File:Currant galls on oak - geograph.org.uk - 1336489.jpg

English: Currant galls on oak These currant galls are translucent and berry-like, and measure less than a centimetre across. They often occur alone, but are sometimes seen in groups, as shown here. They sometimes grow on the catkins rather than the leaves; see 1908818.

They are caused by a Cynipid wasp, Neuroterus quercusbaccarum.

With such gall wasps, there is often an alternation of generations: one generation consists of males and females, and reproduces sexually; the other consists only of agamic females (i.e., they are able to produce offspring without mating).

The agamic females lay their eggs within the tissues of the oak, causing these currant galls to appear. The wasps that develop within the galls, and which later emerge from them, are a sexual generation of males and females; the mated females of that generation in turn lay their eggs in oak tissue, causing another kind of gall: 929354. A generation of agamic female wasps develop within and emerge from common spangle galls, and the cycle begins again.