Sunday, May 11, 2014
Oak apple and Currant galls
This oak, just coming into leaf beside the river Tyne at Wylam yesterday, was carrying both oak apple galls and currant galls.
The big, spongy oak apple is caused by the tiny wasp Biorhiza pallida, which lays its eggs as the base of an oak bud and causes it to swell into this large gall that's home to up to 30 of the gall wasp's grubs. After it matures in July the hatching adults chew their way out, leaving distinctive exit holes.
Oak apples contain either all male or all female wasps but when they emerge they mate and the fertilised females then lay their eggs on the oak's rootlets, when they form a much smaller woody gall. Only females emerge from these and in late winter climb the oak's trunk and lay eggs to form the next generation oak apples.
The small red, spherical galls dangling under the oak apple are currant galls formed in the catkin of the oak, when the gall wasp Neuroterus quercus-baccarum lays its eggs in the male flowers. Each currant gall contains a single grub and either males or females hatch in June. After mating the female lays her eggs on the underside of the oak leaves, where they form the familiar spangle galls that cover the lower surface of the leaf in autumn.
Spangle galls drop off shortly before the leaves fall in autumn, then in spring females emerge and lay eggs in the oak's catkins, producing currant galls again.
For more pictures of plant galls click here