Blotchy patches, serpentine lines, and small black dots inside a clear section of leaf – these are all symptoms I look for when diagnosing leaf miner damage. Leaf miners are the larval form of a few species of flies that live within the upper and lower surfaces on the leaf and feed on the cells inside.
Typically, it is nothing more than an aesthetic nightmare. But on plants used for edible greens, like spinach, it develops into a problem that requires management, as leaf miners can decimate a crop, rendering it inedible.
The leaf miner flies lay eggs on the underside of the leaf, and once the eggs hatch, bore into the inner space of the leaf. Once inside, they munch on the inner cells, leaving a trail behind filled with small black frass (insect poop) inside the tunnels. After maturing, the flies emerge from the leaves and begin the cycle again. In the fall, the larvae pupate and fall to the soil to overwinter. These insects can have multiple cycles within one year.
Depending on the severity of the infestation, there are a number of management techniques available. Spraying insecticides is an option, but the timing is crucial – once the larvae are in the leaf, contact insecticides will not work. Systemic are an option, but have other negative consequences including affecting pollinators and other beneficial insects.
The most effective methods of control in a vegetable garden include tilling the soil to disrupt the pupae in the winter, remove the leaves as mines develop and squish the eggs on the undersides of the leaves. Doing a combination of all of these things can prevent a complete decimation of your fresh edible greens. In a perennial bed, however, most opt for either removing the leaves or just ignoring them.