Marbles on oaks
Midwinter is a very revealing time. With most trees and shrubs stripped of their leaves it is easier to see birds, the shape and architecture of the trees and various other things which are obscured at other times of the year.
Often prominent now are marble galls on oaks, especially young and scrub oaks.
As their name suggests marble galls are hard, round and about two centimetres in diameter.
They are brown and have a generally smooth surface. They are very easy to spot on small branches, sometimes singly but often in groups of three, four or more.
Some saplings appear to be covered in the galls whilst adjacent trees are completely free of them.
The galls are caused by gall wasps, the females of which lay their eggs in buds on our native oaks in the spring.
When the larva emerges it feeds on the bud tissue and the oak tree reacts by producing the gall which is at first soft and green.
The larva sits in the middle of the marble happily munching away. Later in the year it pupates before emerging as a female wasp. It lays eggs (without the need to mate) in the buds of Turkey oak (a species introduced to Britain about three hundred years ago) where much smaller galls form.
Both male and female wasps emerge from these galls, and after mating the females return to native oaks to start the whole cycle off again.
We owe the presence here of the marble gall to the leather and printing trades. Many galls, especially those on oaks, concentrate tannins in their tissues. For centuries certain types have been collected and traded for use in inks and in tanning. Someone thought that marble galls would be a good source of these tannins and deliberately introduced the species to Britain in the 19th Century.
Any attempt to do this earlier would have failed because, as outlined above, the wasps need the introduced Turkey oak to complete its life-cycle.
The wasp prospered, and from its original stronghold in Devon quickly spread throughout the United Kingdom. It did so well so quickly that a campaign was mounted for its eradication.
It was thought that it would lead to the extinction of oaks and that this, in turn, would affect pig farmers who relied on acorns for autumn fodder for their animals.
As it happens the oaks seem to survive the attentions of this, and many other similar gall wasps, without significant harm.
It is also the case that those responsible were mistaken in thinking that the galls were a good source of tannins - marble galls do have some but not enough to make them commercially viable when compared to other sorts, although they are useful for small scale ink making.
So the oak trees are surviving, the pigs are fed, and we are left with a natural history curiosity to enjoy.