A gall formed in Quercus robur by the unisexual generation of the wasp Biorrhiza pallida. Wingless females arise from root galls and climb the trunk of the tree in spring to lay many eggs in the bases of leaf buds. The eggs cause the formation of multilocular, pale pink, spongy galls that resemble small apples. The galls mature by midsummer and give rise to the sexual insect generation whose members emerge and mate in June and July. Mated females penetrate the soil around the tree and lay eggs on rootlets, giving rise to small, brown, rounded galls whose occupants emerge at the end of their second winter. May 29 is Oak Apple Day and commemorates the restoration of the English monarchy in 1660 and the birthday of Charles II; according to legend, the king once hid from his pursuers by climbing an oak tree.
Subjects: Plant Sciences and Forestry — Ecology and Conservation.