We think these are the sawfly larvae having a good old munch on the leaves - they practically stripped the plant bare.
According to the BBC website, sawfly larvae skeletonise (good word!) leaves or eat them to transparency, leaving just the veins remaining. If disturbed, they curl up into an 'S' shape (just as they are doing here).
They are probably under-recorded as a species and come in different groups, often defined by their menu of choice: the gooseberry sawfly, the turnip sawfly; the rose slug sawfly.
What can be done to stop them chomping their way through your lovely leaves? It is worth noting that they are unlikely to kill the plant and they tend to have years where there are more rampant than at other times. However, there are some things that can be done:
- Inspect the leaves every week from April onwards for signs of the larvae and remove by hand
- Encourage birds to the area: they will happily feed on the larvae
- Turn the soil over in the winter a few times - the pupa overwinter in the soil in cocoons and birds will be happy for the source of food (but don't damage the roots in the process)
- Use a soapy insecticidal spray or a limited spectrum oil spray specifically targeted at the sawfly larvae (broad spectrum sprays should be avoided as they will also kill beneficial insects)