Marble Gall

• Its galls, rich in tannins, were for centuries the raw material of iron gall ink, with which manuscripts were written and copied across Europe from the Middle Ages to the nineteenth century.
• It alternates generations: a minute sexual one on one oak species, and another made up only of females that reproduce without males on a different species, a cycle that baffled entomologists for decades.
• Each gall may also house other parasitic and inquiline wasps, forming a community of dozens of species inside a ball the size of a marble.
• Its history in Great Britain is a case of deliberate introduction for industrial reasons. It was introduced in the 1830s because its galls, rich in tannins, were used to make iron gall ink and for tanning and dyeing.
• That ink was Europe's writing ink for more than a thousand years: the ink of medieval manuscripts, of Magna Carta and of a large part of historical archives. And over time it has become a conservation problem, because its acidity corrodes paper.
• It has alternating generations on different hosts: a sexual generation on the Turkey oak, Quercus cerris, and an asexual one on European oaks. Without Turkey oak, the cycle cannot be completed.
• That explains its spread: as Turkey oak was planted across Europe, the wasp was able to colonise whole regions where it had not been before.
• The gall is not a built nest: the wasp chemically manipulates the development of the oak bud so that the tree itself builds it a chamber and feeds it.
• Each gall normally houses a single larva, and dozens of species of inquilines and parasitoids later move in.

Skeleton type

Invertebrate - chitin exoskeleton (insect)

Habitat and environment

Oak and holm oak woods of Europe and the Mediterranean

Gestation / incubation

A heterogonic cycle with alternating generations: the agamic generation develops in galls on Quercus robur and Q. petraea and emerges in September; the sexual generation needs Turkey oak (Quercus cerris) and matures in late winter

Litter / clutch size

A single egg per chamber: each spherical gall contains one female larva of the agamic generation

Sources consulted: Stone et al. (2002), Annual Review of Entomology · British Library, conservación de tinta ferrogálica · Buglife · Ficha de especie de Andricus kollari · RHS, guia de avispas de las agallas del roble · Ficha de especie de Andricus kollari (Wikipedia/iNaturalist)

Best time: The spherical galls are green in summer, ripen in August and stay brown and woody on the tree for years; the agamic adults emerge in September through a circular hole