Gall on Yellow-cress

There’s a large population of Creeping Yellow-cress (Rorripa sylvestris) on the shingle banks of the Tawe River near Pontardawe. This year most of the plants in the population have been infected with the gall midge Dasyneura sisymbrii, which also forms similar galls on Winter-cress (Barbarea vulgaris) and Hedge Mustard (Sisymbrium officinale). The gall is roughly spherical, creamy coloured and about the size of a small pea. Galls are fantastic things. They can be caused by viruses, bacteria, fungi , mites and insects such as beetles, aphids, midges , sawflies and gall wasps. But whatever the causative organism, the plant is stimulated to grow abnormally (like a tumour) to produce a distinctive structure that gives protection and food for the gall former. Gall midges are flies (two-winged insects) whose larvae feed and develop inside the gall structure.