Whitney Cranshaw, Colorado State University, Bugwood.org
Size: 1/4 inch long (Note: There are other flies that are similar in appearance that are not necessarily pests in your garden. Have flies identified to determine their pest status if it is not clear what they are.)
Where/When: Adults lays eggs on the leaves of spinach, beets, chard, lambsquarter, and other plants. Larvae feed between the two leaf surfaces, creating mines inside the leaves. Active April and May. A second generation can be seen in June.
Significance: Their feeding has little impact on plant growth so is not considered a pest to beets. However, they can be quite destructive to vegetables grown for edible greens, like spinach.