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Oak-galls | by Stoutcob
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Marble Galls (Andricus kollari)


"The Oak Marble Gall (also known as 'Oak Nut') is caused by a tiny gall wasp, Andricus kollari. Clusters of Oak Marble Galls can be found on oak twigs. They turn brown as they mature and emergence holes, from which the asexual adults have escaped, can be seen from autumn onwards. The empty gall is left on the twig. The emerging females then lay eggs in the buds of Turkey Oaks which develop overwinter and emerge in spring as a sexual generation of males and females, ready to make the familiar summer gall."


"Oak marble galls develop as a chemically induced distortion of leaf buds on pedunculate oak (Quercus robur), sessile oak (Quercus petraea) or their hybrid Quercus × rosacea trees, caused by the parthenogenetic gall wasp (Andricus kollari) which lays eggs within leaf buds using their ovipositor. The Turkey oak (Quercus cerris), introduced into Britain in 1735, is required for the completion of the life cycle of the gall. The oak marble gall is frequently confused with the oak apple gall, caused by another gall wasp, Biorhiza pallida. "


"There is often a whole community - a mini ecosystem - that develops within and around the gall. This is where some other fascinating players enter the stage. Many galls will host lodgers, which zoologists refer to as 'inquilines'. The term can be applied to many different members of the animal kingdom and comes from the Latin inquilinus, which means 'lodger' or 'tenant'.


The inquiline wasps are closely related to the true gall wasps, but unlike their cousins they cannot create galls. So they do the sensible thing and occupy an existing gall, rent-free! Some inquilines dwell fairly benignly in the tissues of the gall, only modifying their immediate surroundings, and with each occupant minding its own business. Others however, grow in the same chamber as the original occupant, outgrowing and smothering their reluctant 'landlord'.


Again, each kind of gall varies and some of them may have numerous original occupants, and many inquilines. However, before long, both the cynipid larvae and inquilines will need to watch out. Enter the parasitoid wasps... Parasitoids are different to true parasites in that whereas a parasite feeds from its host, usually without killing it, a parasitoid will occupy a host, eventually leading to the victim's death. In the case of the parasitoid wasps, they lay their eggs within the larvae of gall inducers or inquilines. As the invader's egg hatches, the larva develops inside the host grub, devouring it from within.


Naturally, the besieged occupants of the gall have had to evolve to resist such intrusions. In the later stages of the life of a gall, it will often develop a hard exterior, through a process known as lignification (lignin is the chemical compound that gives rigidity to wood). This makes it much harder for parasitoid wasps to penetrate the gall with their ovipositors.


The diverse structures of the galls themselves are largely a result of the need to ward off invaders... In some galls, the chamber is deep enough within the structure that it is just out of reach of the parasitoid. Others have an air space between the outer tissues and the larval chamber. This frustrates the efforts of the invading wasp, as its ovipositor can only penetrate the grub if it has structural support from the surrounding gall tissue. Where these hollows are present, the ovipositor bends and the eggs remain unlaid. One-nil to the cynipid!


Some gall wasps invest in numbers to ensure at least some of their offspring avoid being parasitised. Galls such as the oak apple have numerous chambers within them. While some of the larvae on the periphery may be found and parasitised by an invading wasp, it can't attack all of them, especially those right in the centre. The invader leaves contented and many of the gall wasps still hatch.


It's not just the parasitoids that cynipids have to be aware of. Fungi are ever-present in the forest, and if they invade and decompose the gall, the cynipid larvae will not survive. This is where the tannins come in. Oaks, like many other plants, produce high levels of tannins. These chemicals protect the tree against decay, and also against browsing herbivores, since tannins inhibit the absorption of proteins by animals. In galls, however, the concentrations of tannins can be many times higher than they are in the surrounding plant tissue, which helps to prevent fungal attack, and in some cases wards off parasitoids and herbivores. Interestingly, this concentrated source of tannin has even been used by humans. The oak marble gall (Andricus kollari) was originally introduced to Britain because it yields a black dye, although it was found that the tannin content of galls grown here is actually too low for this purpose.


Even the most aggressive parasitoid is vulnerable, as there are bigger, hungrier mouths about. While effective against smaller foes, the tough lignin exterior of some mature galls is not enough to deter a great spotted woodpecker (Dendrocopus major), which will peck the gall open to extract the soft and juicy prize within. Other gall predators include rodents such as wood mice (Apodemus sylvaticus) and birds including great tits (Parus major)"


" The earliest documents written in iron gall ink on papyrus date back into the first centuries after Christ. Because of it's indelibility, it was the ink of choice for documentation from the Middle Ages to the twentieth century...


Iron gall ink is primarily made from tannin (most often extracted from galls), vitriol (iron sulfate), gum, and water. It was also easily made; the ingredients were inexpensive and readily available. Good quality iron gall ink was also stable in light. It was very popular with artists as a drawing ink, used with quill, reed pen or brush. The coloring strength of iron gall ink was high and it had, depending on its manufacture, a deep blue-black, velvety tone. The range of objects that contain iron gall ink is enormous. Iron gall ink is found on manuscripts, music scores, drawings, letters, maps, and official documents such as wills, bookkeeping records, logs, real estate transactions, etc."

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Taken on December 6, 2014