Small black flies drifting up from your plants every time you water them are almost always fungus gnats, also called sciarid flies. They’re one of the most common houseplant pests in British homes, and they’re worst in winter, when compost stays wet for longer than the plant can use it.
The good news: the adults don’t bite and don’t harm plants. The less good news: you can’t get rid of them by killing the flies you can see, because the next generation is already in the compost. This guide shows you how to confirm what you have, then gives a four-week plan built around their life cycle.
Is it fungus gnats?
Fungus gnats are small, dark, delicate flies, a couple of millimetres long, with long antennae. They’re weak fliers: they lift off the compost when you disturb the pot, flutter about at low level and settle again. Their larvae live in the top few centimetres of compost. They’re thin and legless, whitish to see-through, with a shiny black head, and up to about 6 mm long.
Two things that get mistaken for them:
- Springtails. Tiny grey or white specks on wet compost that jump rather than fly. They feed on decaying organic matter and rarely, if ever, damage indoor plants, according to University of Minnesota Extension. They’re a sign the compost is too wet, and they fade when it dries. No treatment needed.
- Fruit flies. Similar size, but they hang around the fruit bowl and the bin rather than the plant pots, and they fly more strongly.
If the flies rise from the compost and nowhere else, it’s fungus gnats. Still not sure? The bug identifier asks a few questions and tells you.
Check for larvae. Push a chunk of raw potato, cut side down, into the compost surface and leave it for a day or two. Larvae gather underneath it. This is the monitoring method UC IPM recommends, and it tells you which pots are actually infested.
Why you can’t just swat them
The life cycle is the whole story. Females lay their eggs in the damp surface of the compost. The eggs hatch in a few days, the larvae feed for a week or two, pupate in the compost and emerge as adults. At around 24°C a full generation takes about 17 days (UC IPM). The RHS puts it at about a month in summer, and notes they can breed all year round indoors.
So at any moment a pot holds eggs, larvae and pupae that no fly spray or trap can reach. Anything that only kills adults just thins out one generation while the next is already on its way. The fix has to make the compost a bad place to breed, for long enough to outlast every stage.
The larvae are the part that does the harm. They feed on fungi and decaying matter in the compost, and large numbers also eat fine roots. Established plants usually shrug that off. Seedlings, cuttings and young plants can be stunted.
The four-week plan
Day 1: dry it out and set traps
- Stop watering the affected plants until the top few centimetres of compost are dry. For most houseplants in winter that’s safe for a week or more. Check with a finger, not the calendar.
- Empty any water standing in saucers and decorative pots.
- Clear the compost surface of fallen leaves and other debris, which feeds the larvae.
- Put a yellow sticky trap in each infested pot, just above the compost. Traps catch adults, cut egg-laying, and show you whether numbers are falling. The RHS warns they also catch other insects, so take them down once the job is done.
Day 1 to 3: for a bad infestation, go further
If the potato test turns up lots of larvae, or the plant is a seedling, cutting or something you’re worried about, add one of these:
- Repot into fresh compost. The RHS suggests repotting and throwing the old compost away, which removes the eggs and larvae in one go. Use a potting mix made for houseplants. For winter, choose a free-draining mix, and see the potting mix guide for what to add.
- Water in nematodes. Steinernema feltiae are microscopic worms that hunt fungus gnat larvae in the compost. They’re sold by UK biological-control suppliers for sciarid flies and are one of the controls the RHS lists. UC IPM rates them the most effective species against fungus gnats, working best between about 16°C and 32°C in moist compost. That suits a heated home. Follow the pack: they’re usually watered in, and the compost needs to stay damp for a few days afterwards, so time this before you start the drying routine. Check whether the pack suggests a second dose.
The RHS also lists predatory mites (Hypoaspis miles) and a rove beetle (Atheta coriaria). These are better suited to a greenhouse or a large collection than to a single houseplant.
Weeks 1 to 4: keep the surface dry
- Water less often, and more thoroughly when you do. The aim is a dry surface between waterings.
- Water from below where you can. Stand the pot in a tray of water for 20 to 30 minutes, then drain it. The roots get a drink and the surface stays drier. The watering guide covers how to judge when a pot needs it.
- Swap the traps when they fill up, and keep an eye on the count. A steady fall over two to three weeks means it’s working.
- Repeat the potato test at the end of week two. No larvae under the slice is the best sign you’re winning.
Week 4: check, then stop
By now you’ve outlasted at least one full life cycle. If the traps are still filling up, something is keeping the compost wet. It might be a pot without drainage holes, a saucer that never empties, or a plant in a pot much too big for it. Fix that, then do a second round of nematodes.
What doesn’t work, or isn’t allowed
- Bti. Bacillus thuringiensis israelensis is widely recommended online, often as “Mosquito Bits”, and it does kill the larvae. But we could find no Bti product authorised for home use in Great Britain. The ones on the UK register are professional products, and UK supplier Dragonfli warns that Bacillus thuringiensis has no home and garden registration here. You can check any product on the HSE pesticides register. Use nematodes instead.
- Sticky traps on their own. They catch adults, not the eggs, larvae and pupae in the compost, so they only work alongside drying out.
- Fly sprays. Spraying the air or the leaves misses where the problem lives. The RHS lists no pesticide for fungus gnats.
- Cinnamon, hydrogen peroxide drenches and similar home remedies. We found no good evidence that they solve the problem. Drying the compost does.
- A layer of grit or sand on top. It may help a little by keeping the surface drier, but it’s not a fix on its own. If the compost underneath stays wet, the gnats carry on.
Stopping them coming back
- Water for the season. Most fungus gnat problems start in autumn and winter, when plants use far less water. The winter houseplant care guide covers how much less.
- Use a houseplant compost with plenty of grit or perlite, and keep opened bags sealed so they don’t sit open and damp.
- Check new plants before they join the rest. Do the potato test on anything you bring home. The pest field guide has more on quarantine.
- Don’t let water sit in saucers or decorative outer pots.
If your plant is also drooping or yellowing, it may be overwatering doing the damage rather than the gnats. The symptom checker can help you tell the difference, and the root rot guide covers what to do if the roots have started to go.
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