Earthworms eat decaying organic matter: fallen leaves, food scraps, animal manure, and the microorganisms that colonize all of it. A red wiggler, the most common composting worm, can consume up to its own body weight in organic material every day. But worms aren’t just passive garbage disposals. They rely on a surprisingly complex digestive system and billions of gut bacteria to break down what they swallow, and what they produce in return is some of the richest natural fertilizer on earth.
What Worms Actually Eat
Worms feed on humus, the dark, decomposed layer of organic material found in soil. In nature, this means dead plant tissue, rotting roots, fallen leaves, and animal droppings. In a compost bin, it means fruit and vegetable scraps, coffee grounds, tea bags, cardboard, newspaper, and other biodegradable waste. The organic matter itself is only part of the meal. Much of a worm’s nutrition comes from the bacteria, fungi, and other microorganisms coating that material. These microbial colonies are essentially a living film on every piece of rotting food, and worms graze on them heavily.
Not all worms eat the same things, and the differences come down to where they live. Surface-dwelling species like red wigglers (the type used in composting) feed on fresh organic debris right at the soil surface. Soil-dwelling species that burrow in the top 20 centimeters consume a mix of organic material and soil itself, processing older, more broken-down carbon. Deep-burrowing nightcrawlers pull leaves and other debris down from the surface into vertical tunnels, feeding at multiple depths.
How Worms Digest Without Teeth
Worms have no teeth. Instead, they rely on a multi-stage digestive tract that mechanically and chemically processes everything they swallow. Food enters through the mouth and is pulled in by the pharynx, a muscular throat. It then passes through the esophagus, where specialized glands release calcium carbonate to neutralize excess calcium and regulate the worm’s internal chemistry.
From there, food moves into the crop, a storage chamber where it sits until the worm is ready to process it. The real grinding happens in the gizzard, a thick-walled muscular organ that uses tiny stones and sand particles the worm has swallowed to crush food into fine particles. This is why grit is so important to a worm’s diet. Without small, hard particles to act as grinding stones, the gizzard can’t do its job effectively.
Once ground down, the food enters the intestine, where gland cells release digestive fluids and gut bacteria take over. The intestinal wall is lined with blood vessels that absorb nutrients and distribute them throughout the body. What comes out the other end is vermicompost: a stable, nutrient-rich material packed with beneficial microorganisms.
The Gut Bacteria Doing the Heavy Lifting
A worm’s digestive system depends heavily on the microorganisms living inside it. Gut bacteria provide essential enzymes that the worm cannot produce on its own, allowing it to rapidly break down complex organic compounds. The foregut and midgut handle digestion and nutrient absorption, while the hindgut excretes the processed material. Three groups of bacteria dominate the worm gut, collectively making up over 81% of the microbial community. Some of these bacteria are particularly effective at biodegrading tough organic compounds, which is why worms can process materials that would otherwise decompose very slowly.
This partnership between worm and microbe is what makes vermicomposting so effective. The worm provides the mechanical grinding and the warm, moist environment. The bacteria provide the chemical firepower to convert dead organic matter into plant-available nutrients.
What to Feed Composting Worms
If you’re keeping worms in a bin, the best foods are soft, moist, and already starting to break down. Fruit and vegetable scraps, coffee grounds, crushed eggshells, dampened cardboard, and shredded newspaper all work well. A good rule of thumb is to aim for roughly equal parts “greens” (nitrogen-rich food scraps) and “browns” (carbon-rich bedding like shredded paper or cardboard). The ideal carbon-to-nitrogen ratio for a worm bin is approximately 30 to 50 parts carbon for every 1 part nitrogen, but the 50/50 green-to-brown split gets you close enough without needing to measure.
Grit is a small but important addition. Sprinkling crushed eggshells, oyster shell flour, rock dust, or coarse sand into the bin gives worms the grinding material their gizzards need. About two tablespoons per square foot of bin surface, added monthly, provides enough grit while also supplying calcium that worms use to produce their egg cocoons. The calcium also helps neutralize acidity in the bin, which matters because worms and their beneficial microbes thrive in a pH range of 6.0 to 7.0. Acidic conditions damage a worm’s skin, which is how it breathes, impairing its ability to absorb oxygen.
Moisture matters just as much as food selection. Worm bedding should stay between 60% and 80% moisture, roughly the dampness of a wrung-out sponge. Too dry, and worms can’t breathe or move through the material. Too wet, and oxygen levels drop, creating anaerobic conditions that produce foul smells and drive worms to the surface.
Foods That Harm or Kill Worms
Several common kitchen scraps are poor choices for a worm bin. Citrus and pineapple peels are too acidic and will drive worms away until the acidity breaks down over time. Garlic and onion peels repel worms with their strong compounds, though they’ll eventually be consumed once they’ve softened. Too many tomato scraps can also lower the bin’s pH into uncomfortable territory. Avocado pits and other large, hard seeds won’t break down in any reasonable timeframe.
Meat, dairy, and oily foods like salad dressing should never go in a worm bin. They turn rancid, produce strong odors, and attract flies, rodents, and other pests. Processed foods loaded with preservatives are also a bad idea, since those preservatives are specifically designed to prevent the microbial breakdown that worms depend on. Salty foods can dehydrate worms through their permeable skin.
Some materials are outright toxic. Yard waste treated with pesticides or synthetic fertilizers will poison worms. Sawdust from pressure-treated lumber contains chemicals that are lethal to them. Glossy or bleach-whitened paper introduces compounds that don’t belong in a living ecosystem. Animal feces, particularly from pets, can carry parasites and pathogens that thrive in the warm, moist bin environment.
How Much Worms Eat
A red wiggler eats roughly its own body weight per day under ideal conditions. Since an adult red wiggler weighs about one gram, a pound of worms (roughly 1,000 individuals) can process close to a pound of food daily. In practice, consumption rates vary with temperature, moisture, and how well the food has already started decomposing. Worms eat pre-softened, microbially active food much faster than fresh scraps, which is why many composters let food waste sit for a few days before adding it to the bin.
Overfeeding is a more common problem than underfeeding. Uneaten food rots anaerobically, producing acidic byproducts that lower the bin’s pH and create conditions worms actively avoid. If you notice food piling up uneaten, cut back and let the worms catch up before adding more.