Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada
Beta vulgaris host - Pegomya

Beta vulgaris host - Pegomya
Waltham, Middlesex County, Massachusetts, USA
October 6, 2016

Moved from Unidentified Leaf Mines.

I think this is right; a shot of eggs on the underside would confirm.