These are knopper galls, caused by the knopper gall wasp (Andricus quercuscalicis). It is a small solitary wasp which arrived in Devon from the Channel Islands more than 50 years ago and has gradually spread north. I have noticed the gall in recent years but this year it looks as though almost every acorn is affected.
Andricus quercuscalicis has a very strange life cycle. The first generation develops in spring in small conical galls on the male catkins of the Turkey oak (Quercus cerris) and a second parthenogenetic (all female) generation in the acorns of English oak (Quercus robur). As I have several mature examples of both trees in my garden it is perhaps only to be expected that the knopper gall wasp would thrive here.
The gall develops in the summer after the wasp lays an egg in the developing acorn. The gall mainly or completely replaces the normal acorn.
I cut this gall in half last month to show the larva inside.
In the past week or two the galls have turned woody and brown and are beginning to fall off the trees.
I cut this one in half to show the eaten out core of the gall and the cocoon.
This one shows the grub inside the cocoon.
The female wasp emerges from the gall in the spring to start the cycle all over again by laying both male and female eggs in the Turkey oak. You can watch a short video from David Attenborough's programme Life in the Undergrowth on knopper gall wasps here.