Long time ago, I used to accompany my father in the woods. He worked for the French State Forests Department. He taught me all he knew about trees, birds, stars… He stressed the importance of biodiversity and sustainable development.
I enjoyed asking him the name of every plants and animals on our path. One day I found a small ball on an oak leaf. He explained that this was due to a parasite but was not putting the life of the tree in danger. However I was impressed by the name he put on this: oak gall.
Galls or cecidia are a kind of swelling growth on the external tissues of plants or animals. Plant galls are abnormal outgrowths of plant tissues, similar to benign tumors. They can be caused by various parasites, from fungi and bacteria, to insects and mites.
Plant galls are often highly organized structures and because of this the cause of the gall can often be determined without the actual agent being identified. On the picture above, the parasite is the gall wasp, also called gallflie (Cynipidae): Cynips quercusfolii.
Many parts of the tree can develop a gall of very diverse shapes depending on the parasite species and the season. Here on a small branch the parasite is Andricus kollari:
Inside the gall, the larva will be protected and find food during its development. But the gall is also a form of protection for the tree as it insulates the parasite.
If opened, the larva is to be found in the middle of the gall:
Some Cynipidae have two generations every year and two different galls like Neuroterus quercusbaccarum on white oak or Plagiotrochus quercusilicis on Kermes oak.
Other trees can have galls, namely beech, lime tree, elm, dog-rose and maple, like this one, largely infected: