Here's another picture.

Kinda of looks like slugs and if this is happening at night, that would be true. You can try slug bait the iron sulfate is pretty non-toxic to everything but slugs.

It doesnt look like slugs to me. I have never seen slugs leave the leaf ribs. I would spray with an insecticidal soap or stronger.

Could be slugs, but my vote is for June bugs. What part of zone 8 do you lie in? I have damage exactly like that on my beans and peppers. I too thought it was slugs, and put a beer trap out. I want out later that night to see and it was June bugs happily eating away. Knock off and smash. Do you leave your patio light on? That will attract them big time, which is why theyre not on the other plants. But I could be wrong. Only one way to know for sure, go out tonight at about 11 and look. Either slugs or June bugs will do a lot of damage fast. If it is slugs, put out a saucer with beer in it, and they�ll flock to it. Also move the plant away from places where slugs might lurk during the day. Under rocks, buckets, boards, etc.

slugs eat holes, but don't skeletonize leaves
could also be caterpillars

found this:
Bean Leaf Beetle

Bean leaf beetles are chewing bugs that attach themselves to the leaves of the green bean plants and eat pieces out of the leaves. Measuring 1/4 inch long, this beetle is yellowish-green in color and has four black spots on its back. Bean leaf beetles spend the winter in the foliage in wooded areas and emerge in early summer. Keep these green-bean-plant-eating bugs under control by removing dead leaves and foliage from the ground. If you think you may have a bean leaf beetle infestation, confirm the infestation by looking for beetles on your bean plants between 12:00 p.m. and 4:00 p.m., as this typically warm time of day is when the beetles feed.

Read more: Bugs Eating Green Bean Plants : eHow.com http://www.ehow.com/list_7387427_bugs-eating-green-bean-plants.html#ixzz20FFrrUAp

looks like grasshoppers. they usually leave leaf ribs. slugs put rounded holes in the leaves.

Wonder if it's grasshoppers. I noticed some today in the garden.

Thanks for the thoughts. It is definitely happening at night. My vote is gonna be on June bugs - I have TONS of those on my patio at night! I don't leave the light on, but when I take the dogs out after dark, they come from who knows where! I will use an insecticidal soap on the plant. I knew I didn't like those bugs!

Mine looked exactly like that because of caterpillars. If you notice, in your last pic there's a part of the leaf folded over. Unfold the leaf and take a peak, for me, that's where the caterpillars hid. Smaller folds on the leaves are smaller caterpillars. I removed using bt at first, but since the green beans have started producing, I've just been picking them off. Hope that helps!