Avoid leaf miner this year by getting those row covers on now. The solid plastics will build up too much heat later on, as summer approaches, but spunbonded types or screening allow some air circulation, while blocking out the pests. It's worth a try, because the flies can quickly ruin a crop. Beets, cabbage, beans, turnips, lettuce and peppers can also be affected.
Another control measure is dusting with diatomaceous earth. The remains of fossilized, microscopic sea creatures, the dust contains sharp particles, which penetrate the insect's body. The resultant dehydration causes death. Diatomaceous earth is harmless to earthworms, mammals and birds.