How to recognise it
There are two generations each year, each developing in a different type of gall. In the spring ‘currant’ galls can be seen on catkins (and sometimes on leaves), they are spherical, berry-like galls around 4mm, but up to 7mm, in diameter which can be yellow-green to red in colour. They resemble redcurrants.
The second generation develops in small (5-6mm diameter) disc-shaped ’spangle’ galls which appear on the underside of oak leaves summer and autumn. These spangle galls are slightly hairy and start as a yellow-green colour, then reddening. They are raised in the centre, giving them a ‘fried egg’ shape. The underneath of the galls is white-yellow in colour. Each leaf can be infested by up to 100 galls.
Each gall contains one egg or developing larva.
The adults are up to 3mm long with hairless brown or black bodies (the two generations looks slightly different) and long wings.
Female gall wasps, which are parthenogenic (a form of asexual reproduction), lay their eggs in the spring on oak leaves and a ‘currant’ gall develops around each egg. The next generation of adults, which reproduce sexually, emerge from the currant galls in the summer and produce further eggs and ’spangle’ galls. These spangle galls fall off the leaves in autumn, often creating a carpet of galls underneath the tree. The larvae continue to develop within the galls over the winter, the adults emerging in the spring.
Why it’s a problem
Generally speaking the galls aren’t a problem, although the leaf coverage of a heavy infestation might check the growth of a very young oak tree.
Where you are likely to find it
On oak trees, particularly Quercus robur and Quercus petraea.
How to deter it
There is nothing you can do to deter an infestation from this flying insect.
How to get rid of it
The galls do little damage to oaks so control isn’t necessary, although a severe infestation might decrease the growth rate of young oak trees. Collecting and destroying the fallen galls in autumn can reduce numbers and gradually eradicate them.
Is it good for anything?!
Other useful information
Despite commonly being called ‘wasps’, gall wasps don’t resemble wasps and should be accurately be called ‘cynipids’.
Over 100 different species of gall wasps have been identified on oaks.
The appearance of the galls is similar to scale insects and the two problems are often confused.