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?

This entry was posted in Asteraceae, Flowering Plants, Galls, Insects, Plants. Bookmark the permalink.

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