Awesome a tort right under my nose! Thanks for the ID!

I thought that was an ironic twist!

I was basing it on Marshall 2006 where he states that the recently introduced Chirosia betuleti creates swellings on ferns. I did my undergrad at Guelph, and if my memory isn't failing me, he also showed a picture of the gall which looks similar to mine.

I found this paper:

von Aderkas, P. and B. V. Peterson. 1987. Chirosia betuleti (Ringdahl) (Diptera: Anthomyiidae) a gall-former on the ostrich fern, Matteuccia struthiopteris, with notes on other insect-fern associates. Proceedings of the Entomological Society of Washington 89: 532-547 (here)

and that species does indeed cause damage similar to this. The larvae feed externally, on the adaxial surface of the rachis (though sheltered by the curl of the distorted frond). I was considering the possibility that, as Paul Beuk suggested, the drosophilids were not the cause of the galls I found, but since I found larvae mining within the rachis of the ferns I investigated, it seems that Chirosia betuleti was not involved, and the possibility remains that fern galls like this can be caused by different types of flies. I plan on investigating this further this spring--I missed my chance last year. According to the above paper, Chirosia betuleti is closely associated with ostrich fern in eastern Canada, but is known from Pteridium and Athyrium (and not ostrich fern) in Europe. According to a post I found here, no Chirosia species has been recorded from Dryopteris, which is what my ferns were. I've seen similar damage on ostrich fern and various other ferns in New England.

Sadly I'll be in Edmonton this Spring (where large ferns are scarce) so I wont be able to look for them as well. Interestingly the post that you linked to was posted by Cosmin, whom I was in contact with about collecting sites in Romania. Please let me know if you get any more insights on this interesting gall-maker.