Most galls are caused by fungi (particularly rusts and smuts) or invertebrates. Prominent among the latter are aphids, mites, psyllids, gall-midges (Cecidomyiidae), gall-flies (Tephritidae), gall-wasps (Cynipidae) and sawflies, but a wide range of other invertebrates are included. Galls can also be caused by viruses, bacteria and phytoplasmas.
Where can I find galls?
Almost anywhere that plants grow. Galls can occur on the stems, leaves, flowers, fruits, seeds and roots of plants, and a wide variety of plants are affected, including many very common species. Oaks and willows are particularly rich in galls.
That’s all very interesting, but are they actually GOOD for something…?