Oak Galls

And finding things when you stop looking

Last Sunday I co-hosted a tree walk, right after we had the inaugural meeting of a community group I am setting up (along with some other wonderful people) called Wild South London Community. I'll share more about that over the summer so you can all get involved and join in the fun!

I spent the whole walk looking for oak galls to show people because I think they're fascinating. I’d also started to run out of tree facts, but I knew these little objects would fascinate people. My wise friend Andrew, who was on the walk, said: "You need to stop looking and you'll find one". Sure enough, as soon as almost everyone had gone home, I spotted a tiny oak tree out of the corner of my eye which looked like it might have some galls on, right by where we'd been standing at the end of the walk. I swore loudly and ran over for a closer look. What I found was not one, but TWO types of gall on this young tree. Typical.

Oak marble galls and Oak apple galls, spotted when I stopped looking

Here you can see Oak marble galls (the three brown balls close to the bottom of the frame) and Oak apple galls (the two to the right that look like…err… apples). Both are parasitic growths caused by a specific species of gall wasp. In spring, these wasps lay their eggs on a dormant leaf bud. This triggers a genetic change in the bud, causing it to develop into a gall. The tiny wasp larvae feed on the oak tree's tissues from inside the gall, which becomes thick and woody over summer and autumn. Eventually, the following spring, adult wasps emerge from the galls and the process starts all over again (hope I got all that right, but correct me in the comments if you know otherwise please!).

There are tons of other types of galls, and while the other one I’ve most often seen is also created by a wasp - the oak knopper gall (if you’ve ever seen an acorn that looks a very weird shape it’s likely a knopper gall) - some other galls are created by other insects, bacteria, fungi or viruses.

Marble galls were traditionally used to make ink and, as far as I am aware, if you're collecting galls for ink, you want to make sure you collect ones with a visible hole in, because this means you can be confident the wasp inside has departed. However, I just watched a video which showed a fig wasp with a giant zinc-tipped 'drill' (three times the size of its body!) that it used to drill into a marble gall and lay its own eggs. And now I don’t know what to think!  Nature continues to blow my mind and confuse me every single day…

Why not spend some time this weekend looking for galls? Or perhaps you should follow Andrew’s wisdom, and not look at all, it seems you may be much more likely to find some!

P.S. Look out for the Nature Chat thread this Sunday, where I’ll be asking you to share some of your own nature highlights from May.