I decided to see if I could find some ragweed beetles today, and ended up seeing at least 8. And guess what? None were on ragweed! In fact, about 90% of the time I've found them on something other than ragweed, usually on Bidens sp. Another interesting thing about these beetles is that in sunlight they look very different than when in shade. here, they appear to have a reddish cast to them. But in the next photo, they look completely black and white.

These are members of the Chrysomelidae, leaf-eating beetles. At one point they were exported to Russia as a biocontrol agent for our native ragweed which is an invasive species in Russia. The idea was that as they are closely associated with ragweed, they would control it. This didn't work. Or it has not worked yet. You could say it is not working here either, given the amount of ragweed around. But perhaps without these native beetles we'd have twice as much. And don't forget the ragweed fly which also has some impact on ragweed.