Rats, flies, fruit flies, and ants: preventing and dealing with compost bin pests

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A backyard compost bin is a small living world: countless microorganisms, along with worms, sow bugs, and larvae, work day and night to turn your scraps into compost. Among all these residents, a few visitors are much less welcome: rats and mice, swarms of fruit flies, and ants in large numbers.

Their presence isn't inevitable. It almost always signals the wrong kind of scraps, food left uncovered, or a moisture imbalance, so a few simple habits will usually fix it. Do you have a worm composter? Its visitors and remedies are a little different: you'll find them in our guide Worm bin pests.

Rats and mice: what really attracts them

A rodent is looking for two things: easy food and dry shelter. A compost bin can offer both if you aren't careful. What attracts it first and foremost are scraps of meat, fish, and dairy products. Saucy dishes, greasy leftovers, and large amounts of bread have the same effect.

The type of compost bin matters too. An open pile is more exposed to cats, dogs, and rodents than a closed bin. And a well-kept compost bin, with varied, covered additions, generally doesn't attract pests.

Meat and fish are also among the scraps that composting experts disagree on. We explain why we advise against putting them in the compost bin, and what you can do with them instead, in our guide Controversial waste. The best solution for these scraps is often bokashi, which ferments them in a sealed bucket before returning them to the soil.

Protecting your compost bin from rodents

A few precautions are enough to make your compost bin far less appealing:

• Bury fresh scraps in the center of the pile or bin rather than leaving them on the surface. It's an effective deterrent.
• Cover each addition with a good layer of brown material, about 4 in (10 cm) thick: dead leaves, wood chips, straw.
• Choose a closed bin with a lid that fits securely. A cheap, poorly fitted compost bin full of gaps lets rodents in more easily.
• Lay wire mesh under the bin if the visits continue: galvanized metal mesh with small openings, spread on the ground and extending a few inches beyond the bin all the way around.

A compost tumbler is another answer: its closed, raised drum keeps rats at bay, making it a good option in the city, near a patio, or in a neighborhood with lots of rodents. Our Maze 180L dual-chamber compost tumbler is one example. Just keep in mind that worms can't live in it: the compost will need to finish maturing in a pile.

Willy's tip

For the mesh, choose a fine gauge, about 0.4 in (1 cm): a mouse can squeeze through surprisingly small openings. Bring it a few inches up the inside walls of the bin and remember to check the corners. Earthworms, on the other hand, pass through easily and can keep moving up into your compost.

If, despite everything, a colony settles in for good, don't wait: empty the compost bin, move it if necessary, and contact your local city office or a pest control professional.

Flies and fruit flies: cover fresh additions

A few fruit flies around a compost bin are normal, especially in summer. A swarm, however, almost always means too much sweet, moist fruit left exposed, or compost that's too wet. These tiny vinegar flies love melon rinds, bruised peaches, and peels from ripe fruit.

• Mix fruit scraps into the top layer of the compost as soon as you add them;
• cover them right away with brown material: wood chips, dead leaves, dry grass clippings;
• check the moisture with the squeeze test, and add dry material if water runs between your fingers;
• keep your food scraps in the kitchen in a lidded compost pail, such as our Kaylis 7L compost bin, and empty it regularly.

If you see large flies with a metallic blue sheen, the cause is different: they're attracted by scraps of meat or fish, or by animal waste. The remedy is simple: stop putting these in the compost bin.

Ants: often a sign of compost that's too dry

The rule of thumb is simple: a few ants, nothing unusual; lots of ants, or an actual nest, and the compost is short on water. This is a hands-on observation rather than a scientific rule, but it's the first thing worth checking.

There's no need to reach for insecticide. Water the compost while turning it, add moist green materials (peels, vegetable tops, thin layers of grass clippings), then check with the squeeze test. Once the moisture is back, the ants will leave on their own. For other signs of compost that's too dry, see our guide Compost that smells, is too wet, or too dry.

The "good bugs" to leave to their work

Before worrying about a creepy-crawly, take a good look at it: most of the residents of your compost bin are your allies. They cut up, shred, and digest scraps, and play a direct part in making compost.

• Sow bugs (woodlice), small gray armored crustaceans, feed on old leaves and plant debris;
• springtails, tiny jumping white dots, and compost mites, which have nothing to do with household dust mites;
• worms, which come up from the soil on their own when the compost bin sits directly on the ground;
• millipedes with cylindrical bodies, which eat softened plant matter;
• flower chafer grubs, large curled white larvae, which refine the material at the end of composting.

To learn more about these little critters, our guide Life in a worm bin describes the most common residents, which you'll also find in a backyard compost bin.

Did you know?

The flower chafer grub looks a lot like the May beetle grub, but it doesn't attack roots: it feeds on decaying plant matter. To tell them apart, look at both ends. The flower chafer grub has a small head and a wider rear end; with the May beetle, it's the other way around.

In a shared compost bin: common rules

In a compost bin at the foot of an apartment building or in a neighborhood, a single unsuitable addition can attract pests for everyone. That's why the rules at these sites are often stricter than at home: in some cities, for example, meat, fish, bones, dairy products, bread, and large amounts of citrus fruit aren't allowed in community compost bins. Follow the list posted at the site, even if you put these scraps in your own compost bin.

Always cover your addition with the dry material provided next to the bins, close the lid tightly, and report any sign of rodents or any fly infestation to the site coordinator. We explain how these sites work in detail in our guide Shared compost bins: how they work.

In summary

Compost bin pests almost always have a simple explanation: scraps of meat, fish, or dairy for rodents; sweet fruit left exposed or too much water for fruit flies; and a lack of moisture for ants. Bury and cover each addition, choose a tightly closed bin, add wire mesh or switch to a compost tumbler if rodents keep coming back, and keep an eye on moisture. As for sow bugs, worms, springtails, and flower chafer grubs, let them do their thing: they're the ones making your compost.

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Other topics in the guide

Vermicomposting30 fact sheets →Bokashi24 fact sheets →EM microorganisms26 fact sheets →