I picked these goldenrod galls on
October 3 in Tommy Thompson Park, then kept them in a ziplock bag at room temperature. I had a look on November 19 and then they had hatched; I don't know exactly when they did it, certainly not before the end of October when I last had a look.
There were two females in the bag, they walked in all directions, particularly backwards and this was very striking, it looked as if they had some threatening pincers/jaws.
See in A. Dale's
"A Nature Observer′s Scrapbook", UK, a particularly good example of a cut up gall with 3 puparia; now I regret not having had a closer look of their emergence holes, but our stay was coming to an end [I hope you realize that I did all this in a hotel room]! I must add that I also cut a couple of goldenrod spherical galls and the larvae were still there, see
this photo taken at the same time.
This fly has been introduced in north America to control the impact of Canada thistle, coincidentally I found in the area one of their galls on thistle; they look remarkably similar to the goldenrod spherical galls.
See
Has anybody reported this cross-over yet? The plants are in the same family, Asteraceae, and of course gondenrod is extremely common, this was a thing waiting to happen...
Another question, goldenrod
Solidago spp. have been introduced in the UK where they thrive and I wonder if these flies have felt the same urge the other side of the pond?