[H]ost records indicate that A. thoracica is nearly monophagous on [three] species of Baccharis [B. pilularis, B. sarothroides, and B. emoryi ... The gall of A. thoracica is clavoidal (Fig. 6E) or ellipsoidal (Fig. 6C), depending respectively on whether or not the apical bud is killed by larval feeding ... Often, the expanded base of an axillary branch arises like a webbed thumb on one side of a gall, giving it an asymmetrical, "mitten-like" shape (Fig. 6 C and F). The galls are smooth and yellow-green when young but are streaked with red, darken further to brown, and become roughened and corky with age. [Galls collected ranged from 5 to 17mm long and 2 to 7mm wide.]

Adults emerge in January through March from galls on B. sarothroides, live, and probably oviposit through most of April in southern California.

[see pg. 75 for gall photos]

- Headrick, David H. & Richard D. Goeden: (1993) Life History and Description of Immature Stages of Aciurina thoracica (Diptera: Tephritidae) on Baccharis sarothroides in Southern California^©

Reference: https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1002&context=hcs_fac