Open your compost bin and you’re likely you’ll see a variety of critters scatter as the light exposes their busy activity.

Sow bugs scurrying under the debris, worms pulling their heads back into the soil, or a fruit fly aiming for your nose. Gross might be the word that comes to mind, but before you swat, crush, or blaspheme the residents, take another look. Sure, you expect to see worms transforming your kitchen scraps into healthy, rich soil, but they’re not the only ones doing the job. In fact, there are a plethora of organisms working together to achieve the same goal.

Static or Active Compost?

The intensity of your composting practice is what will determine who you’re likely see when you look inside your compost bin. A static compost (one that you add to but don’t turn) will have more organisms than an active compost. That’s because an active compost (one that you turn routinely) generates a lot of heat during the initial stages of breaking down. Heat isn’t conducive to larger decomposers, so their numbers generally don’t become more abundant until the pile begins to cool.

For the sake of covering all organisms, we’ll be discussing who you would find in a static compost. From smallest to biggest, you’ll be surprised at who does the most work, and whose presence indicates when your pile needs help.

ActivityWho's Involved
Primary ConsumersOrganisms that eat organic residues
Bacteria, fungi, actinomycetes, soldier flies, nematodes, some types of mites, snails, slugs, earthworms, millipedes, sowbugs, whiteworms.
Secondary ConsumersOrganisms that eat primary consumers and organic residue
Springtails, some types of mites, feather-winged beetles, nematodes, protozoa, rotifera, soil flatworms.
Tertiary ConsumersOrganisms that eat secondary consumers
Centipedes, predatory mites, rove beetles, fomicid ants, carabid beetles.

Micro Creatures Do Macro Work

The primary workhorses of the compost are the microorganisms that you can’t see with the naked eye. The first to arrive are the mesophilic bacteria, who begin to break down the soluble and readily degradable materials. The process of their work causes the temperature in your compost to rise rapidly. Once core temperatures reach above 105 degrees F (or 40 degrees C), populations of mesophilic bacteria recede and the thermophilic bacteria take over.

These thermophiles are made up primarily of the genus Bacillus and will work until the compost reaches above 140 degrees F (60 degrees C). At this time the population of Bacillus declines and the thermus bacteria take over. These are the same bacteria that are found in the hot springs of Yellowstone National Park and the thermal vents deep in the sea. This period can last from just a few days to a few months, depending on the size and content of the pile.

As the pile begins to cool and enters the maturation phase, the Actinomycetes really get to work. These are the microorganisms that break down the tough, complex materials of lignin, chitin, cellulose and proteins. They are also responsible for the sweet smell of soil you enjoy when harvesting the finished product.

Compost Helpers


The word ‘fungi’ might bring to mind the plump button mushrooms you find at the market, so it’s no wonder you might not recognize fungus in your compost. A mushroom is only the fruiting body of fungi, tasty for sure, but this organism is much more admirable for the work they do as mycellium in the compost or soil.

Mycellium are made up of strands and observed as an explosion of fine, white filaments reaching into the debris to create a dense mat. Though delicate to the touch, mycellium have the strength that few others do. They can break down lignans, one of the toughest natural materials growing on earth. Ever so slowly, mycellium reach out, pushing their tips into the dry stalks of a sunflower or branch where they will release enzymes that separate the cellulose chains, converting them into soluble forms that are easier for organisms to ingest.


Nematodes and Mites

Nematodes are the most abundant of the physical decomposers, feeding on fungi, bacteria, protozoa and other nematodes. If you had bionic vision, you would observe a hundred thousand nematodes in a handful of soil. The second most abundant invertebrate found in compost will be mites, and they too have a wide diet. Some mites feed on organic matter, while others will act as predators, feeding on other insects in the environment.

Sowbugs and Pill Bugs

Although you might call this creature a variety of different names (sometimes preceded by an expletive) when you find them munching on your seedlings, they have an important job to do in your compost pile. Equipped with the right enzymes, pill bugs can feed on the very tough walls of plants rich in cellulose, like twigs or brown leaves. The waste they produce, or poop, becomes a food for the microbes. Being a member of sea-loving crustacean family, these bugs love water. That means they can also be a very good indicator of too much moisture in your compost pile. If you are seeing an abundance of pill bugs when you pull off the lid, it’s likely time to turn your pile. This will help reduce moisture and improve the airflow.


The vegetarian millipede has a thousand legs to move it in and out of your compost. Burrowing and consuming very quickly, the millipede is a highly effective decomposer. But don’t confuse them with the highly predaceous centipede, who moves across the planet on a hundred legs eating earthworms and beneficial insects in your compost. To tell the difference, look at the shape of the body and how many legs per body segment. Two sets of legs per segment is a millipede, while the centipede only has one set. Fortunately, centipedes are quite territorial, so you shouldn’t see many in one place. When you do, be quick! With a gloved hand pinch them between your fingers and put them in a place where their work will be appreciated.

Millipedes have a cylindrical body.
Centipedes have a flat body.

Black Soldier Flies

While many people confuse these creatures with the reviled maggot, black soldier flies are the next best composters to red wrigglers. In fact, some composting enthusiasts will set up a compost specifically for soldier flies. Efficiently converting food waste into rich biomass, the soldier fly will stop feeding and climb to higher ground before entering its pupal stage. You may have observed these pupae collecting on the sides of your compost bin. If you raise chickens, these pupae are a great treat for your feathered friends.

Criminals of the Compost

Rove Beetles

Not every bug in your compost is a good one. Even those that we would consider beneficial in the garden can do great harm in your bin. Predators like the rove beetle are a great example of this. Although we encourage them in our crops to eat root maggots, cutworms and aphids, in our compost bin they will consume the processors that are doing the work. This doesn’t mean you have to squish the ones you find, just simply transport them with your gloved hand to an area where they can prey on those we want them to.


Ants in the compost indicate that your pile is too dry. An undisturbed, static compost that does not have enough moisture becomes the ideal place for an ant nest. Unfortunately, this nest will displace decomposers and put them under threat of the defensive ants. The best way to manage ants in the compost pile is to keep it nice and moist—all the way through.

Earthworm Mites

Though the majority of mites in your compost bin are probably beneficial, the earthworm mite is not. These reddish brown mites will cover decaying fruit and waste, depriving worms of their much-needed nutrients. Their presence will be obvious on decaying material and is something every composter should watch for. The red mite is one of the most destructive, since it preys upon worms themselves. Their piercing, sucking mouth-parts are a deadly weapon against red wrigglers and their eggs. A little harder to distinguish, the red mite first appears as white or grey clusters resembling mold, but when observed under magnification you will see the various stages of mites clamouring around.

To reduce the presence of both of these mites in your compost:

  1. Expose the pile or bin to sunlight for a few hours each day.
  2. Place slices of sweet fruits on top of the bed and allow the mites to collect on them. They especially like sweets. Dispose of these slices in a bucket of soapy water.
  3. Heavily water the compost bed but do not flood. While worms enjoy the high moisture and will move into the bed, the mites will scurry to the top. Using a propane torch, scorch the top of the pile to kill the mites. Repeat this method a few times.

Keeping Your Compost Healthy

A healthy compost is a diverse ecosystem of insects and organisms working together. When in process, your compost should smell sweet and not rotten. The material should be moist enough that your hand would be wet if you squeezed it. A foul smell suggests too much moisture causing anaerobic conditions. Additionally, the colour should be like dark cocoa.

Your compost is a live organism made up of millions of other organisms doing a job. Get to know your buddies and do what you can to help them work efficiently.


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