Caught in the act!
I nearly missed this little critter. He's almost the same shade of green as the bean pods that he was happily boring holes in, thinking, no doubt, that he was well camouflaged and would avoid detection.
I have no idea just what he is, although in trying to identify him, I've learned a great deal more about caterpillars. I looked here and thought he might be a cabbage or soybean looper or a green cloverworm, but he doesn't have the right number of leg segments or otherwise fully fit the description. Then I went here, and became fascinated by the diversity of caterpillars. Some I'm familiar with, like the tomato hornworm that nearly wiped out all my tomatoes one year. But being mesmerized by the images on the first two pages and spending much more time than I'd planned, I gave up, so still don't know what my little bean borer is. If you recognize him, please let me know.