Nobbly knopper galls

Welcome to the fascinating world of knopper galls and their maker...

It's that time of year when you might spot something odd on an oak tree. If you think you know what an acorn looks like, you might want to rethink. Look again for those smooth, lime green oval nuts that sit in a rough cup and you might see some that definitely don't look how you'd expect them to. Welcome to the world of knopper galls.

Galls are abnormal growth forms that have been caused by a third party organism. The plant, in this case an oak, reacts to the presence of this third party by enlarging or proliferating its cells or tissues to provide both shelter and food for the gall causer. Perhaps even more amazing is the world in miniature that can exist within a gall, with parasites of the gall-causer, other species that eat the gall tissue and yet others that move in once the gall is empty.

Galls can be created by insects, fungi and bacteria but knopper galls are homes to the larvae of tiny gall wasps Andricus quercuscalicis. There are more than 30 species of wasp that create galls on oak trees; none damage the tree and although acorn crops can sometimes be reduced in a year with many wasps, this is cyclical and in some years there are far fewer wasps.

Abnormal growth of a knopper gall on an oak tree - knobbly green uneven growth of an acorn - with normal growth of acorns on the same branch (photo by Wendy Carter)

Female knopper gall wasps lay their eggs into the buds of developing acorns in early summer and the chemical signal of the hatching larvae prompts the growth of the gall. The acorn transforms into a larger, ridged structure that houses the developing wasps; you might find multiple galls on one acorn.

The galls are initially green and sticky but they become reddish then brown - look underneath oak trees for freshly fallen brown, woody galls. Inside are pupating wasp larvae, snuggled down in their protective cases until spring arrives. Each and every emerging wasp will be a female and these seek out the catkins of Turkey oaks on which to lay their eggs. A much less obvious gall is formed here but the wasps that emerge from this in early summer are female and male - it's this second generation of wasps that mate, lay and form the nobbly knopper galls on our native oaks. We've come full circle.