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Knobbly acorns


by Richard Jones

Walking back from the Horniman Museum last week took me past a large oak tree growing just inside a front garden...What caught my attention were all the broken knopper galls lying on the pavement.


Richard JonesWalking back from the Horniman Museum last week took me past a large oak tree growing just inside a front garden. The tree looks like an old pollard and must pre-date the early 20th century houses hereabouts. What caught my attention were all the broken knopper galls lying on the pavement. A knopper gall is a strange wrinkled and knobbly growth that distorts the acorn into a large sticky folded mass. It is caused by a minute midge-like creature, the gall wasp Andricus quercuscalicis. As it lays an egg into the acorn, it also injects a cocktail of chemicals that causes the acorn's growth to be interrupted. Then as the grub feeds inside it too secretes chemicals that alter the normal development of the acorn forcing it to grow into the knopper.

The galls on the ground appear to have been broken open. Normally they fall with the leaves in autumn and by that time they are hard and dry and brown. But these were still greenish orange and the cavity inside each, where the pale grub should have been feeding, was manifestly empty. I waited a few minutes but saw no sign of activity up in the tree except the ubiquitous grey squirrel. Although squirrels eat acorns I can't believe that they would hunt out the grubs. I can only think that it must have been a bird of some sort. I'll keep looking.



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Gardeners' World Web User 28/11/2011 at 18:29

If the galls are split from top to bottom, yes it would be the squirrels. The galls are also opened by mice once they are on the ground. Mice will tend to bite through the bottom part where the larval chamber is located. Cheers, Karsten