Urophora affinis is a tephritid fly that forms a unilocular receptacle gall (Fig. 16) in [plants belonging to a] Centaurea subgenus [that] contains the weeds C. diffusa and C. [stoebe], diffuse and spotted knapweed. The fly is native to Europe ... Freshly hatched larvae chew down a floret, consume the ovule, and then feed on the inner layer of the ovary wall. After about 12 days, they penetrate the base of the ovary to feed on the receptacle. Cells of the ovary wall and receptacle proliferate to enclose the larva in a thick layer of parenchyma. Vascular bundles extend from the receptacle into this layer ... [After] about 25 days ... a sclerenchyma layer forms around the gall except for the apex ...
The corolla is suppressed or absent in galled capitula and the stomata in the bracts are not enlarged ... The florets tunnelled by larvae of U. affinis are destroyed and adjacent ones abort ...

- Harris, P. & Shorthouse, J.: (1996) Effectiveness of gall inducers in weed biological control^©