Answer: It sounds like you are describing the larval form of the Mexican Bean Beetle! Both the adults and the larvae will skeletonize the leaves from the underside. Mexican bean beetles are best controlled early in the season by hand-picking before they lay eggs, then by hand-picking both beetles and egg masses. However, if the infestation is high, you might wish to resort to pyrethrins (botanical insecticide) applied according to the the label instructions.
To minimize next year's attack, be sure to clean up the garden completely at the end of the season. Remove all plant waste and mulching materials and leave the ground bare for a couple of weeks to allow birds to dine on any remaining adult beetles.
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