These tortoise beetles are found almost entirely on Canada thistle, which is not a native species! And is very invasive. This is the larval stage. Normally, the larva would have a 'shield' made up of fecal matter which it would hold over its body, no doubt to deter predation! Here is a photo of one with a fecal shield: http://www.pbase.com/laroseforest/image/156274554, And here is a photo of the pretty, bright green adult: http://www.pbase.com/laroseforest/image/152082479