Nature on our doorsteps: Galls galore

Galls are unusual growths that occur on the buds, stems, leaves and fruit of many plant species.

They are caused by an insect, or by a viral, bacterial, or fungal infection.

An obvious gall on Wild Roses is the Robin’s Pincushion Gall which looks like a clump of red spiky hairs.

This gall is caused by a very small insect, a gall wasp.

In May, the female gall wasp lays her eggs in a leaf bud. On hatching, the larvae emit a chemical signal which causes the bud to grow rapidly around them. The larvae grow and develop inside this safe place, emerging as adults the following spring.

Oak trees support very high numbers of insects, all of which nibble its leaves, buds, twigs, and acorns.

Oak Apple Galls provide a safe place for the developing larvae of the gall wasp

This diversity also includes different gall insects, all of which form particular types of galls.

The back of oak leaves are often peppered with the small flaky discs of the Spangle Gall, or the small doughnut shaped Button Gall.

The spherical Marble Gall and Oak Apple Gall occur in leaf buds and on twigs.

Most insect galls do not affect the general health of the tree.

Knopper Galls, however, are different.

These are caused by a fungal infection which distorts the development of the acorns.

This can seriously impact on the tree’s ability to reproduce successfully.