Thistle gall: what is it?

Wending my way to the Earley Gate on the evening of Thu. 13th, I found a small plant of Creeping Thistle (Cirsium arvense) in grassland with Lady’s Bedstraw (Galium verum) not far from the Psychology building. What caught my eye was a swelling in the thistle’s  stem about 3/4 of the way up. I’ve seen similar swelling on thistles several times before but this time I was inspired by the `Galls on Campus’  blog to investigate further….

The gall (if such it be) was a “prolate spheroid” (like a little rugby ball)  approx 2 cm long and  1.5 cm across. I took it home to look inside.  Just below the soft, green surface it had an amazingly tough, hard and woody shell (protecting the content from birds?) With a lot of pressure I managed to section it with my penknife.

The white splotch at about 8 o’clock from the centre of the gall was a fat, pure white maggot with a clearly-defined head. The body was a couple of mm thick but also only a couple of mm long…I fear it may have been sectioned along with the gall. So, any ideas? Is it a gall-insect? If so, which one?

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4 Responses to Thistle gall: what is it?

  1. pehatcher says:

    This is a good example of a thistle stem gall caused by the fly Urophora cardui (Diptera: Tephritidae), luckily this is only species that produces galls such as these on Cirsium arvense. It has a very interesting lifecycle: the adult female lays a batch of eggs in mid-summer between immature leaves in an expanding bud on the stem. The larva, on hatching (it hatches as a second instar larva, having already moulted once whilst still in the egg), tunnels into the stem, skirting the meristem until it reaches the zone where the vascular tissue for the new shoot is about to differentiate. Then it moves to the inside of the new vascular tissue and excavates an individual chamber. The gall tissue differentiates around this, new vascular strands (connected to the plant’s vascular system) form around it and patches of lipid- and protein-rich nutritive cells form near the larva, in response to larval feeding. The entrance tunnel made by the larva closes up with callus produced by the damaged cells, and so the larva is walled into its gall, which by September has become lignified and woody. There are often several larvae (often four, and up to 12 have been recoded), each in its own chamber, in one gall. There is some evidence of cooperation between larvae, rather than the competition you might expect – the more larvae the bigger the gall, the larger the sink on the host plant (which can become very stunted), and the larger the individual larvae become. The larger the larva the better chance of survival and reproduction it has.
    However, the larva has one problem, how does it get out of this enclosed woody gall? Your larva looked pretty well-grown, it will shortly pupate in the gall and when the thistle dies back the gall will fall to the ground. Here it overwinters, and the fly relies on the gall, and in particular the callus tissue filling the larval tunnel, becoming rotten during the spring thaw. If this does not happen the young adult fly cannot escape the gall, and dies within it.
    This species is patchy distributed in the southern third of the UK, but is gradually spreading north. Because it looks impressive, and can stunt thistles, it was introduced into Canada in 1974 as a biocontrol agent against creeping thistle, which is a serious pasture weed there. Although it spread, it did not cause enough damage to control the plant. I have also read that the galls used to be carried in the pocket as a charm to ward off haemorrhages!

  2. drsol says:

    Thanks for the identification, Paul! I looked up the adult fly, which is a pretty little thing with black-banded wings. From your account, I guess the second dark patch above the midline of the gall could be the remains of an already-pupated larva. (It did seem to contain fragments of a pupal case, also somewhat the worse for my violent intrusion).

    • pehatcher says:

      Yes, some of the adults in this fly family can be quite pretty – they are sometimes called map- or picture-winged flies. I think you are right, there does appear to be two chambers visible in your sectioned gall. These flies pupate inside their last larval skin, using this as a cocoon, called a puparium. This makes telling larva from pupa rather difficult, if it is not feeding it is probably a pupa.

  3. Back in 2010 I got some photos of Urophora cardui in Whiteknights Park. See here.

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