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Thanks to all that commented on my previous hazel question, by the way the stakes are still growing!

Something else I had forgot about though is that about half of last years hazel nut crop had a single tiny round hole in the shell. I'm assuming it was a bug/insect that had laid an egg on or in the nut at a very early stage and then ate the kernel and wormed its way out. Any ideas? It's probably too late for this season but I would like to be prepared for next!

I know this is going to surprise you - what you probably have is an infestation of the Hazelnut Borer. Told you it would be a shock

There is some work in progress somewhere about the development of a biological control. Apart from that, though, I know very little.