Following part 1 of the winter tree ID blog - can anyone tell me what these brown balls are? It's a young pedunculate oak tree and you can find them on the public footpath by the silt lagoons. Answer to follow next week....
No guesses....they are oak marble galls (a.k.a. bullet gall, oak nut) and are produced by a small gall wasp called Andricus kollari (no English name I'm afraid!). The galls are chemically induced responses by the tree to the presence of the wasp larvae developing inside the gall.
The wasps have a complex lifecycle that involves both sexual and parthenogenetic (asexual) generations. A sexual female lays her egg in a leaf bud of pedunculate (or sessile) oak, forming the gall during the summer. The developing larva inside is a parthenogenetic female (she doesn't need to mate to reproduce). She emerges through a small hole when adult in September and then lays an unfertilized egg in the leaf buds of turkey oak (an introduced species). The galls on turkey oak are much smaller and less conspicuous. The generation that emerges from the turkey oak galls are sexual males and females. They mate to produce fertilised eggs and the cycle begins again.
The wasps themselves are only 1.5-2mm long and are a dark brown/black colour.