What are these structures on the lateral and terminal leaf buds of pedunculate oak, Quercus robur? This is a young tree by the edge of silt lagoon 6. The structures are around 10mm in diameter, but beware - they are not usually this size! Look out for the answer next week....
Sorry for the delay in answering this - it's been a hectic two weeks preparing for the RSPB Council's visit to Langford last Sunday - see the latest blog for details of the great day we had.
So, with the extra time, did anyone get this one? If you said a gall, you were correct and this time the causal organism is a wasp of the family Cynipidae, called Biorhiza pallida. You may know the galls by the common name of oak apple.
Like many species of gall wasp, Biorhiza pallida has two generations - one a sexual generation and the other parthenogenetic (the females don't need to mate to reproduce). Both generations take place on pedunculate oak, Quercus robur, with the familiar oak apple galls appearing on the lateral and terminal leaf buds and the other generation's galls forming on the roots. The oak apple galls start to form in May each year as a response to the presence of wasp larvae. The galls are at first a cream/pink colour, but darken and harden with age. There are several wasp larve inside each gall. After metamorphosis, the male and female adult wasps emerge in July - September and comprise the sexual generation, they will mate in order to reproduce. The females then burrow into the soil and lay their eggs on the roots of oak trees. Here the parthenogenetic generation forms galls, which are often clustered together, with one wasp larva in each gall. On emergence, the wingless females climb up the trunks of oak trees to lay their eggs on leaf buds and start the porcess again.
Interestingly, in the picture above, the oak apple galls are 3-5 times smaller than they usually are (indeed they can reach up to 50cm across). A possible reason for this was discovered on dissection of one of the galls. The wasp larvae inside had been parasitised by another species of wasp, whose eggs are laid inside the gall and whose larvae feed upon the developing Biorhiza pallida larvae inside. There are several species of parasitic wasp that complete their lifecycles in this way. And indeed about a week later, the imago (adult) parasitic wasps emerged from the galls and were released back onto Phase 1!