Answer: It sounds like you were invaded by the Mexican Bean Beetle! These are a very common pest on beans. Adults overwinter on plant debris; they soon begin laying their yellow eggs on the undersides of the leaves. In about 2 weeks, the blob-like larvae begin munching on bean leaves, leaving only the skeleton veins behind.
For now, I'd be extra diligent about cleaning up garden debris, since the beetles overwinter there. But your real work will begin next spring--the key to controlling this pest is catching them early in the season. Plant your early crop as usual, but inspect these plants carefully every few days for both the adult beetles and egg masses. Be sure to check the undersides of the leaves. Destroy any of the pests you see. If you are very thorough about this, it may be possible to control future infestations. If your later crop is still affected, you can either try to battle them with hand-picking. As you've seen, they are difficult to control with insecticides. Instead, you might experiment with different planting times--this is one pest that can be foiled by carefully timing your bean plantings to avoid their peak populations. That, combined with patrolling early in the season for emerging adults/eggs, should do the trick.
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