Nature on our doorsteps: Unusual structures on oak leaves
Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into nature on our doorsteps and the beautiful biodiversity of its plants and wildlife.
OAK trees provide shelter and feeding to hundreds of different insect species.
Some feed on the Oak’s leaves, stems, or leaf buds.
Others nibble on twigs and bark, while some will feed on the wood of the tree itself.
When oak leaves fall to the ground in autumn and early winter, the effect of one particular group of insects is easier to see.
A close look at the back of these fallen leaves might reveal the presence of unusual looking structures that look as if they are stuck to the underside of the leaf.
There are many different types of leaf galls to be found on oak trees
These structures are small swellings known as galls and they are caused by the action of gall wasps.
These are tiny black insects that are not much bigger than a few millimetres in length.
There are over 30 species of oak gall wasps, and each one causes the formation of a different shaped swelling.
The swelling is caused after the female gall wasp lays her egg.
Some gall wasp species lay these eggs just underneath the skin of the leaf.
Other species lay the eggs in leaf stems and buds, on oak catkins, or in developing acorns.
When the eggs hatch into tiny larvae, they emit a chemical which alerts the oak to their presence.
The tree responds quickly and causes the plant cells around the larva to grow very rapidly.
In this way, the grub becomes completely enclosed in plant tissue, preventing it from spreading throughout the tree.
This, however, is precisely what the grub wants. It now has a safe place in which to live and grow, with plenty of plant tissue to feed upon.
With most leaf gall species, each leaf gall contains just one developing grub.
There are many different types of leaf galls to find.
Spangle Galls are small, yellow-green, circular discs on the back of the leaf, while the doughnut-shaped Silk Button Gall is smooth and golden brown
