The Crumbug is not much of a threat by itself as it is defeated in one hit and can only eat the Pikmin located before it. However, the Crumbug can become a threat if it is fought at the same time as othe enemies. They can also be summoned by the Emperor Bulblax during its Boss battle.
Hey! Pikmin log
"Its only move is to walk straight toward you without stopping. Is that so it can eat us, or so we can eat it?"