Why do acorns suddenly explode?

I noticed these curious knobbly acorns growing on some oak trees today. Below is a comparison of how the baby acorns should look at this time of year.

You probably won't be surprised to learn these growths - known as galls - are caused by the actions of a parasite. 

There are over 70 different species of gall wasp in the UK but they all do the same thing - they lay their eggs inside new acorns or leaf buds. The tree’s reaction is to increase the production of its growth hormones, which causes the deformation. Meanwhile the gall protects and feeds the wasp larva. 

If the wasp lays its eggs inside an acorn, you get an acorn cup gall (also known as a knopper gall) like the photo above. If it lays its eggs inside a developing leaf buds instead, you get a different kind of gall - a marble gall or oak apple. Here are a group of fresh ones from late last Summer, plus an old one I found on the ground today.

Oak galls, as I mentioned in a previous post about oak trees (here) were very important at one time. They are full of tannins and when they are ground up and mixed with iron sulphate you created a chemical reaction that produces an intense black. Mixed with gum arabic, it could then be used as ink. From the Middle Ages to the early twentieth century, iron gall ink was the main medium used for writing in the Western world.

 

Different species of wasp create slightly different galls. The common hard round brown marble galls are created by Andricus kollari. The slightly flattened spongey textured oak apples - usually brownish white, tinged with pink - are made by Biorhiza pallida. But you'll also see galls that are yellow or red and even one that looks like an artichoke. The acorn cup gall is made by Andricus quercuscalicis, which only arrived in the UK in the 1970s but is now very common and widespread. They'd certainly been busy on this tree.