Typically, worm bedding, as the first "layer" of the bin, is a carbon-based material that will break down more slowly than the feed. It acts as a place for the worms to escape to if the upper layers of the bed becomes too acid or is otherwise uninhabitable.
The bedding layer is usually a coarse material that has little protein or mineral content (unlike feedstock), won’t pack tightly and therefore maintains air pockets within the growth chamber.
The worm bedding material also helps to absorb excess moisture from feedstock.
Any bedding material used MUST be moistened to the feel of a damp sponge. If it's not, then any watery feedstock on top of it (most fruit and vegetables) will be releasing moisture as they decompose and can overwhelm the bedding's ability to absorb the excess.
When that happens, worms die.
Worm Bedding Requirement
A 2'x2' bin will need approximately 4-6 lbs of initial bedding;
a 2'x3' bin will require 9-14 lbs.
The aim is to lay down from 4-6" of bedding depending on the size of the bin. Too little and there won't be enough material to absorb excess moisture; too much and it will take up space for more nutrient-rich feedstock.
Good Choices for Bedding Material
Shredded black/white newspaper (no glossy or lots of color)
Shredded junk mail, cardboard (I like to soak mine to soften the glue between the layers) – then use the water to moisten the finished bed as needed as it is high in protein from the glue.
Aged manures (at least 6 months) Horse, rabbit or goat are fine, but horse manure is the best bedding to use as it's not as high in nitrogen as poultry manure and a lot drier than cow manure. Plus the hay and straw keep it light and aerated.
And while we're talking horse manure, finding a one-horse stable could more than fill your requirement, at least for those who have a large worm bin. A horse produces 50 pounds of manure every day. That's 8 tons per year.
For those concerned about Ivermectin given to horses to kill intestinal worms and have been hesitant about putting the manure in your bin,
Dried, crushed corncobs (for those living in the corn belt)
Crushed dried leaves (maple,elm,cottonwood and willow are best). These are best piled up and run over with a mulching mower so they won't pack together as whole leaves will.
Chopped pea or bean vines
While some might think that straw would be suitable as bedding. It's not. Even though it's dry, it does not have the ability to absorb water. It takes forever to decompose and would have to stay in the bin longer after the rest of the feedstock is gone. Put it in your poultry nesting boxes, just not in the worm bin.
Dangerous Bedding Materials
Aromatic leaves (citrus,laurel,eucalyptus,oak,black walnut,sequoia and pine are a few)
The first 3 items are extremely high in nitrogen, mineral and moisture content and should be classed more as feed stock if properly pre-composted.
Aromatic leaves should not be used at all as these materials decompose slowly and contain oils, tannic acids and resinous saps harmful to earthworms or may drive them from their beds.