Some of the most most complex and diverse galls found on oak trees are caused by tiny gall wasps (Hymenoptera, Cynipidae, Cynipini), with each species producing a characteristically shaped gall. I came across some nice little, currant-like galls this Spring, which were new to me, although very common it seems (see below).
These currant galls, as they are called, are induced by the cynipid wasp, Neuroterus quercusbaccarum, which has both sexual and asexual generations (something called heterogony) that cause morphologically different galls to occur on oak trees at different times of the year.
In April, parthenogenetic female Neuroterus quercusbaccarum (L.) (Hymenoptera: Cynipidae) wasps emerge from spangle galls (see below) where they have spent the winter, and induce the rapid formation of the small round currant galls, c. 5mm in diameter, which form around the egg (see below).
Currant galls are not restricted to leaves and may appear on male oak inflorescences, bracts, or young stems. Sexual males and females emerge from these currant galls.
Later on, in the summer, common spangle galls ( yellowish brown discs c. 4mm in diameter) appear on the the underside of leaves. In June, male and females (there are two types of females!) of the bisexual generation emerge from currant galls, and after mating, fertilized eggs are laid in the oak leaves, inducing the formation of flat, circular, spangle galls. These spangle galls usually appear in groups, with up to100 galls per leaf. These leaves fall to the ground, and are covered by leaf litter, which protects them from drying out and from frosts. Inside the spangle gall, the larva matures and pupates, ready to emerge as adults in the Spring.
Oak gall wasps (below) are tiny but can have a massive impact on the tree.
There are of course, may other types of galls on oaks, including oak apple galls.
Jankiewicz, L. S., Guzicka, M., & Marasek-Ciolakowska, A. (2021). Anatomy and Ultrastructure of Galls Induced by Neuroterus quercusbaccarum (Hymenoptera: Cynipidae) on Oak Leaves (Quercus robur). Insects, 12(10), 850.
Plantard, O., & Hochberg, M. E. (1998). Factors affecting parasitism in the oak-galler Neuroterus quercusbaccarum (Hymenoptera: Cynipidae). Oikos, 289-298.
Stone, G. N., Schönrogge, K., Atkinson, R. J., Bellido, D., & Pujade-Villar, J. (2002). The population biology of oak gall wasps (Hymenoptera: Cynipidae). Annual review of entomology, 47(1), 633-668.