Yeah we get something like this here on Australia's east coast, usually on cabbages and leafy vegetables. Mostly it's green grubs which are the larvae of the white cabbage moth. It lays clusters of tiny yellow eggs on the underside of the leaves and when they hatch they eat through the leaf sometimes leaving mines or hundreds of pinholes. As they grow they can demolish a whole cabbage in a few days. We use derris dust to control them here in Australia but you have to reapply it after rain or heavy wind. I'm aware that derris dust (Rotenone) is banned in many US states because it kills fish; perhaps you can find an alternative. Cheers