Worm composter pests: how to recognize them and get rid of them

Unlike the little critters we describe in our guide The other inhabitants of the worm composter, some organisms do call for real action on your part. The good news: most of them share the same cause — too much acidity or moisture — so they can be dealt with using the same simple steps.

Fruit flies, the most common pest

This is by far the most common problem in vermicomposting. The first thing to understand is that fruit flies don't show up because you have a worm bin. They're already buzzing around your fresh fruit bowls or houseplants — it's only when your worm bin becomes welcoming to them (an environment that is too acidic or too wet) that they decide to move in. Keeping the right acidity/moisture balance is still the best prevention.

As soon as a dozen or so fruit flies start coming out of the worm composter, take action instead of waiting for things to get worse:

• Pause feeding: in the active tray, add 2 to 3 tablespoons of RegulaWorm or crushed eggshells, then cover the entire surface with a good layer of cardboard torn into small pieces, 0.8–1.2 in (2–3 cm) thick — this creates a real barrier against fruit flies. Leave this tray alone for 2 to 3 weeks;
• Homemade trap: pour some cider vinegar and a drop of dish soap into a small dish. The fruit flies are drawn in by the vinegar, then trapped by the stickiness of the dish soap;
• Preventive repellent: wipe a little black soap over the outside walls of the worm composter so they stop hovering around it.

Willy's tip

If your worm composter is outdoors or the summer weather allows, leave the lid open for 2 to 3 days, even overnight, to let as many adult fruit flies as possible escape, then apply the cardboard barrier to keep them from coming back.

Black soldier fly larvae, more conspicuous cousins

Under the same conditions as fruit flies — exposed fresh scraps, an environment that is too acidic or too wet — you may also see black soldier fly larvae appear: they're larger (up to 0.8 in, or 2 cm), grayish, and far more visible than fruit fly maggots. Rest assured, they don't attack your worms, but they can compete with them by quickly devouring large amounts of food scraps. Since they're a good size, they're easy to remove by hand; then follow the same steps as for fruit flies (cardboard, a break from feeding) to keep them from returning.

Ants, the opposite signal: too dry rather than too wet

This is the opposite of all the previous visitors: ants in your worm composter, most often one kept outdoors, usually signal an environment that has become too dry rather than too wet, since ants are drawn to sweet food and are looking for moisture. The solution is purely physical and requires no products: place a small container of water under each leg of your worm composter, or coat the legs with petroleum jelly — the ants will no longer be able to get in. Indoors, the problem is rare.

Small red or black spiders, not to be confused with ordinary mites

You may call them spiders, but they're actually mites, red or black depending on the species — nothing like the harmless mites in our guide to helpful little critters. These can literally cover the food when the environment is too wet or too acidic, to the point that your worms refuse to eat. If you notice an unusual concentration of them, remove the infested food and leave the lid open for a few days to dry out the bedding: this is usually enough to make them disappear.

Rarer predators: centipedes and flatworms

Two occasional visitors do attack your worms directly — but they remain rare in a well-kept worm bin. The centipede, flat with one pair of legs per segment (not to be confused with the harmless millipede; see our guide to helpful little critters), is a predator you simply need to remove by hand if you come across one. Predatory flatworms, which digest their prey through their skin, are even rarer: if you spot one, it's best to remove it by hand as soon as possible rather than wait for it to multiply.

Snails, slugs, and small rodents rarely find their way into a worm composter started with shredded paper or cardboard bedding — they're far more attracted to a traditional outdoor compost pile. If you do come across any, simply removing them by hand is enough; for rodents in particular, the best preventive habit is to never put meat or bones in your worm composter (see our guide What earthworms eat), which is the basic rule anyway.

In summary

The vast majority of worm composter pests can be dealt with using two opposite but complementary approaches: rebalance the acidity and dry out the environment with cardboard when there's too much moisture (fruit flies, black soldier fly larvae, red mites), or, conversely, restrict access with a physical barrier when the problem comes from an environment that is too dry (ants). Keep an eye on your worm composter, act at the first signs, and these unwanted visitors will remain the exception rather than the rule.

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