Geranium Plants

I have a fair few Geraniums in the greenhouse ready to be hardend off. Over the last few days I have noticed that several have had their leaves eaten. I have put down Slug Pellets but to be honest there is no sign of them. Any ideas please on what the culprit could be?

They are prone to getting green caterpillars - just the same shade of green as a geranium leaf so you have to look hard.  They lay along the leaf stems to hide