These green tortoise beetles are a European species, accidentally introduced here over a 100 years ago. But, according to Bug Guide, introduced more recently into a few US states to control invasive thistles. Another name for these beetles, therefore, is Thistle Tortoise Beetle. I made it my mission to look for this species today as I'd never seen one before and really wanted to! By searching Canada Thistle plants (despite the name, a non-native, often invasive species) I eventually found this one, sitting very conveniently on a dead leaf caught on the thistle plant.