A galling discovery

It was whilst saying goodbye to a volunteer group recently that one of the party produced an Oak leaf he’d found, with lots of mysterious round growths on the underside. We had to admit to not knowing what they were, so set out to investigate and found ourselves entering a mysterious and very alien world. The world of Galls.

No, not France, but the abnormal growths found on many plants usually caused by some sort of attack or penetration into the plant’s growing tissues, making it reorganise it’s cells. Galls can be caused by many different agents such as viruses, fungi, bacteria, insects and mites and there is a huge variety of them. They’re often very distinctive though so the causer can easily be identified by the gall.

A good example of the possible variety of the growths can be seen in two galls you may well be familiar with – the so called ‘Oak apple’ caused by a small wasp and the ‘Witches Broom’ seen on Birch trees and caused by a fungus.

Witches Broom - picture from Trees for Life.org

Generally, these growths aren’t just caused for aesthetic reasons, but may provide their inhabitants with food, shelter or protection from predators. It’s often a parasitic relationship, causing harm to the host plant but this isn’t necessarily so. Sometimes it gets very complex too, which is what we found when investigating the aforementioned Oak leaf.

A load of old Galls - the Oak leaf with it's occupants

 The wasp causing the Oak Apple gall was just one of many different species of ‘Cynipid’ wasp –  all causing different types of gall in Oak trees and our leaf was another one of them. If you must know, the wasp in question this time is Neuroterus quercusbaccurum. I feel confident in saying there’s probably not a common name for the wasp, but the flat disc galls produced this time is a common Spangle gall.

Alien worlds! a close up on some of the Galls

However, these flat discs are just part of the story. They contain the developing eggs of the wasp and in autumn will drop to the forest floor where the grubs will develop over winter under the cover of fallen Oak leaves. In Spring an all female generation of ‘agamic’ wasps emerges (meaning they can reproduce without mating) and lays their eggs in oak buds. These in turn produce an entirely different ‘currant’ gall in catkins and leaves, with male and female wasps emerging in June. These mate and fresh eggs are laid on the undersides of leaves, developing into more Spangle galls.

It goes to show that few things in nature are as simple as they first appear and even a pile of fallen leaves can have a lot more to it than meets the eye ….

By Rob Clarke, High Wray Basecamp volunteer centre