Galls among the flowers or modified flowers...
Gall ovoid, white-haired, larger than florets...Rhopalomyia spp.

Galls are sessile and widest at base, 4–7 mm long by 3–5 mm in diameter, one-celled, ovate, pointed at apex, and green with white down. Adults have been reared from the galls in fall. The three species listed are too incompletely known to determine whether they are synonyms. ...similar galls have been taken on an aster in Nebraska.

R. crassulina Cockerell. Host: A. crassulus. Distr.: Colorado.
R. gemmaria (Stebbins). Host: A. ericoides. Distr.: Massachusetts.
R. lateriflorae Felt. Host: A. lateriflorus. Distr.: Massachusetts and New York.