Holes in my nuts.

[FB.]

I've found earwigs living inside nuts with holes like that. I've also heard of people seeking the earwig-resistant varieties of hazels, which might suggest that earwigs really are the cause of the problem.
But maybe the earwigs are just taking advantage of a ready-made home that a weevil had vacated, or whether the earwig chewed it's way into the nut when it was soft and green......