The beet or spinach leafminer, (Pegomya hyoscyami), is a gray fly about 1/4 inch long. It emerges in April-May and lays white eggs on undersides of leaves of beet, spinach, chard, and other plants including weeds. Eggs develop into pale maggots which feed between leaf surfaces, making winding mines at first, then widening and merging them to form large, light- colored blotches.
Holding infested leaves to the light reveals the maggots. Maggots may move from leaf to leaf. They are about 5/16 inch long when mature, at which time they usually enter the soil to pupate (change into flies). Some pupation occurs in leaves or among debris on the soil surface. A few weeks later, flies emerge. There are several generations each year.
Control: Remove infested leaves and weeds as soon as noticed and place in the garbage. Using "floating row covers" or otherwise screening plants should help if screens are in place before flies emerge and if the crop is in a different area of the garden from the previous year. No pesticides are recommended.
For more information contact your local WSU Extension Office.