The mighty oak
Nature Notes for 20th November, 2014
Our oak tree is bare: all that luxuriant summer foliage in the compost bin or wind-blown to the edges of flowerbeds. Sad – but it does allow a clearer view of the birds that use the tree. A friend told me how she’d looked out of her flat window and seen a jay in the large tree outside. Jays are much in evidence at present and what handsome birds they are. Their brown plumage has a distinctive pink tinge and the bright blue flash in the wing and the speckled black and white punk hairstyle give them a jaunty air. We are currently seeing two in our garden. I suspect they are the gawky adolescents that hopped long-legged around the lawn in the summer, now grown-up and, presumably, knowing all about caching acorns.
They’ll be disappointed here, though. After producing big acorns in 2013, our oak seems not to have fruited this year. I’ve heard reports of similar failures from friends elsewhere but it’s too early to say whether this is widespread. It’s quite normal for oaks to have good and bad years. However, something else has been happening in our tree. Instead of acorns, we have been finding strange spherical objects –brown and lumpy – rather like those posh chocolates you see advertised on TV being handed round on a silver salver – but very hard. I have two in front of me as I write, one cut in half to reveal a cavity and a hard round inner gall and, inside this, the soft white body of a tiny grub.
It turns out that these are Knopper galls, the work of a parasitic wasp. I knew about oak apples, but this gall was new to me. The wasps lay their eggs at the bud stage of the incipient acorn. As the larva develops it feeds on the tissue and distorts the acorn into a lumpy mass that hardens and darkens. The adults emerge in the following Spring and – here’s the fascinating bit – they’re all females. Without needing to mate, they seek out a Turkey oak and lay eggs on the catkins. These form galls as they develop into a sexual generation needing a native oak like mine to complete the cycle.
One summer’s missed acorns is unimportant in the long life of an oak. Its progeny, new saplings of assorted sizes, are already visible in the garden – thanks to earlier generations of jays.
Wharfedale Naturalists Society
Photo by – Evelyn Simak [CC-BY-SA-2.0 (http://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons