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Photo#581640
stem gall on white snakeroot #1 - Neolasioptera eupatorii

stem gall on white snakeroot #1 - Neolasioptera eupatorii
Little Sioux Unit, Loess Hills State Forest, Pisgah, Harrison County, Iowa, USA
September 27, 2011
Size: 9 mm X 4 mm
Swelling on upper stem of White Snakeroot (Ageratina altissima [Eupatorium rugosum]) in bur oak woods. Several plants had this gall, but most did not. I saw one plant with two of these galls, but most had only one. I kept this specimen to rear, so I dissected a second individual and linked those photos to this posting.

Moved
Moved from ID Request.
I'm basing this ID on the location of the gall on the stem and the assumption that adults haven't emerged yet. I think that galls that ultimately become woody may start out succulent like this one.

 
Very fresh
The gall appeared very fresh to me (no exit holes; unweathered, pristine condition) so it may well have been newly formed and thus in a pre-woody stage.

 
That might also explain
why your cross-section didn't reveal any larvae. In my experience, Neolasioptera larvae are bright orange, although I don't know for sure that they all are.

Dissected specimen...
...on second plant shown here:

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