Question: I have leaf miners on my Swiss chard and wonder if there is an organic method of killing them.
Answer: Leaf miners are the larvae of flies. They hatch out of eggs laid on the undersides of leaves. The maggots get in between the top and bottom layers of the leaves and feed there. Even if you wanted to use a poison, it wouldn’t affect the leaf miners, at least not until they’d done their damage. The damage shows up as mottled leaves or light patches. Destroy the leaves that contain the miners or crush the maggots. If the problem is severe, try to figure out what is making the plant susceptible to the bugs: nutrient imbalance in the soil, unusual weather, killer smog, or some other stressor. You can also try lightweight floating row covers early in the season to keep leaf miners from decimating your plants. They let in enough light that the plants can photosynthesize sufficiently and are light enough not to impede their vertical growth.