Though there are many more galls to be found on oak trees than the six I have covered in this and my previous two posts (knoppers and artichokes, and currants and spangles), I’ll make this the last lot for now. Otherwise, it might be just too galling for words!
So, to finish off this mini series, today we have Marble and Apple galls. Let’s start with the Marble galls and another tiny wasp, Andricus kollari, which lays its eggs on the twigs of any species of oak. This causes small perfectly round spheres to develop on these twigs. The spheres start off green but brown with age and will often remain on the twigs for a year or more. You can tell that the wasp has fled its larval home when you see tiny holes in the sphere. And although these do look just like brown marbles, I’m not sure you could use them to play the once-popular childhood game – they’re a little too light to shoot with.
And so to Oak apples. With their basic green colour and pink tinges, these do resemble immature apples but their surface texture and spongy feel are all wrong. The wasp Biorhiza pallida is the culprit this time, and these ‘apples’ contain several larvae, not just one.
Like many such wasps, both the Marble gall wasp and the Apple gall wasp have a sexual and an agamic (asexual) reproduction cycle. I have not seen the sexual galls produced by Andricus kollari which, interestingly, are produced only on Turkey oak (Quercus cerris) – the Marble galls are produced by the agamic (all female, no mating required) generation. Oak apple galls are produced by the sexual generation of Biorhiza pallida: the agamic generation lay their eggs on the roots of oak trees, so I haven’t seen those yet either.
I find the whole concept of two different types of reproduction and, indeed, the way these wasps can cause such galls to form very intriguing!