Last Sunday I walked across the heath with my ex-next door neighbour, Matthew – he of the splendid garden at Troyes House that I visited back in the early spring. (See this post for a walk around it complete with some super vocal birds.)
As we passed the massive hollow-trunked oak tree near my path up to my new house that I wrote about back in May Matthew pointed out that among the proliferation of egg shaped acorns that now festooned its branches were a number that had sprouted tiny little ruffs from their pointy end. He said that they were oak galls.
As far as he could remember from an episode in David Attenborough’s 2010 Life in the Undergrowth there were a particular family of wasps who sought out acorns with a tiny blemish so that they could burrow inside and lay their eggs. Happy ensconced in their new home the larvae grow and multiply, feeding happily on the contents of the acorn. In due course, when they are ready to face they world they burst out of the acorn, scattering it to the four corners – and get on with their lives as wasps.
So, on the way back I stopped off to video the acrons and hopefully a wasp at work. Well, I got the video of the about-to-be blasted acorns – but the only wasp I could find seemed far more interested in grooming him/herself on a leaf than in laying eggs in an acorn. So sorry….
(If you want to see the videos you will need to click onto the blog as the email notification does not include the video.)
Anyhow, when I got home, I investigated a bit further – and the RHS were most helpful.
It appears that they are called oak gall wasps and that there are loads of different ones – around 70 in Britain alone. They have delightful names: Oak apple gall wasp, Oak marble gall wasp, Common (and smooth) spangle gall wasps, Silk button gall wasps, Oak cherry gall wasp, Knopper gall wasps….. They also seem to have rather complicated sex lives….
In general oak gall wasps alternate between generations that are either asexual (all females) or sexual (males and females). The generation that emerges as adults in summer has both sexes, whereas the generation that develops as adults in late winter-spring is all female.
They can also affect all different bits of the tree. Not just the acorns but the twigs, the underside of the leaves and the leaf margins. But wherever they make their homes, the oak trees seem totally relaxed about them so, as the RHS says:
As Oak gall wasps have little or no impact on the tree’s health and growth, control is not necessary nor is any available.
For a really amazing selection of pictures of the various different galls, see this post on the Woodland Trust site. This is their image of Lime Nail galls – courtesy of Roger Griffiths /Wikimedia.
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Fascinating! Never knew that. I shall look out for the frilly acorns now!
I see galls on oaks here in California all the time, but never see the wasp. My understanding is that they’re tiny.
I also see somewhat similar galls on maples trees as well…
Well, maybe that wasn’t a gall wasp at all Tom just a wasp…. Which would explain why it was just buzzing around and not attacking acorns. Actually, you are right – I have just looked up gall wasps and they are tiny – 1-8 mm only and look to be red and black rather than yellow and black. To quote Wikipiedia:
Like all Apocrita, gall wasps have a distinctive body shape, the so-called wasp waist. The first abdominal tergum (the propodeum) is conjoined with the thorax, while the second abdominal segment forms a sort of shaft, the petiole. The petiole connects with the gaster, which is the functional abdomen in apocritan wasps, starting with the third abdominal segment proper. Together, the petiole and the gaster form the metasoma, while the thorax and the propodeum make up the mesosoma.
The antennae are straight and consist of two or three segments. In many varieties, the backside of the mesosoma appears longitudinally banded. The wings are typically simply structured. The female’s egg-depositing ovipositor is often seen protruding from the tip of the metasoma.
Errr…. right……