but this reminds me strongly of galls made by Eutreta sp. flies (Tephritidae) on certain composites in northeastern Iowa:

Joe pye weed (Eutrochium)
Leafcup (Polymnia canadensis)
White snakeroot (Ageratina altissima) (unsuccessful rearing; guide placement to Eutreta just an educated guess)

My experience with the Eutreta galls is that they are filled with black crumbly stuff (mostly frass I guess) and they tend to split open right around the time when the adult is ready to emerge from its puparium (coincidence? I think not...). If you come across a gall from which the adult has already emerged, what you generally find is a bunch of black crumbly stuff inside and a spent puparium (but sometimes the spent puparium has already fallen out).

Of course at least once in NE Iowa I've also come across a Papaipema burrow in giant ragweed that resulted in a swollen, split-open stem filled with black stuff. So, like I said, not an ID... :)