Caterpillars munching, caterpillars crunching...

I'm loving the macro setting on my camera at the moment! I've been using it for a while to take close ups of my jewellery, but recently I've taken it into the garden too. A couple of weeks ago I caught a bumble bee on a poppy, and today I've got a mug shot of the little beasties who are destroying my favourite rose bush.

This is a Zepherin Druin - a climbing rose with a heavenly scent - and I chose it because there's been one growing beside the kitchen door of my parents' house for years. But recently I noticed the leaf damage you can see in the background of this photo, and here are the culprits:

My limited research suggests they're the caterpillars of some kind of white butterfly - a large white perhaps? If anyone knows any better, please do tell!

Update - it's now a week later, and there's only one caterpillar... but he's now a big fat caterpillar:

I'm tagging this as part of Mum's the word's Closer to Nature linky: