Tiny flies around my houseplants: are these fungus gnats?

You water a plant and a little cloud of dark specks lifts off the soil. Later one lands on your laptop screen. This is the most common houseplant call I get that is not about a leaf, and I want to reframe it before we talk about killing anything, because the gnats are not really the story.

The most likely cause: soil that stays wet at the surface

Fungus gnat larvae live in the top inch or two of potting mix and feed on fungi, algae, and decaying organic matter. They need that layer to stay consistently moist. A pot whose surface dries out between waterings is simply not habitat, which is why some plants in a house are infested and others three feet away are not.

So a fungus gnat problem is a moisture readout. It is your soil telling you something before your roots do, and that is genuinely useful information: the same conditions that suit gnat larvae are the conditions that lead to root rot. I have found more early root problems by following gnats than by any other single clue. If you take one thing from this page, take that: treat the gnats, but treat the wet soil first, or they come back in a month and the roots keep quietly declining underneath.

Air conditioning makes this worse in a way people do not expect. Our AC keeps indoor air relatively dry, so leaves lose water fast and owners compensate by watering more often. Meanwhile the same cool, dim indoor conditions slow how fast the pot actually dries. More frequent watering into slower-drying pots is exactly the recipe.

What the larvae do, honestly

• On an established houseplant: mostly nothing you would notice. They are feeding on organic matter, not on your monstera.
• On seedlings, cuttings, and stressed plants: real damage. Heavy larval populations feed on root hairs and the tender roots of young plants, which shows up as stalled growth, wilting, and seedlings that simply fail.
• As a vector: they can move soil-borne pathogens around between pots, which matters most in a collection that already has a rot problem.
• On you: nothing. They do not bite and they do not carry human disease. They are just relentlessly present.

How to tell them apart from what else it might be

• Fungus gnats are dark, delicate, and mosquito-shaped with long legs, and they run across the soil as much as they fly. They scatter upward when the pot is disturbed.
• Fruit flies are rounder, tan or brownish, often with red eyes, and they aim for fruit, drains, and recycling rather than pots.
• Shore flies look stockier and darker with clear-spotted wings and are also a wet-media insect; the treatment overlaps, so it rarely changes what you do.
• Whiteflies are white, cluster on leaf undersides, and fly up in a cloud when you brush the foliage. Different problem, different treatment: those feed on the plant.
• The sticky card test. Lay a yellow sticky card flat on the soil surface. It confirms the ID, and counting the catch week to week is the only honest way to know whether your treatment is working.

What to do this week

1. Fix the moisture. Let the top inch or two dry before watering again, and check with a finger rather than a schedule. Take every pot out of any cachepot holding water, and empty every saucer.
2. Bottom-water where you can. Standing the pot in a basin for twenty minutes lets the roots drink while the surface stays dry and inhospitable.
3. Yellow sticky cards, flat at soil level. They catch egg-laying adults and they measure your progress.
4. BTI for the larvae. Mosquito bits or granules steeped in your watering water deliver Bacillus thuringiensis subsp. israelensis, which targets the larvae and leaves everything else alone. Repeat with each watering for several weeks.
5. Top-dress with a dry layer of coarse sand or fine grit to make the surface an unattractive place to lay eggs.
6. Clear the organic debris: fallen leaves on the soil surface, spent flowers, propagation cups of murky water.
7. Be patient for two to three weeks. You are outlasting eggs that were laid before you started.

When to call a professional

Call when the gnats keep returning after you have genuinely dried the soil down, when several plants have them at once, or when you are also seeing yellowing and wilting: that combination usually means the real issue is a watering routine or a set of pots that do not drain, and the gnats are just the symptom that got noticed. A pest management visit deals with the infestation across the whole collection at once and, more usefully, changes the conditions that produced it. If it turns out the roots are already involved, we handle that in the same visit.

Related: root rot is what wet soil leads to next, and spider mites are the opposite problem, thriving in the dry air an AC-conditioned Florida house produces.