Monday, 8 September 2014
Thistle amaze you
I saw this swelling on a thistle stem today at Filnore Woods. Looking it up I discovered it is a gall caused by the Thistle Gall Fly (Urophora cardui). The eggs are laid in the thistle stem during summer and a fat brown grub develops. The thistle supplies it with food by growing this gall.
I've never seen this fly but found a picture of it on the NatureSpot website. Thanks to photographer Barbara Cooper for the image below.
These clourful little flies are about the size of a house fly and are around between May and September. The larvae grow inside the gall and overwinter inside it.