Two insects can cause this kind of feeding damage on corn and grasses- 1) armyworms and 2) grasshoppers. The distinctive part is that the leaves are eaten inward from the edge and only the tough midrib is left. You can see an early stage of damage on this corn plant. It is early in the season and there is a killed grass cover crop visible in this picture. Armyworm moths laid their eggs on this grass and moved to corn when the grass was killed with a herbicide. The armyworms may be hidden in the dead grass or they may have crawled on looking for more food.
Grasshoppers feed on grass leaves from the sides but may chew rounded holes in the leaves of other plants. Grasshoppers would be too small to cause this much damage so early in the season.
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