What is an Oak apple gall?

This weekend I discovered and learned something new, Oak Apple Gall is now added to my vocabulary and stored in my visual library. This all started with a walk in the park on a sunny day in May. I was looking up at the trees I saw these weird shaped things hanging from a tree. I thought ‘that looks like a potato’. One had fallen on the ground, I picked it up and squeezed it, it felt smooth and spongy. I decided to take a few photos and then continued my walk.

A lot of people would have probably left it at this and just enjoyed the rest of the walk. Well… my brain doesn’t let me… So while enjoying the walk, in my head it’s going like: hmmm, weird potato shape, it looked like a fruit… or the start of new leaves, but why are some so big? I will spare you the details but this goes on in my head until I come home.

First thing I do when I’m home is take a good look at the photos. Then next thing I know, i’m on the internet finding out what tree this is (yes I’m one of those people that doesn’t recognise a tree by its leaves). It soon became clear this was an oak. So I started looking for ‘Oak tree blooming’ in Dutch and I soon came across photos of similar looking shapes as in my photos. That’s when I discovered these ‘things’ are called Oak Apple Galls (potato gall in Dutch).

What are they exactly and how do they get on the oak tree?

There are a lot of different galls. On the oak tree alone there are about 40 different kinds of galls! They can grow on the roots, leaves and bark. The one I spotted is caused by the Biorhiza pallida, one of the many gall wasp species that excist.

Their lifecycle is interesting as well. In summer the males and females mate, the females lay eggs on the oak roots. In the Spring these eggs hatch into agamic females (females that can reproduce without mating with a male). The agamic females can’t fly and have to climb all the way up to the young leave buds of the oak tree. There they lay eggs and inject venom as well in the leave buds. This venom causes the leaf tissue to swell and soften. The eggs hatch and the larvae secrete further substances that encourage plant growth and a globular gall is formed. The gall provides a nutritious, protective environment and there may be as many as thirty larvae developing inside! Males and females emerge from different galls after two to three months. And the whole process starts over.

The galls have been around for a long time as they were used as ingredient to make Iron Gall Ink. Iron gall ink is a purple-black or brown-black ink made from iron salts and tannic acids from vegetable sources. The tannic acids was usually extracted from the oak galls. It was the standard ink formulation used in Europe for the 1400-year period between the 5th and 19th centuries and is still sold today.