Look on oak trees in autumn and you may see a variety of strange growths known as galls. Galls are caused by changes in the oak’s tissues, in response to the activities of small gall wasps.
On a single leaf you can see different-looking galls that are produced by different species of these tiny wasps. However, exactly how they provoke these specific responses from the same tree isn’t known. What is known is that they provide food and shelter for the gall wasps.
Many oak gall wasps have alternate sexual and asexual generations. One of the commonest oak-leaf galls is the spangle gall, which looks like a tiny flying-saucer. In some years this can cover the underside of the leaves.
Spangle galls are made by the grubs of the asexual generation of the gall-wasp Neuroterus quercusbaccarum. Each gall, containing a single wasp grub, falls from the tree in autumn and overwinters in leaf-litter.
The wasps that emerge in spring lay their unfertilised eggs on oak catkins to induce currant galls. In summer the currant galls produce a sexual generation of male and female wasps, which create the next batch of spangle galls.
Alongside spangle galls you may see silk-button galls, which look as if they’ve been woven from golden threads and are made by the asexual generation of the wasp Neuroterus numismalis.
Galls can occur on the acorns too. The commonest of these is the Knopper gall, made by the asexual generation of the wasp Andricus quercuscalicis.
It looks as if the acorn has partly exploded and is strongly ridged. When the knopper gall first appeared in the 1960s, naturalists were worried that it would affect oak regeneration, but so far their fears have been unfounded.
On oak twigs look for the round marble galls, made by the asexual generation of the wasp Andricus kollari. Although each round woody gall contains one wasp grub, it’s also home to a range of other insects seeking food shelter or even the grub itself.
Top image: Peter O / Peter aka anemoneprojectors