Oak Marble Gall

When: Year round, but more obvious in autumn and winter

Scientific name: Andricus kollari

Height: Up to 2cm in diameter

Where: Growing on oak tree branches

Galls are one of the stranger things you may find whilst out and about in the Scottish countryside. At first sight, it could be mistaken for a misshapen bud, or a gnarled piece of wood. But in fact, these galls are the product of a gall wasp, Andricus kollari, that uses the tree as a nursery for its young. 

Oak marble galls have been found on oak trees since the 1800s, thought to be an import from the Middle East. They are formed when the Andricus kollari wasp lays eggs in a leaf bud, injecting the bud with chemicals to initiate the growth of the protective gall. The structure is made up of plant tissue and acts as a shelter and food source for the next generation of wasps, which develop inside the woody growth. Initially, the galls start out as a small, green and spherical, hence the "marble" connotation, eventually turning brown as they age. Galls that have successfully hatched will have a small, woodworm-like hole, from which the fully grown adult wasp will emerge.

Different generations that hatch from the galls alternate between being asexual and sexual; the fresh, green galls that you are more likely to see in summer have been created by a wasp from a sexual generation, ovipositing eggs that will develop into asexual wasps. These asexual wasps are usually responsible for the galls that develop over the winter, which are more commonly found on Turkey oak trees.

Although there were concerns that these galls may cause negative effects on the native oak's growth in the UK, populations remain healthy and humans have managed to find many uses for them since their arrival in the UK. The presence of tannic acid in the galls means that when they are crushed and mixed with other chemicals, they form ink. This kind of ink has been found in some of the most treasured documents in human history, from Leonardo Da Vinci's drawings to Mozart's compositions, and many biblical and political texts in between! Crushed galls mixed with lard were also said to be an effective cure for piles. and to this day the tannic acid found in the galls is used in deodorants, thanks to its antibacterial properties. Who would have thought that the reproductive habits of such a wee wasp could have such an array of useful functions?

To find out more about the marble oak gall and the Andricus kollari wasp, visit https://en.wikipedia.org/wiki/Andricus_kollari

Image: Roger Griffith, Wikimedia commons