Oak Marble Gall Wasp - Andricus kollari, galls, click for a larger image
Oak Marble Galls
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Oak Marble Gall Wasp - Andricus kollari
Also known as - Bullet Gall, Oak Nuts

Oak Marble Galls are frequently mistaken for Oak Apple due to their superficial resemblance, they develop as a chemically induced distortion of leaf buds on Pedunculate and Sessile Oak caused by the parthenogenetic Gall Wasp - Andricus kollari, laying eggs within leaf buds.  The Turkey Oak introduced to Britain in 1735, is required for the completion of the life cycle of the gall.

The Oak Marble Gall Wasp has two generations per year, the first is sexual, whereas the second is agamic, needing no males for reproduction.  Reproduction therefore is a two stage process requiring two species of Oak in which to breed.  The first stage (sexual) is found as Galls on the buds of Turkey Oak (Quercus cerris) during January to February, these Galls are apple green with a pink tinge on one side.  These galls are very small about 3.0x1.5mm (0.12x0.06in) and are mature around the end of March, with each gall producing one male ♂ or female ♀ wasp.  After emergence mating occurs after which the females look for buds of the English Oak in which to lay her eggs.

The second stage agamic galls, found in the UK mainly on the buds of Common Oak - Quercus robur, as well as several other Oak species, are usually seen from June onwards.  The gall is smooth and round growing to about 20mm (0.8in) in diameter, occasionally larger, green in colour and quite soft to start with.  This second stage gall can be lumpy.  The white legless larvae live within a central chamber surrounded by a sponge like mass.  As the gall matures the outer surface turns brown and hardens to a very dense structure, usually during August to September and is when the wasp larva pupates.  Adult Wasps chew an exit tunnel out from the centre then fly off in search of a Turkey oak to start the whole process again.

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