Catkinlike growth, a foreshortened stem, with narrow leaf tips along length of gall...Rhopalomyia strobiligemma (Stebbins)

Galls are polythalamous. Adults have emerged from the galls in September. Host: A. paniculatus [syn. of Symphyotrichum undulatum according to POWO, but may more likely refer to S. lanceolatum here; Gagné uses "Aster undulatus" elsewhere in this book]