Can anyone tell me what this swelling is on the stem of creeping thistle, Cirsium arvense. This is last year's plant and the swelling is around 30mm long and 15mm wide at it's widest point. Answer next week....
And another quick apology for the delay in answering this (see the last quiz!), but it does provide you with more thinking time for each ID conundrum!
So, as you may have guessed we have another gall here, this time caused by a species of true fly, or Diptera to be scientific, called Urophora cardui or commonly, the thistle gall fly. And what a pretty little creature it is too, a member of the family Tephritidae, otherwise known as the picture-winged flies, they are characterised by their pigmented patterning on their wings.
The female flies lay their eggs on the stems of creeping thistle, Cirsium arvense, during the plant's growing season. The larvae remain inside the egg until they are ready for their first moult (skin shedding in order to grow), they then hatch shortly afterwards and the larvae burrow into the stem to feed. Here they complete one more moult and proceed to overwinter in their third instar (an instar is the period between each moulting). In the spring, the larvae pupates inside the gall and metamorphoses into the imago (adult) fly. The imago then emerges through a pre-prepared hole in the gall in June - October.