Spangle Galls

Rosie has been rather ignored this past week, so I decided to take her for an early morning walk before taking Chris to the station and then setting off for my last day of survey work on the current project. Chris is heading to Abisko in Sweden for just over a fortnight, participating in some research on the impact of global warming on the tundra.

It was rather grey first thing, and the world is entering that rather drab late summer state, that I find photographically uninspiring. But a new plantation of oaks provided some subject matter, with many of the leaves being prolifically covered in Spangle Galls. There were two distinct species present in approximately equal quantity.

The hairy red discs are young Common Spangle Galls, caused by a species of gall wasp Neuroterus quercusbaccurum. As they mature the disc flattens and enlarges somewhat, retaining localised tufts of red hairs. The galls drop to the forest floor, where the grubs develop over winter under the cover of fallen oak leaves. In the spring an all-female generation emerges. These are 'agamic', meaning that they are able to reproduce without mating. They lay their eggs in oak buds, producing Currant Galls on the catkins and leaves. The sexual generation of male and female wasps emerge from the Currant Galls in June, mate, and then lay their eggs on the undersides of the leaves. Spangle galls develop, and so the cycle continues.

Silk Button Galls are caused by the closely related gall wasp Neuroterus numismalis which also has two generations per year, one being sexual and the other agamic. The sexual generation causes Blister Galls on oak leaves whereas the agamic generation causes Silk Button Galls on the underside of oak leaves. These look like a thick, rolled edge disk with a deep central pit and gold hairs, there is no mark on the top of the leaf. It is a single cell gall holding one wasp and can be seen from August to October, until the leaves fall in autumn. The wasp larva will mature in August but remain in the gall on the ground throughout the winter, emerging the following year from February to April.

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