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Oak galls

A gall is an abnormal growth on a plant that is made of plant tissue, but created by another organism. Galls can be caused by insects, bacteria, mites, fungi or viruses. The organism triggers the reaction in the plant which then causes it to grow a gall. Curiously, it is not the laying of the egg which causes the gall, but the hatching of that egg. (Robin Williams, "Oak-galls in Britain"). A gall is used as a source of food and shelter for the developing larva inside, but can also host other insects called inquilines. They use the existing gall to lay eggs and use the gall's food storage, while the initiating insect is still in residence. In most cases, they share available supplies with no harm caused to either of the species. However, more damage can be done to the insect's larva when the parasitoids gain access to its gall, as they use the insects in the gall as a direct source of food and eventually cause death.

I’ve come across oak galls during my art residency with the Museum of Loss and Renewal this summer. I was very fortunate to spend two weeks in Abruzzo and Molise region in Italy, which gave me plentiful opportunities to explore and study local flora. Despite the abundance and richness of the biodiversity in the area, I could not focus on any other species in the same capacity as on those beautiful gall formations, which once noticed, seemed omnipresent.

Once back in Orkney, I continued researching, and drawing various oak galls with the prime focus on: oak marble galls, oak apple galls, and oak artichoke galls. Depending on the species of wasp, the galls can vary from small disks to large ridged woody growths, with some wasp species going through two parts of their life cycle and producing different galls in different parts of the tree, depending on the stage of their life cycle. Oak galls have a high tannin (gallotannin and gallic acid) content inside to protect the larva, especially during the colder months. It is a substance that was used throughout the world to produce traditional medicines, hair dyes and tanning agents, including the manufacture of gall ink. From around the 5th century to the early 20th century, crushed oak galls were mixed with water, iron sulphate and gum arabic to make gall ink, which was the main medium for writing and drawing in the Western world. Medieval monks relied on gall ink to copy many of the surviving manuscripts from the Middle Ages. Old music scores, drawings, letters, maps, wills, book-keeping records, ship logs, etc. were produced with gall ink. (

Oak tree has been widely present throughout the major cultures of Europe, where people have held it in high esteem. The Gaelic word for oak is 'darach' and remains in place names such as Glac Daraich (oak hollow/small valley) in Glen Affric. Almost every part of the oak tree had a symbolic, folkloric, medicinal or industrial use. Mistletoes, which frequently grew on oak trees were seen as the most potent and magical plant by the Druids. Oak leaves were added to the ancient kings' crowns as a symbol of the god they represented as kings on Earth. Successful Roman commanders were presented with crowns of oak leaves during their victory parades. Oak wood was praised for its strength and durability and became a part of the distinctive Tudor timbered houses, artists used the wood for carving and turning. The bark, with its high tannin content was used in the leather tanning industry. During the Industrial Revolution large amounts of the oak bark were sent from north-west Scotland to Glasgow for this purpose. Harness sores on horses were treated with a tonic extracted from boiling the oak bark. Oak galls were used to create ink. Children would wear oak leaves (or oak apples) as part of a custom which officially lasted until 1859. In fact, some texts mention that the tradition continued well into the 20th century, drawing again the connection between the symbol of oak leaves and royal traditions. (

Oak marble galls (created by Andricus kollari wasp), can be found mainly on oak twigs, with a woody, marble-like, spherical shape (often seen in clusters). They can be also referred to as ‘oak nuts’. Inside the gall, the larvae of the wasp feed on the tree’s tissues, with very little damage caused to its host. In Autumn time, as the oak marble galls mature, they turn from green to brown, with tiny holes, from which the asexual adults escape. The empty galls often stay on the twig and can persist for several years. Oak marble galls have alternate generations -asexual and sexual.

The asexual (female) wasps form a hard and round marble gall on English or Sessile oaks. The alternate generation is fully sexual, with males and females performing their usual role, while the tiny bud-gall is created on a Turkey oak. The sexual female leaves the Turkey-oak and flies to a Sessile or Pedunculate oak, where she lays her egg in a leaf bud, on a twig. The hatching of the egg stimulates gall growth and the larva feeds on surrounding gall tissue forming a smallish central cavity, before pupating and then emerging in the pitch-black of the interior. The marble gall has an extremely hard exterior and the adult insect has to bite her way through this before emerging onto the surface. Her first action is to clean off the debris and sawdust before flying off to look for the Turkey-oak, where she lays an egg inside a bud to start the whole process once again.

Andricus kollari wasp was introduced to England from the Middle East around 1840 by a Victorian entrepreneur for the manufacture of inks, dyes and cloth dying. Within 40 years, the wasp had spread to the north of Scotland, and it is now found across the Great Britain.

Oak apple gall (formed by Biorhiza pallida tiny wasp) resembles small, slightly flattened apple, up to 40mm in diameter. Brownish in colour, with a hint of white and pink, they have a soft, spongy texture and appear in spring on oak twigs. They tend to be redder when young, and turn brown as they age and the tannin level increases.The internal side of the oak apple gall is divided into a number of chambers, each housing a larva which eats its way out. Males and females emerge in mid summer and eggs are laid on oak roots. The next (asexual) generation produces marble-like galls on the roots, from which females emerge in late winter to lay eggs in buds on the twigs.

Oak artichoke gall (caused by Andricus fecundator) appears after wasp lays a single egg in a young leaf bud. At the start the larva develops inside a shiny brown gall, with hardy texture, which looks like a miniature acorn. The galls get bigger during summer, and when they reach their maturity, the enlarged galls look just like tiny artichokes. There is a cavity inside containing the inner gall which is ejected in late summer. The single larva that is housed inside will pupate in the fallen gall, emerging next spring.The following generation in spring develops small, hairy pale green and brown galls on the oak's male catkins. The gall wasp which emerges from the gall chamber in spring will always be an asexual female. She will go on to lay a solitary egg in the male flowers of the oaks, which will cause the formation of the 'hairy catkin galls.'

"Oak-galls in Britain" by Robin Williams (


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