Knopper galls are new to me, but there is an oak tree near our house which has lots of them. They are caused by a gall wasp, Andricus quercuscalicilis, which lays an egg in developing acorns. In turn this leads to a chemical reaction with the result that the acorn becomes grossly misshapen and host to the insect grub.
The wasp is an alien, and was first recognised in the UK in the late 1950's and early 1960's. Judging from NBN data it now seems to be present throughout most of the UK, though remains commonest in the south and east.
I had a brief foray into the literature and there is some interesting work from the 1990's by Mick Crawley and others about the ecology of the wasp see eg. Hails and Crawley, where they were seeing as many as 60% of the acorns were affected. Our tree, a native English, pedunculate oak, had approximately a third of its acorns affected by galls. The gall wasp does not damage the tree, just reduces the number of fruits.
At this time of year the grubs are just developing in what looks like an oval egg-case inside the disfigured acorn. Often other parasites are found in the gall but I found only the gall wasp in the two galls I cut open (and getting a very sticky mess in the process).
The really interesting thing though is that the wasp needs not one but two host species. It has two generations, one asexual the other sexual, each dependent on a different tree. In addition to the native oak where the asexual wasp lays its eggs, there is a second generation where the emerged insect mates and eggs are laid in the buds and catkins of the introduced Turkey oak.
I therefore spent an hour this afternoon looking for Turkey oaks around the village. The nearest I found was a big old tree at the school, next to a younger native oak, whose acorns were affected. The distance between the Turkey oak and our oak is 320 metres if the gall was flies in a straight line and does not get diverted. How does the gall wasp find the host?