Have you spotted something unusual growing on a tree recently? Mystery growths on trees is one of my favourite subjects! Galls are a somewhat specialist topic; there aren’t many scientists dedicated to their research, and much of the information on them is collated by enthusiasts. Unfortunately, this also means that many galls don’t have a common name, only a scientific name. But don’t let this put you off! Many common galls in the UK are easy to spot and identify, and once you’re familiar with a few of the species, you’ll start spotting them everywhere.
It looks like it might be a bumper year for galls, so here are 7 you might see this summer.
What is a gall?
A gall is defined as a growth on a plant that is made of plant tissue but caused by another organism. These organisms might be insects, bacteria, fungi or viruses. Insect galls are the most common. There are estimated to be around 133,000 gall-causing insect species in the world, and that’s only for one type of gall causer (the organism that causes the gall) – less is known about bacteria, fungi and viral galls, so the true number of gall types will be even higher. As you’ll see in the examples below, galls vary in size and shape; nature is very creative!
All galls are formed for the same sort of purpose: the deformity is a deliberate mechanism by the gall causer to use the plant for its own purposes. In ecological terms, it’s known as a parasitic relationship. The gall causer manipulates the plant tissue for itself, but the plant receives no benefits in exchange. Most galls don’t harm the plant though and will have no effect on the health of the host trees.
So, what can you look for?
1. Oak apple gall