Spectacular year for oak leaf cherry galls

 I don't think I can recall a year when oaks hereabouts have hosted so many different kinds of galls, in such abundance. These oak leaf cherry galls, caused by the cynipid wasp Cynips quercus-folii, are growing on an oak near Blaids wood, just south of the city of Durham.

There will be a single gall wasp larva inside each, which will pupate and then emerge in late winter as a mature wasp, after the gall has spent the winter in the leaf litter.

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Sharing a leaf with a severe infestation of silk-button galls caused by Neuroterus numismalis, and a single pea gall caused by Cynips divisa

Sharing a leaf with spangle galls caused by Neuroterus quercus-baccarum

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