Cabbage slug

Cabbage slugs are tiny brown and white creatures that are able to reproduce quickly. As a group, they can destroy entire cabbage patches in less than a week. These pests are sometimes hard to spot as they bore holes into the cabbages and eat them from the inside out, so by the time their presence is realized, it is often too late.

History
Supposedly, a single cabbage slug can destroy the entire ecosystem of a large city, such as Ba Sing Se; as a result, the cabbage merchant's cabbages were confiscated and destroyed by a platypus bear, much to his dismay. This apparently benefited Ba Sing Se's safety, as in fact there was a slug in the cabbage.

Connection
Real-world garden slugs enjoy eating cabbages. Slugs, which are like snails without shells, are a type of mollusk related to clams and oysters, though most slugs live on land. Because they are prone to drying out and subsequently dying, they secrete a slimy mucus coating, feed at night or on rainy days, and take shelter during the heat of the day, tending to hide in moist, dark, cool places, such as under rocks and boards and in piles of decaying organic matter.