Creeping thistle usually occurs in large masses and so spotting something different about one of them is not always easy. Taking a close look at thistles is not everyone's idea of an afternoon either so there is a pretty good chance you have never seen this very common gall or, if you have, you probably did not give it a second thought!
This gall that occurs on creeping thistle is the work of a picture wing fly called Urophora cardui. It is an attractive fly with black and white wings, the problem is that it is very small and difficult to see let alone recognise. The eggs are laid on the shoots of young creeping thistle plants and swell up as the plant grows. Inside each gall is a number of chambers and a lava can be found in each chamber. As the plant flowers so the gall hardens and becomes woody and the larva spend the sinter safe inside. In the spring the gall disintegrates and the now adult flies escape and the the process starts all over again.
It is quite common and once you know what it is it becomes easier to find!