If it was a Boxelder bug you would probably expect to see them on Boxelder or very near.
The shape suggests a nymph in the family Lygaeidae, a seed bug. You can see one which looks similar on this site .. "Seed bug, nymph, 5th instar"
http://colinlmiller.com/wildli...
Some bugs use a specific plant host, Neacoryphus bicrucis appears to use Senecio sp. but the nymph doesn't seem to match that.
http://onlinelibrary.wiley.com...
If I could find a list of insects which use Senecio as a host plant it might help.
https://www.google.co.uk/searc...
I found a Plant-Feeding Insect Database for Illinois which may be of some other use but not necessarily for cultivated plants.
http://www.illinoiswildflowers...
