I found this beetle on a Ragweed plant. It was a fairly defensive critter in that every time I moved the plant to get a better shot it would just let go and fall to the ground whereas other similar beetles would have clung to the plant. In the third and fourth pics you can see it is in its defensive posture with head and legs tucked under. It's in this position because I couldn't get a decent shot of its back because it kept getting scared and falling to the ground (kind of like those fainting goats on the Discovery channel :-D). Then I would have to wait for it to climb back up so I could try again. I finally just took it and put it on a leaf (third and fourth pics) to get the back pics.

Please help with ID.