This small fist-like ball on the end of this fern frond is caused by a fly called Chirosia grossicauda. The larvae tunnel into the central veins of the pinnules in late summer and cause them to roll downwards from the tip. The solitary white maggot feeds on the main vein by mining. Mature larvae most likely pupate in their galls. Widespread and fairly frequent in Britain.
I have always been fascinated by these strange yet sometimes most beautiful growths. Plant galls are something of an oddity when some folk first encounter them, others don’t even know they exist. Plant galls come in all shapes and sizes, and are formed by another organism using the plant as a host, using it for shelter and for food.
They are caused by insects or mites, fungi or bacteria, and cause a biological reaction within the plant which causes these odd lumps and bumps to form of their tissues. They affect both herbaceous and wood plants, and there are at least over 1,000 species in Britain alone.
It is most unlikely you would see the mite or insect which causes the majority of these galls for they are very small, some even microscopic, but the species can be identified by the galls they produce.The study of plant galls is called cecidology.
This strange yet beautiful growth is the result of a tiny gall wasp called Diplolepis rosae laying its eggs in a wild rose bud in springtime. Also called the ‘Bedeguar Gall Wasp’, the females appear in the spring just in time to lay their eggs in the fresh young buds. Males are a rarity, and most females lay fertilised eggs without mating.
The gall mainly grows on the stem of the plant, and it can spread up to 7cm across. The gall has a woody core each surrounded by branching red or green hairs. The core usually has multiple chambers in which each a wasp larvae develops. The galls turn brown in the autumn and lose many of their hairs.