We learn from Lorelai that Jackson cultivates his own mealworms to help fertilise his plants.
Mealworms are the larval form of the mealworm beetle (Tenebrio molitor), a type of what is called “darkling beetles”. Originally from the ancient Mediterranean region, they have spread all over the world.
Mealworms can be raised to produce composted garden fertiliser from their castings (manure), in the same manner as worm farms. This seems to be why Jackson is cultivating them.
The Annotated Gilmore Girls 