If you see acorns with bizarre, alien-like growths all over them as summer comes to a close, don’t pick them up and break off a piece – unless you want larvae in your lap. Spotted by NOTCOT’s London-based editor Justine Aw, these strange-looking mutated acorns have been impregnated with eggs by the gall wasp.
The growths occur as a chemical reaction in response to the wasp laying its eggs. The larvae develop inside, turning into full-grown females which will emerge from the Knopper galls in the spring. The cycle of the gall wasp depends on a sort of parasitic relationship with two trees, the common oak and the Turkey oak, so Knopper galls tend to only be found where both of these trees grow.
As you might imagine, this relationship isn’t exactly good for the trees. They interfere with the trees’ own growth and reproduction. The wasp was introduced to the British Isles in the 1960s and can now be found well into Scotland. See lots more close-up pictures of a Knopper gall dissection on NOTCOT.