The mystery of oak galls

1 WHAT MAKES ACORNS GO KNOBBLY?

Knopper galls, as they’re known, are caused by the larvae of a tiny wasp, first recorded in Britain in the 1960s. It belongs to a famous family of master architects, the gall wasps, which are parasites whose larvae develop inside plants. There are thought to be around 90 species in Britain and nearly all of them use oaks, giving rise to a weird and wonderful range of galls.

From ‘artichokes’ to ‘apples’, they’re found throughout the year.

2 BUT HOW DO GALLS FORM?

We used to think trees formed them as damage limitation, but now know the wasp larvae are effectively hijackers. They seem to secrete chemicals that somehow manipulate their host oak tree into producing abnormal growth. This includes snug chambers for the larvae, concentrations of nutrients in surrounding tissues for them to eat, and a tough layer of lignin to protect the gall. Many types of gall have extra defensive features, such as thickened outer layers, false internal chambers or spines.

3 DO GALLS HELP THE TREE?

Probably not. But some galls secrete nectar, which attracts ants. The ants might provide defence against leaf-eating insects, so in that sense could benefit the tree to an extent. And the wasp larvae aren’t alone in their safe houses. Galls can be home to rich communities, including the larvae of beetles, moths, other gall wasps, and other parasitic wasps and flies, not to mention fungi. Quite a crowd! So those species benefit, even if the tree doesn’t.