Using worms to make compost, called vermicomposting, is an effective way of creating rich soil from your leftover kitchen scraps. Worms in a compost bin eat the scraps and excrete a nutritious type of soil called worm castings. Worm castings are full of nitrogen and make an excellent organic fertilizer for your garden and indoor plants. However, not all worms are suitable for compost bins, so you should learn to identify them to ensure successful vermicomposting with thriving and breeding worms.
Check the surface of the compost for the most common type of compost worm. Its Latin name is Eisenia foetida, and some of its other common names include red wiggler, manure worm, tiger worm and brandling worm. These worms are reddish in color with a lighter band near the head. Red wigglers are usually no larger than 3 inches, and they are about a quarter-inch in diameter. They like warm temperatures, moist environments, and they feed on organic matter near the top of the soil.
Dig a little deeper in the compost to find another common compost worm. Its Latin name is Lumbricus rubellus, but some of its other common names are the same as those of Eisenia foetida. These worms feed on organic matter a little deeper than Eisenia foetida, and they are browner in color and slightly fatter.
Look around the bottom of the compost bin to find Eisenia hortensis---also known as a European nightcrawler, even though it is not a true nightcrawler. This compost worm feeds on soil rather than organic matter, and prefers the wetter parts of the compost. These worms can be up to 5 inches long, though their color is similar to that of red wrigglers. They are more often found in compost piles than worm bins.
Hold the worm in your hand to check if it's the right type of worm for a compost bin. If the worm wriggles around but doesn't really go anywhere, it is probably OK for vermicomposting. However, if the worm is large and can sit up or crawl along your hand, it will die in a composting bin. Worms that can crawl are usually nightcrawlers, and they prefer to burrow deep in cool soil.
Avoid using worms from your garden in a vermicompost bin. Unless you are an expert at worm identification, it's hard to select worms that will thrive in a container. Most garden worms prefer cooler temperatures than the 70 to 75 degrees F favored by common composting worms. Buy worms that are effective vermicomposters at garden stores and bait shops, or ask a friend with a healthy vermicomposting bin to give or sell you some extra worms.