Just a hunch.

The nice neat frass lines here look similar to what the Marmara in this series left behind but that's really all I can say. I'll keep an eye on the host plant as the summer continues.

Any sense for whether the Marmara spp. in herbaceous stems tend to be hostplant specialists or generalists? And whether they pupate in the mine or in the ground? (I'm especially curious about the latter as I try to rear.)

I thought I was seeing agromyzid frass in one of those photos before, but I can believe they're Marmara now that I look again.

Not enough Marmara spp. have been reared (and critically examined) to say how specialized they are; M. gulosa is polyphagous but known only from the Southwest, and as far as is known all other species are confined to a single genus or maybe a few genera in the same family. Have I sent you my paper with the hostplant table?

Marmara auratella was described from cultivated Rudbeckia, but Dahlia appears to be the normal host. It pupates under a flap of epidermis at the end of the mine, as do some of the species on woody plants. Most species, including others on herbaceous plants, exit their mines and spin cocoons--on the hostplant if you're lucky, but often not.

I just re-read your Marmara paper; very helpful. In reading it again it was neat to be reminded of the way M. viburnella goes from the leaf to the stem. These are some seriously cool critters. I am especially charmed by the unique form of the larvae, and by seeing their feeding apparatus in action.

It's interesting that you're finding all these mines on Asteraceae now. We recently found Marmara mines on Iva frutescens that do the same leaf-to-stem thing (except the stem mines are always invisible due to the bark characteristics). Julia somehow managed to find a larva in a stem anyway, and we're going to try to barcode it along with a viburnum one to see if they might be the same thing.
I too have been mesmerized by watching those bizarre mouthparts opening and closing!

Host sounds like a cool plant, I'd never heard of it so looked it up. Good luck with the barcoding!