Canada Thistle Gall Fly
by Ian Bushell
This afternoon, I found these galls on the Creeping Thistle in the second set-aside in Village Green. They are caused by Canada Thistle Gall Fly, Urophora cardui. This is a very distinctive fruit fly which, despite its name, is indigenous to the UK and Europe.
The process starts with an egg laid on the stem of the Creeping Thistle (Cirsium arvense). When the egg hatches, the grub burrows into the stem and forms a gall. The larvae grow and overwinter in the swelling and in early spring they pupate. The adult fly then chews its way out and escapes from the gall.
The adult fly is quite distinctive with its black body and white wings with a black W on them. They fly from May to August/September; now I know what to look for next year!