I was having a mooch around an area of rough ground on the edge of some local recreation grounds when I spotted a healthy and robust-looking Spear thistle, so wandered over for a closer look as thistles often provide a home for interesting insects. And I was in luck for there, on top of the first flower I peered at, was this handsome little gall fly, Urophora stylata.

This was my first sighting of this species but, fortunately for me, the Urophora gall flies have quite distinctive patterns on their wings and they each have a preference for a particular species of thistle. Unfortunately for some of you, this gall fly is mostly seen in southern Britain but, according to my internet research (see, for example, this guide from the Montana State Government), it is also present in parts of North America, where it was introduced as a biological control for the Spear thistle, which has been classified as an invasive species in some locations.

After mating, as per my voyeuristic photo above, the female fly lays her eggs, as shown below, after inserting her sturdy ovipositor into the flower heads of the Spear thistle (Cirsium vulgare). The actions of the subsequent larvae cause woody galls to grow within the flower head. The Naturespot website says that these galls are not obvious to the naked eye but ‘can be felt as hard lumps by squeezing dead flower heads’. How to explain that to a passing stranger who politely asks what you’re doing?