Wild in the City: The strange houses of the gall wasp
Galls are now big enough to be quite easy to spot, particularly on oak trees. The insects that create these strange little houses for their young laid their eggs on the buds, flowers and emerging fruits earlier in the year. Now many of the deformations caused by those wasps — the galls — are ready to fall to the ground.
The oak apple gall, a round, reddish growth around eggs laid on leaf buds by the wasp biorhiza pallida, may have already fallen as its young are on the wing from June, but the round houses can stay on the twig where they were laid much longer. Some, like the oak marble gall, stay on the tree all winter, long after the wasp has hatched. The woody outer shell protects the larvae from cold and harsh weather. This is a smooth, round, light brown gall with a dimpled surface. The insects will bite out a hole in the gall in the autumn.
Oak gall wasps, andricus kollari, are not native to the UK originally but were brought here from the Mediterranean in the 1800s because their galls are high in tannins and therefore useful for making inks. The US constitution was originally penned using oak galls. An even stranger-looking gall that also grows on an oak is the knopper gall, laid by andricus quercuscalicis. These wasps are tiny, but the nurseries their young grow in are big knobbly things.
Pedunculate oaks — the native species in the UK — can host hundreds of them, with the ground underneath the tree becoming very bumpy indeed. They grow on acorns so you will start to find them littering pavements, woodland floors and parks from now on.
Other trees host alien-looking galls. The weirdest is the lime nail gall, laid by the mite eriophyes tiliae. Chemicals injected by the insects into lime leaves cause them to distort and develop bright red tubular growths like painted fingernails. As with many other galls, they do no harm to the tree. In fact, they jazz it up even more.