Stem miner (Diptera: Agromyzidae) in Brassica

I noted several stem mines, one containing an actively feeding larva, on upper stems of the host plant in mid-August, when the host was in flower. I also discovered several black puparia overwintering under the epidermis of dead stems of the host in winter. It appears the puparium may be the typical overwintering stage, but the only thing I have reared from an overwintered puparium is a parasitoid wasp.

The identification of the host plant is tentative. Photos of the host plant are available here:

• Flowers: https://www.inaturalist.org/observations/336331883
• Flowers and fruit: https://www.inaturalist.org/observations/192430392
• Seedhead in winter: https://www.inaturalist.org/observations/192430583