Scientific name: Andricus kollari Gall
Common name: Oak Marble Gall
This gall appears first as a pink-yellow round ball up to 2cm growing on oak. With age the gall becomes brown and woody. This is home to the Andricus kollari wasp. The larvae (present from June-September) grow inside the gall until they hatch into the adult wasps. The wasps then excavate a hole through the wall of the gall to escape. There can be several larvae in each gall. The exit holes that the wasps have made can be seen in Fig.1. The wasp lays its eggs at bud points of the plant. The tree responds to his by mutating its growth forming a gall. The wasp encourages this by injecting the bud with an irritant. Several galls are usually situated on the same stem. The kollari Wasp is thought to have no males in the species reproducing by females alone.
Effects of the pest:
The gall has little effect on the tree. They appear due to the mutation of bud growth as a response to the wasps egg laying. If the outbreak of wasps was so severe that a large percentage of buds were mutated, it would be likely to have a detrimental effect on the trees growth however such a large infestation has not been found yet.
Fig.1 – Andricus kollari gall close up showing exit holes.
Fig.2 - Andricus kollari galls on oak twig.
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