|Female Andricus kollari of the asexual generation|
Oak marble galls
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, (Hartig, 1843)) 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. Oak marble galls are also known as the bullet gall, oak nut or Devonshire gall. Synonyms for the species are Cynips kollari, Andricus quercusgemmae, A. minor, A. indigenus and A. circulans.
The physical appearance of the gall
The gall growth first appears as a rounded mass of green plant tissue on the leaf buds of the oak, later becoming hard and brown, being up to approximately 25 mm / 1 inch in diameter. Although nearly spherical, the galls often have a number of little flattened nodules. The rounded growths are filled with a spongy mass and a single wasp larva is located in a hard seed-like cell in the centre. The word 'marble' derives from the gall's shape, which is a marble-like rounded structure. As stated, although normally distinctive the oak marble gall can, under some growth conditions, be mistaken for the oak apple gall, caused by a number of gall wasps, such as Biorhiza pallida. This may be due to the observer's unfamiliarity with the true oak apple gall which grows to be somewhat larger, has red markings, but does grow on the axillary or terminal buds. The galls sometimes coalesce. The non-parasitised specimens are at the largest end of the size range.
The wasp life-cycle and arrival in Britain
The marble gall has alternating sexual and asexual generations, often taking two years to complete, especially in the north of Britain. The familiar summer gall develops from eggs laid by a sexual female in the developing buds of the two native oaks in May or June; the host trees often being immature or retarded, scrub-oak, specimens; they are rarer on older healthy trees.
The developing spherical galls are green at first, brown later, and mature in August. Each gall contains a central chamber, with a single female wasp larva of the asexual generation, which emerges through a 'woodworm-like' hole as an adult winged gall-wasp in September. These asexual (agamic) females lay unfertilized eggs in the embryonic bud leaves of the Turkey oak, with galls slowly developing during winter, and are visible in March and April as small oval structures between the bud scales, looking like ant's eggs or pupae. The emerging adult gall-wasps in spring are the sexual generation, producing both males and females, which fly to the common oaks to initiate the formation of the summer marble gall.
The abnormal buds develop during summer and the bud is wholly replaced by the gall growth. Marble galls may remain attached to the tree for several years. The level of attack by the insect varies greatly from year to year.
Gall forming insects
Some herbivorous insects therefore create their own micro-habitats by forming usually highly distinctive plant structures called galls, made up of plant tissue but controlled by the insect. Galls act as both the habitat, and food sources for the maker of the gall. The interior of a marble gall, formed from the bud, is composed of edible and other structural tissues.
Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts. Galls may also provide the insect, its parasites and inquilines, with a degree of physical protection from predators.
Predators, inquilines, parasitoids and fungi
Mature galls are sometimes broken open by vertebrate predators to recover the larva or pupa. Woodpeckers, such as the lesser spotted woodpecker (Dendrocopus minor), as well as other birds or squirrels have been suggested. In the territory of former Czechoslovakia, both bank voles and yellow-necked mice feed on larvae and pupae extracted from oak marble galls.
A number of insect inquilines live harmlessly within the oak marble gall and some of these, as well as Andricus itself, are parasitised by insects referred to as parasitoids. The chalcid wasp Torymus nitens is an example of a parasitoid in oak marble galls. The presence of these inquilines and parasites is often visible on older galls by the presence of fine exit-holes, smaller than that of the gall wasp itself.
A gall can contain the cynipid wasp as the host that made the gall; up to five species of inquilines (Ceroptres clavicornis, Synergus gallaepomiformis, S. pallidipennis, S. reinhardi and S. umbraculus) eating the hosts food; as well as up to thirteen parasitoid species (Eurytoma brunniventris, Sycophila biguttata, S. variegata, Megastigmus dorsalis, M. stigmatizans, Torymus geranii, T. auratus, Caenacis lauta, Hobbya stenonota, Mesopolobus amaenus, M. fasciiventris, M. sericeus, Eupelmus urozonus) living on the host, inquilines and each other.
Many old galls bear numerous dark brown excrescences, due to the fungus Phoma gallorum.
Uses of oak marble galls
The galls contain large amounts of tannic acid, which was used for making iron gall ink and for dyeing cloth. According to recent research, traces of iron-gall ink have been found on the Dead Sea scrolls and on the 'lost' Gospel of Judas. Iron-gall ink may have been used for 1,800 years, but it does not withstand the test of time well. Over the course of centuries, the ink fades, and discolours and damages the paper. Other waterproof formulae, better suited for writing on paper, became available in the 20th century. Iron gall ink is manufactured chiefly by artists enthusiastic about reviving old methods or possibly forgers of old documents.
A recipe for preparing the ink is as follows: Take one lb. of bruised galls, one gallon of boiling water, 5½ oz of ferrous sulfate in solution, 3 oz of gum arabic previously dissolved, and a few drops of an anti-septic, such as carbolic acid. Macerate the galls for 24 hours, strain the infusion and add the other ingredients.
Oak Marble Gall extract is used in deodorants because of tannic acid's anti-bacterial properties.
Infestations of oak marble galls
Removing and destroying galls before they dry and the wasps emerge from a hole may help to reduce the infestation. While fairly large, spectacular, and sometimes present in quite large numbers, they cause no measurable harm. The galls were the subject of considerable press controversy in the mid-nineteenth century when it was thought that the acorn crop would be ruined and its rapid spread would deprive farmers of valuable pannage (fodder) for their pigs.
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