Cabbage white caterpillars

I thought I'd got a lovely big pot of pak choi, one of mizuna too, but they are being eaten, particularly the pak choi. I have small cabbage whites on them, but I'm really struggling to see the green on green and pick them off. Old eyes. 

Any tips please? As its a pot i can move it. Would investigation under electric light help? Torch? Can they be attracted away to something else like slugs and beer? 

Give up and sow new?