In simple terms, a plant gall is the plant’s response to something that another organism does to it. The overall effect is a little bit like what your body does when a nettle stings it, but gall production is more complicated and still isn’t fully understood. There are hundreds of kinds of galls in Britain, and over 50 of them can be found on oak trees: our two native varieties, pedunculate and sessile oak, together with the introduced turkey oak. August is a good month to start looking for the ones we’ve found at St Nicks, and you can follow their development through the autumn. They are all caused by tiny wasps that lay their eggs in or on the tree. As the gall develops, one or more larvae hatch and feed inside it until they are ready to emerge as new adult wasps.
Neuroterus quercusbaccarum (sorry – it hasn’t got an English name yet!) specialises in oak leaves. Look underneath the leaves of trees along the Bund and near the Sustrans entrance, and you’ll soon come across round, scale-like growths ranging from yellow to dark red in colour. These are Oak Spangle Galls. They don’t normally have any obvious effect on the leaves, but sometimes there are so many that they distort them. The galls fall to the ground in autumn and overwinter among the decaying leaves. The wasps that emerge in spring are females that can reproduce without mating. They lay their eggs in new buds, creating Currant Galls on the new leaves and catkins. You’ll have to wait till May to see those. The larvae in the Currant Galls develop into male and female wasps that mate and set off the cycle all over again.
Another little wasp, Andricus quercuscalicis, has the same sort of life-cycle, but lays its eggs in oak flowers so that the developing acorns turn into bizarre Knopper Galls. These start off bright green, turning dark brown and hard with age. Look for them along the path north of the John Lalley wood or near the Sustrans entrance. Again the galls drop off to spend the winter among the leaf litter, but the new female wasps need to fly away from St Nicks in spring, because they lay their eggs in the flowers of Turkey Oaks. Knopper Galls were first found in Britain during the 1960s, and it seems that any species using Turkey Oaks is likely to be a relatively recent arrival.
Andricus kollari seems to like the trees alongside the meadow, where it lays its eggs in leaf buds. The buds swell into hard little balls known as Marble Galls that tend to stay on the tree long after the new wasps have hatched. By autumn when they emerge, you’ll be able to find the little exit hole. Again they have to travel quite a long way in search of Turkey Oaks to lay their eggs. Cola Nut Galls are similar to Marble Galls, but smaller and scaly-looking, and are caused by another member of the same family, Andricus lignicolus. You’re most likely to find these along the Bund path. Yet again the next generation live on Turkey Oaks. We don’t know where all the wasps that hatch at St Nicks go to lay their eggs, but the nearest Turkey Oaks that we know about are up the road at the University. They send us volunteers; we send them gall wasps!
We have tentatively identified two other kinds of oak gall – a spotty, pea-like one produced underneath the leaves by the wasp Neuroterus anthracinus,and a scaly swelling on either side of the leaf caused by Andricus curvator. These are much harder to find, and there are bound to be others that we’ve missed so far, so if you spot anything that looks interesting, please make a note of where you find it and report it to the staff!
As well as the wasps themselves, many galls play host to other species that exploit the safety and food sources in the gall – but that’s a whole new story.
Source: Michael Chinery 2011 Britain’s Plant Galls (Wild Guides Ltd)
Further information: www.british-galls.org.uk
The photographs were taken at St Nicks during 2013.