What eats cardboard and leaves residue that looks like dirt?

Cardboard is primarily composed of cellulose, which can be consumed by certain types of pests, particularly insects like termites and some species of beetles. These pests break down the cardboard as they feed, leaving behind frass, a pellet-like residue that resembles dirt. Additionally, some fungi and mold can also degrade cardboard, producing a similar residue as they digest the material.