This is a great time of year to spot galls. There are lots of gall types but most common are the knopper and the rose gall.
They are both produced by wasps laying their eggs into the plant.
You will find the rose gall (or robins pincushion) on the dog rose that grows in many parts of the forest. The parthenogenic (asexual) wasp lays up to 60 eggs within the terminal bud or unopen leaf of the rose which causes a chemically induced reaction when the larvae hatches and lives safely within and survives on the nutrients provided by the gall as it grows. (These galls actually promote nutrient cell growth within the rose.) Here they have 5 laval instar stages (skin moults) while in the gall and they stop eating in October where they overwinter before emerging as adults in May.
The knopper gall wasp requires the pendunculate and Turkey oaks to complete its 2 cycles of reproduction.
This type of gall wasps lays it eggs in the acorn bud of a pendunculate oak in late spring and as they develop the chemical induced reaction distorts the shape of the acorn, the more larvae sharing the acorn the more weird and wonderful the shape. Like the rose gall wasps they grow and within the gall while feeding from the nutrients inside. In autumn the oak galls turn brown and drop from the tree where the larvae overwinters before emerging as parthenogenic adult females in spring. These female then have to search out the catkins of the turkey oak where they will lay their eggs, this second phase produces males and females who then produce a sexual generation to start the cycle all over again!