I went for a walk the other day to the local Wildlife Garden. I was taking photos of autumn berries and when I went to see how the acorns were doing I couldn't find a single intact one. The handful of oaks in the hedge were covered in all sorts of galls on leaves, stems, buds and acorns. Some galls are quite attractive, others just plain bizarre. These galls are produced by the larvae of tiny gallwasps (Cynipidae). These wasps are one of the few a very group of organisms (with waterfleas, rotifers and aphids) that alternate in their mode or reproduction between a sexual phase (with male and females, usually in spring) and an asexual phase (females which reproduce parthenogenetically usually in the summer). Both reproductive phases often happen in the same oak species, but in some cases they occur in two different ones. The gall is a response of the tree tissues to the presence of the wasp larvae and the shape of the gall differs depending of the species of wasp. The gall offers protection and food to the growing larvae, which after pupation, emerge as adults through a hole. There are over 30 gall wasp species in britain in the common oak alone. Despite their tiny size, gallwasps have their own parasitoid species, also wasps!
All the galls in the photos are produced by the summer, asexual generation on the pendunculate, or common oak,
Quercus robur.
Marble Galls, produced by the larvae of
Andricus kollari. The exit hole is visible in the middle. The sexual generation uses the Turkey Oak,
Q. cerris. This wasp species was introduced in the 19th century and is now widespread.
Knopper Galls on acorns. Produced by
Andricus quercuscalicis. Adults emerge in spring to produce the new sexual generation on catkins of
Q. cerris (a non-native oak). They first appeared in the UK in 1960 and they are now widespread. I think the little wasp at the left of the centre of the photo is the gall wasp.
Common Spangle Galls produced by Neuroterus quercusbaccarum. The sexual generation forms Currant Galls in young leaves and catkins.
Silk Button Spangle Galls produced by
Neuroterus numismalis. The sexual generation causes minute galls on leaves in spring.
Artichoke Galls produced by
Andricus fecundator which lays eggs on terminal or leaf buds.
