Cabbage slug

The Cabbage Slug is a tiny creature able to quickly reproduce. Groups 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 may well be too late. 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, much to his dismay. This, supposedly, benefited Ba Sing Se's safety, as there was in fact a slug in the cabbage.

Connection
The Cabbage Slug could refer to a regular garden slug, as these slugs enjoy eating cabbages. They are a type of mollusk and are related to clams and oysters with the difference being they are land based. Essentially, slugs are very much like snails, except they do not have a shell. They feed at night or on rainy days and take shelter during the heat of the day. That is why slugs are seldom seen, as they like to hide in moist, dark, cool places, such as under rocks, boards and piles of decaying organic matter. They are coated in a slimy substance, most likely a type of mucas, that helps to protect their bodies from drying out.