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Scutellum flattened, with distinct marginal rim; the apical pair of bristles approximated; mesonotum yellow with shining black stripes; legs, except the tarsi, entirely yellow. Sabrosky 1935

Probably identified by Tony Irwin at diptera.info. Mistakes are mine.
Loaded from my soon to be retired Diptera photo gallery: http://xespok.net/diptera/main.php
More accurate location information may be added later.

Japanagromyza viridula. Rock Creek Park, Washington, DC, USA.

Prescutellar acrostichal bristles present, 2 pairs of dorsocentral bristles. Frons not projecting above eye, matt black, lunule conspicuously silvery; 2 equal upper orbital bristles, 2 lower orbital bristles.

The Japanagromyza key is a bit confusing here. The couplet that separates J. viridula from inaequalis and aequalis claims that the mesonotum is greenish in inaequalis /aequalis, black in viridula. It also claims that the mesonotum of viridula is "moderately shining, with virtually no trace of metallic coloration." I think this specimen clearly has a greenish mesonotum, and the flash definitely brings out a hint of metallic coloration. However, I'm pretty sure this is viridula because inaequalis /aequalis don't occur this far north, and neither does their host plant. Maybe the greenish coloration isn't as obvious in museum specimens of this species.