Nature on our doorsteps: Oak Gall wasps

Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into the natural heritage around us and the beautiful biodiversity of the plants and creatures.

Oak trees provide shelter and feeding to hundreds of species.  At this time of the year when leaves and acorns fall to the ground, the effects of one group of insects, oak gall wasps, are easier to see.  


Gall wasps are tiny insects, some no bigger than a few millimetres.  

Some species insert their eggs just under the skin of the leaf, others in leaf stems, on oak catkins, or in developing acorns.  

While over 30 species of oak gall wasps can occur, they don’t usually affect the tree’s vigour.  

When the eggs hatch into tiny larvae, they emit a chemical which causes a growth or swelling to form around them, providing a safe place for the grubs to feed and develop.

Different oak gall wasp species produces different types of gall structures.  

Also, every second generation of gall wasp produces only females and the types of galls they produce are different to the ones produced as a result of male and female interactions.

Check fallen acorns for the Knopper Gall which is a distinctive, ridged, growth where the nut should be.  

You might also see small flaky discs on the back of the fallen leaves which are Spangle Galls, while the Silk Button Gall is smooth and doughnut shaped.


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