Mutating trees, parasite danger, unwanted lodgers and every other generation 100% female – welcome to the bizarre world of the knopper gall wasp!

Knopper gall

Photo courtesy of Nature's Home reader Ann Collier 

Nearly all the oaks in the area where Nature’s Home reader Ann Collier took this photo were covered in these galls.

Galls are formed from plant tissue transformed by chemicals added by gall wasps. They come in a variety of shapes, such as round, hard oak marbles or hairy robin’s pincushions. Like the jays in last week’s photo gall wasps are closely associated with oaks. Of around 1,300 known types of gall wasp around 70% lay their eggs in these trees.

The galls in Ann’s photo are caused by knopper gall wasps. These get their name from the German word ‘knoppe’, meaning swelling or button, were first recorded in the UK around sixty years ago. They’re tiny insects, less than 2mm long.

Knopper gall wasps have an extraordinary life cycle: they need two different types of oak to survive and every other generation is exclusively female. We begin the knopper gall wasp story among the catkins of the Turkey oak. This originally comes from south and east Europe (including Turkey, or course!) and was introduced to the UK in the 18th century as an ornamental tree. You can tell a Turkey oak from our native oak trees by the acorn cups which appear ‘hairy’, and the pointed lobes of its leaves.

Turkey oak acorns and leaves

Turkey oak - photo by Alvesgaspar / CC BY-SA (

Small galls appear on the Turkey oak catkins. From these, male and female knopper gall wasps hatch between April and June.

They mate and the males die shortly afterwards. The females must then find English (also known as pedunculate) oak trees, and lay their eggs in the pollinated flowers where acorns will form.

Female knopper gall wasps laying eggs on English oak

Female knopper gall wasps laying eggs on English oak - photo by Sally Jennings / flickr Creative Commons

The larvae hatching from these will grow mostly protected by the gall that swells up around them. I say ‘mostly’ protected as there are several parasitic wasps which may inject their eggs into the galls too, and the young of these will eat the gall wasp larvae!

As well as parasites, other insects may move into the gall too: these include other tiny wasps known as ‘inquilines’. They may live alongside the gall wasp, but can also kill it to claim the gall for themselves. And they can also get parasitised by the parasitic wasps!

The surviving gall wasps, all female, spend the winter inside the hardened galls, emerging in spring. These can reproduce asexually – no males or mating! They’ll search for Turkey oaks to lay their eggs on, and the cycle continues.

So next time you spot one of these unusual growths on a tree, or amongst the leaf litter, imagine the drama that may be happening inside! As ever, nature never fails to amaze.

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