Have you ever grown Swiss Chard, Spinach or Beets and found their leaves looking like this? When I first saw this happen to my Swiss Chard I had no idea what it was. It completely wiped out my entire crop. I did some research and learned that it was caused by leaf miners.
Leaf miners are larvae that live between the layers of the leaves and feed. The adults overwinter in the soil and then lay their eggs on the undersides of leaves. If you start to see damage from them it’s important to start checking for more eggs. You will find small masses of white eggs on the undersides. They are elongated and in groups of 4-10 eggs. They are easily scraped off with your thumbnail. Once the eggs hatch, however, there isn’t anything you can do but pull the leaf off and dispose of it. We feed ours to the chickens and goats.
The best thing to do is prevent them which is threefold. First, make sure to rotate your crops. When the adults emerge from the soil they won’t have far to go if you plant one of the effected crops in the same place they were last year. Also, planting late in the fall or early spring helps miss them. If you plant them in a new place you can use row covers to create a physical barrier that will keep adults away from them. At our previous house we used this method with great success. We didn’t use it this year but plan to this fall. Fortunately we have been proactive about removing eggs so they haven’t done too much damage.