English: Acorn knopper galls on oak. Knopper galls grow on acorns, and are caused by the gall wasp Andricus quercuscalicis [or is it Andricus quercusradicis?]. The group shown here includes one ungalled acorn, for comparison (this also shows that the acorns themselves are not yet very well developed). The tree on which these were growing was one of several oaks that stand next to an ascending footpath between two fields.

An acorn will sometimes be seen to have more than one of these galls on it; for example, several acorns at this location had two galls on them, pressed together and shaped rather like the two halves of a walnut. Because of the life-cycle of the wasp, appearance of the gall in a given area depends on the presence of both turkey oak (Quercus cerris) and pedunculate oak (Q. robur). On the particular tree shown in the photo, many if not most of the acorns were galled. Depending on their maturity, some galls were green, while others, like those in the photo, were tinged with red; as they age, they gradually become brown.

They appear to be fairly common in this area, and can be found in several locations within a few kilometres of this point. This kind of gall is a fairly recent arrival in Britain, having first been recorded in 1962 in the south of England, from which time it has been gradually spreading northwards.

(Acorn knopper galls are not to be confused with acorn cup galls, which are caused by the gall wasp Andricus grossulariae, and, as the name suggests, usually grow from the cup rather than the acorn itself.)