Today two new beetles showed up on our beans.  They look a lot like lady beetles (beneficial), however their arrival coincides to some significant leaf damage.  Are they here to save the plants from some unseen threat or are they themselves the ones to blame?

According to Avant-Gardening they are most likely the culprits:

MEXICAN BEAN BEETLE
These are related to ladybugs but are not beneficial. The adults have sixteen black spots on their back and the Larvae are fat, spiny yellow grubs 1/3 inch long. Both feed on foliage, leaving plant leaves a skeleton of veins. Adults overwinter on plant leaves, in late spring or early summer lay clusters of yellow eggs on the undersides of leaves, and there can be one to four generations a year depending on different climates.

More information from North Carolina Integrated Pest Management Information.

For now, it seems we will have to settle for picking them off and destroying their eggs.