Andricus quercuscalicis

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Andricus quercuscalicis is a small gall wasp that makes knopper galls on acorns of oak trees. It has a two-phase life cycle that uses two kinds of oak: Turkey oak (Quercus cerris) and pedunculate oak (Quercus robur).

In spring, females lay eggs on the male catkins of Turkey oak. The eggs hatch without mating (parthenogenesis) into a generation that includes both male and female wasps. This generation mates, and in autumn the females lay eggs on pedunculate oaks, where the knopper galls grow on the acorns.

The knopper gall is about 2 cm across and starts as greenish tissue that swells into a ridged mass between the cup and the acorn. If many larvae are inside, the galls can become very contorted; with fewer larvae they may be simpler folds. Some knopper galls can look like acorn cup galls caused by another wasp, Andricus grossulariae.

Inside knopper galls live other insects (inquilines), and some of these, along with A. quercuscalicis itself, are attacked by parasitoid wasps.

This species spread across Europe after Turkey oaks were planted widely and reached Britain and Ireland in more recent centuries. It likely arrived naturally rather than being introduced. Knopper galls mature in autumn and fall from the tree; the adult female emerges through a vent at the top of the gall in spring. The level of attack varies from year to year. While knopper galls can reduce acorn fertility, they are not considered a serious threat, and no control is usually needed.