Gall put back together; note the emergence hole and the frass around it.
As it happened I dissected it in such way that the emergence tunnel previously prepared by the gall was not visible.
In the fall a mature Eurosta solidaginis larva prepares an exit tunnel before entering the winter diapause but it does not cut the skin of the gall. Obviously, the humid conditions in which was kept inside the ziplock bag favoured the gall decomposition. For more about the life history of Eurosta solidaginis visit Eurosta page.

In contrast, this humidity was what probably favoured the pupation and subsequent emergence of Urophora cardui kept in the same conditions.
See and compare with ; all galls were kept under the same conditions.