Pegomya hyoscyami

Pegomya hyoscyami, the beet leafminer or spinach leafminer, is a grey fly about 6 millimetres (0.24 in) long. It emerges in April–May and lays eggs on the undersides of leaves of beet, spinach, chard, and other greens. Eggs develop into larvae that burrow into the leaf hollowing out large patches of the leaf between leaf surfaces, often killing large parts of the leaf.