Worms may be found in compost because they migrate from the surrounding soil and organic matter into manure piles or compost bins in direct contact with the ground. For successful composting, a better method than depending on accidental discovery or migration of worms is to introduce them into the compost yourself. Red wiggler worms (Eisenia fetida) are the most highly recommended worms for composting. The type of worm you use for composting is important because different worm species prefer dissimilar types of food, may not tolerate potential temperature changes and reproduce at different rates. Epigeics like Eisenia fetida live along the surface of the ground and feed among the layers of leaf litter and organic waste available. They are prolific breeders if they are well fed, so a small clew (colony of worms) can generate enough worms within a year to supply many bins. The worms do not get their nutrition from the decaying material itself, but from microbes living in the rotting matter. The worms promote good bacteria and help process out the harmful bacteria in compost through their digestive activity. Since they don't spend time building underground burrows and they prefer the several inches of organic matter that will line your bin, the surroundings are ideal for them to thrive.
Worms may be moved to a new pile by introducing material from an older pile or by introducing organic matter or soil gathered in areas where compost or manure have been stored. It is possible to obtain your own Eisenia fetida by handpicking a few at a time from localized areas of decaying matter in your yard, but other worm varieties are easily mistaken for Eisenia fetida and the process is labor-intensive and slow. Because they are not readily found for collection, the simplest way to begin a clew in your compost is to purchase a batch of worms from a breeder. If you cannot locate a breeder, your county extension agent should be able to help.
Several other species of worm are considered acceptable for vermicomposting, but because they come from subtropical climates, they do not adapt to colder temperatures. The only other species which does adapt to cold does not breed as successfully as Eisenia fetida.
If you know someone with a successful pile, you may divide out worms from it. The juvenile worms process larger amounts of waste than mature or newly hatched worms. Bins should contain 2 lbs. of worms per pound of household food waste generated per day. Because of their small size, eggs may occasionally be unknowingly left behind in a bin and hatch to form new worms. They may also be accidentally introduced along with other material. You may also look for the worms' egg cocoons. These are pale yellow to brown, about 3 mm long. They can be sorted from older compost piles and added to new bins, where they will hatch and establish a new clew.