Following on from yesterday’s More galls, part 1 …

This second gall’s host is not a tree, it’s a thistle, Creeping thistle (Cirsium cardui) to be precise, and the gorgeous little fly that causes the gall is Urophora cardui, the Thistle gall fly, a member of the Tephritidae or fruit fly family. Interestingly, this fly has been introduced into Canada, from populations originating in Austria, Germany, France and Finland, in an attempt to control Creeping thistle, which is there called Canada thistle and is regarded as an invasive weed. The gall production initiated by the flies disrupts the development of flowers and also significantly reduces the vigour of the thistle by diverting nutrients into the gall.

In Britain, the adult Urophora cardui fly can be seen between May and September when, after mating, it lays its eggs on a nearby Creeping thistle. The larvae burrow into the thistle’s stem, often causing a single gall to develop to house several larvae. The larvae also pupate inside the gall, emerging as flies between June and October. I’ll be checking all the Creeping thistles I see for a glimpse of them.