A small but striking fly. The male's wings have darker markings.

The fly causes distinctive swollen stem galls on Creeping Thistle.  These are green and fleshy to begin with, and persist on old stems.   Several chambers are inside.  Adults fly in June, laying eggs on the thistle.  The larvae hatch and crawl down into the stem, causing the gall.  They feed to late summer, then overwinter in the gall.  They pupate inside the gall, and emerge in early summer. 

Adult Gall

Grasslands etc. where Creeping Thistle is present.

It flies from May to August/September.

Widespread in the south of England.

Fairly common in Leicestershire and Rutland