Wasp larvae are on the gall to survive
I RECENTLY noticed an oak tree with small, wooden balls growing in clumps from its twigs.
Curious as to what they were, I looked them up and discovered that they were called galls.
Have you ever looked at a plant or tree and noticed that it’s a bit lumpy and bumpy or spotted something unusual growing on it?
Well chances are you’ve been looking at a gall; a growth or deformity made up of plant material but caused by another creature.
Most galls are caused by insects but some are formed by bacteria, viruses or fungi.
Not a lot is known about galls but we do know that whatever makes them does so for their own benefit.
The galls I found on the oak tree were oak marble galls, named so because they are smooth and round like marbles.
They are formed by the tiny oak marble gall wasp.
Inside the gall is a larva which feeds on the tissue of the oak.
Oak marble gall wasps that emerge as adults in winter are all asexual females.
They crawl out of their gall, leaving little holes behind and lay eggs into the oak tree’s buds.
When the eggs hatch, the legless wasp larvae start making a chemical that changes the oak’s normal growth pattern, tricking the oak so that it instead forms a protective gall around the larvae.
This next generation of oak gall wasps will emerge in the summer as a mixture of males and females.
The oak gall protects the young larvae because it is full of tannins.
Tannins are chemicals produced by oak trees and many other plants to protect the plant from decay and to put off any animals that may want to eat them.
In galls, the levels of tannins can be much higher than the rest of the tree, making sure that nothing nasty will nibble on or attack the larvae.
This is effective against smaller predators, but the larvae are not completely safe.
An oak gall is no defence against a great spotted woodpecker, who can use its powerful beak to peck a gall open and eat the little larva inside.
In the past, humans have also taken advantage of these tannin-rich galls as well, crushing them up and mixing them with iron substances to produce inks and dyes.
There is evidence of this practice going as far back as the Roman Empire and the famous Magna Carta was even written with oak gall ink.
In fact, the oak marble gall wasp was introduced to the UK in the 1840s by a Victorian entrepreneur interested in manufacturing the black dye produced by the oak gall.
As it turned out, the ink wasn’t great, because there wasn’t enough tannin in the galls produced in the UK.
With less vegetation about on the trees at this time of year, you may find galls easier to spot, so keep your eyes peeled when you next go for a walk in the woods.
Once you start looking, you may even start noticing other strange deformities - they come in all sorts of shapes and sizes, depending on what has caused them.
There are more than thirty species of oak gall wasps, including the oak apple gall wasp (which form galls that are soft, brown and white with a pinkish tinge to them) and the oak artichoke gall wasps (which produce hairy green and brown galls on male catkins), but plenty of other plants get galls too.
If do you find lots of galls, don’t worry about their host trees; most galls are harmless, so you can admire them knowing that they don’t have any impact on a tree’s health.