Round clusters of marble shaped ones on the branches. Disc shaped ones on the under side of the leaves. Bizarrely shaped green ones on the acorns. Oaks do have a lot of galls associated with them. The most recognisable and common of these are caused by by the oak cynipids (Cynipini). These are wasps that develop as obligate herbivores in galls that they initiate on various growing parts of oaks (Hayward and Stone, 2005 Basic and Applied Ecology 6 pp. 435-443). The Cynipini lifecycle is cyclically parthenogenic, involving obligate alternation between a spring sexual generation and a summer/autumn parthenogenic generation (Stone et al., 2002 Annual Review of Entomology 47 pp. 633-668).
Occurs in Pedunculate Oak (Quercus robur). Caused by Andricus quercuscalicis. Subsequent generation develops in galls induced in catkins of the Turkey Oak (Quercus cerris).
Occurs in both Pedunculate Oak and Sessile Oak (Quercus petraea). Formed by Neuroterus quercusbaccarum, they appear as discs on the underside of the leaf. Subsequent generation develops in galls induced in oak flowers.
Occurs in both Pedunculate Oak and Sessile Oak. Form clusters of marble like galls caused by Andricus kollari. Subsequent generation develops in galls induced in bud leaves of the Turkey Oak.
Gall function may be either for food or protection. The gall may provide enhanced nutrition over other feeding modes, it may protect the larva from unfavourable environmental conditions or it may protect the larva from attack by natural enemies (Stone and Schonrogge, 2003 Trends in Ecology and Evolution 18 pp. 512-522). It is likely that it may be a combination of all three.
The mechanism for production of galls by Cynipini is not fully understood. Gall production by bacteria involves the bacterium exporting plasmid DNA, with gall induction resulting from host expression of theses genes (as in the case crown gall tumors induced by Agrobacterium tumefaciens) or the production of signal molecules such as nod factors (Rhizobium spp.). Cynipini larvae also secrete gall-inducing stimuli, yet their mode of action is unclear. What is known, however, is that gall morphologies represent the extended phenotypes of galler genes.