What's wrong with my blackberry bush?

[StarlitTrees]

I'm growing a blackberry bush in a pot. Planted last year, this year starting to show signs of producing some fruit.
There is a large shoot growing out of the top, presumably one that tries to shoot off and start another bush. But the leaves are full of holes. I can't see any pests on the plant. The older leaves lower down on the plant are almost all fine. A few random ones with holes, but mostly ok.
What is causing this? Is there anything I can do do make it healthier?

caterpillars of some description by the look of it...if you can't see any and the munching has stopped then no need to do anything. But if they're still there and you want rid of them, then make up a weak solution of soap in water (or crushed garlic in water) and spray the leaves, particularly on the underside.