When I found this (and another one with it) I thought they were caterpillars. I love the frilly Pierrot-style it has!
I identified it by searching for 'wrinkly white caterpillar with black dots'.
Quite a pretty specimen I hope you agree. Figwort is a very common weed on our allotment site so no wonder these larvae are on site too.
The parents, like all sawflies, are carnivores feeding on various small flies. They look like wasps, with long dangly legs. In fact, I think this is an adult Figwort Sawfly, rather than the Digger Wasp I identified it as!