The marvels of leaf galls

article written by Caitlin Coombs, Berkshire Biodiversity Officer

During a recent habitat survey in West Berkshire, I was trudging around a boggy woodland when I was distracted by what I thought was a seriously infected oak tree. From afar, I could make out the discoloured leaves, covered with rather unhealthy-looking bulbous blotches.

Slightly worried, I clambered over an exposed rootplate, waded through a sea of pendulous sedge and ducked under a fallen tree, then ventured closer to the oak. Upon closer inspection however, I realised that the tree itself was in pretty good nick. The trunk was sturdy, the branches were strong, and new shoots were healthy. I reached out for a branch and pulled it towards me so I could get a proper look at the leaves.

It was at that moment, I realised what the strange-looking bulbous blotches were. They were leaf galls!

So, what exactly are leaf galls? Are they borne from the tree itself? Are they beneficial? Are they physically damaging? As they seem to dominate the entire surface area of the leaf, do they prevent the leaves from carrying out essential processes like transpiration and photosynthesis?

A leaf gall is a growth on a plant that is made of plant tissue but caused by (and controlled by!) another organism, such as 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!

The gall I had come across is known as the ‘oak spangle gall’, caused by a species of wasp (Neuroterus quercus baccurum). The female wasp lays her eggs on new oak leaves in spring, the gall develops around the eggs, and the larvae emerge when the wasp is mature during the summer.

So, is this scenario a win-win situation?

Not really. This is known as a parasitic relationship: the insect manipulates the plant tissue for its own benefit, but the plant receives nothing in exchange.

Benefits to the insect: Galls provide the insect with habitat, food sources, and physical protection from predators.

Benefits to the tree: None. Most galls don’t harm the plant though and will have no effect on the health of the host trees.

How do galls develop?

  1. The female wasp lays her egg in some part of the tree (for example a leaf bud, catkin, or even the roots) using a special egg-laying device called an ovipositor.
  2. The eggs release chemicals, which trigger leaf cell growth, creating a chamber and the perfect microhabitat for the wasp grub- thus, creating the ‘gall’.
  3. The gall continues to develop for the larva or larvae to grow in.
  4. The larva also stimulates the plant to direct more nutrients, such as proteins and sugars, to the cells immediately surrounding the gall; providing a ready supply of food to speed it towards maturity.
  5. When mature, the adult wasp emerges from the gall.

There hundreds of species of oak gall wasps (cynipids), each of which creates a unique structure. Keep your eyes peeled!

I also came across a similar structure on the leaves of an alder (Alnus glutinosa) tree. After doing some research when I got home, I identified it to be caused by the mite Eriophyes laevis. The galls start off shiny yellow, becoming green then red, purplish or brown.

There is still a lot to learn about these strange growths and their causes, chemistry, and life cycles of organisms causing them. Regardless, it is clear that galls make a huge contribution to ecosystem function.

What is your favourite leaf gall?

Take a look at other galls here.

Posted: October 20, 2020