A small fly that lays eggs on the underside of leaves (especially beets and chard). The larvae burrow into the leaves creating brown filmy damaged patches on the leaves. Put yellow sticky traps around affected plants to trap adult flies. Remove damaged leaves as soon as they appear.
They cause the burn-like damage on Swiss chard and beet greens. You can save your plants by removing any damaged leaves right away, and inspecting the underside of healthy leaves and remove any eggs.
It is important to keep on top of leaf miner early in the season. The leaf miner can wipe out your chard or beet crops if left unchecked. Remove affected leaves and destroy. Check backs of healthy leaves for eggs and scrape off with your nail.