I have some bush beans planted and about 4 inches high. Something is eating the leaves off the plants starting atone end of the row and working straight down. The next row over is snow peas and nothing is touching them.
Had a rabbit do that to our beans last year. stayed straight down one row til the end. every morning there would be another row gone. rabbit box worked for us on that one. my sister has it as a pet now.
If there is no sign (during the day) of any insect, I would suspect earwigs, especially if the plants are seedlings. These feed at night. Set traps for them. You could also go out late at night and spray them directly with insecticidal soap.
if you see any tiny dark beetles that hop off the plant when disturbed that could be the culprit (flea beetles). Dust the plants and the ground around them with coffee grounds, or try a garlic and hot pepper spray.
Neither of these will be controlled by Bt.
I'm assuming the problem isn't Mexican bean beetles or bean leaf beetles because you surely would have seen some obvious sign of the beetles or larvae.
Cabbage loopers will sometimes feed on beans, but you should have been able to spot the small inch-worm type caterpillars if that were the case.
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