can recover from the infestation (and the necessary leaf-plucking).
• Keep your garden clean and free of weeds or debris that can harbor insects and diseases.
• Some garden insecticidal sprays (such as those with carbaryl or pyrethrins) are labeled for use with leaf miners. Research gives mixed reviews on such products for leaf miners, and you certainly should consider whether you want insecticides on edible plants. Read the labels carefully before using.

Further Information

• Tomato Leaf Miners (University of California)
• Photo of Leaf Miner on Tomato (Clemson University)

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