Got Fungus Gnats? This Easy Potato Trick Lures Hidden Larvae Out of Houseplant Soil

You water your houseplant, and a little cloud of tiny black flies lifts off the soil. A minute later, they're on the window. Fungus gnats turn up in many homes once fall arrives, and they can hang around all winter. The next batch is already growing in the pot, and a fungus gnat potato trap is an easy way to find them.

Before you read up on how to get rid of fungus gnats, take a look at what's going on in the pot. The adult flies you keep swatting don't feed on your plants, but their larvae are a different story. Those live in the damp potting mix just under the surface, out of sight unless you go digging.

One raw potato saves you the digging. Lay a slice on the soil, and the larvae come up to feed on it. Lift it a couple of days later, and whatever is in the pot is often feeding on the underside. Odds are there's a potato in the pantry already, so finding out costs you pretty much nothing, really.

Why Fungus Gnat Larvae Are the Real Problem

A fungus gnat's whole life happens in or around the pot. They're small flies, often in the genus Bradysia, and a female can lay up to 200 eggs in damp potting mix. Once they hatch, the larvae eat for a couple of weeks and pupate near the surface, and new adults show up a week or so later. At room temperature, that's roughly three to four weeks per generation – so a few flies in October can be a lot more by the holidays.

It's the larvae doing the eating. Their usual food is fungi and rotting organic matter, but they'll go after fine root hairs too. While a minor infestation won't usually do any harm, enough of them can damage roots and stunt growth, especially in seedlings and young plants. Watch for something like a tiny worm: clear or whitish, no legs, a shiny black head, ¼ inch (6 mm) long at most.

How to Use the Potato Trick to Find Fungus Gnat Larvae

Cut a raw potato into slices about ¼ inch (6.4 mm) thick. Set one cut side down on the potting mix, and press it lightly so it sits flat. That contact is the whole trick. Larvae move up to the potato as a food source and gather on the moist underside where it touches the soil.

Give it a day or two before the first look. Flip the slice and check its underside, then the soil where it sat, for tiny clear larvae. Against dark, wet potting mix, they're hard to pick out, though, so a magnifying glass, like this Rongon 10X folding pocket magnifier from Amazon, helps a lot. Nothing there yet? Set it back down for another day or two. Sometimes larvae can take a few days to find the potato.

Don't leave it there for good, though. Raw potato breaks down in damp soil, and a rotting slice is just more of the decaying material larvae feed on. Toss the old one in the trash, larvae and all, and put down a fresh slice. Used like this, potato slices for fungus gnats let you track whether the numbers are dropping. Eggs won't show up on it. They don't feed, so they stay put in the mix.

Does the Potato Trick Actually Get Rid of Fungus Gnats?

So, do potatoes kill fungus gnats? Not really. The potato doesn't poison anything – it just draws feeding larvae up from the roots, and tossing a slice with larvae on it removes those ones. Eggs, pupae, and the adults flying around the room aren't affected.

Use the potato to check on things, then reach for something else to fix it. Say the slice comes up clean. That's nice, but it doesn't mean the pot's clear. Maybe there are only a few larvae, or they weren't feeding much. If flies keep hanging around, put down another slice.

What to Do If You Find Larvae

Too much water keeps a lot of fungus gnat problems going. Short days and cool rooms slow houseplants down in fall and winter, so they use less water. Water like it's still July, and the mix stays wet, which eggs and larvae love. Let the top inch (2.5cm) dry before watering again. Empty the saucer, and make sure the pot drains. Old mix holds extra water as it breaks down. If a plant's due for repotting, look for a fresh mix with perlite for drainage, like the Back to the Roots Indoor Potting Mix, available from Walmart.

Yellow sticky traps are for the adults. Push a few into the pot – you can buy sticky traps cheaply from Amazon. Each female stuck to one won't lay any more eggs, and the count tells you if things are improving. Down in the soil, the larvae aren't touched. If the gnats won't let up, try Bti (Bacillus thuringiensis subsp. israelensis), a naturally occurring soil bacterium. Despite the name, it isn't the Bt sprayed on caterpillars. This strain targets only fly larvae.

Summit Mosquito Bits use Bti to control both mosquito and fungus gnat larvae. Rather than scattering the granules on the soil, it's best to soak them in water for about 30 minutes, then apply that water to your houseplant soil. Repeat application about every five days, since Bti doesn't last long indoors.

Beneficial nematodes (Steinernema feltiae), such as BioLogic's Scanmask nematodes, are another organic option. These microscopic roundworms seek out fungus gnat larvae in potting soil. They enter the larvae and release bacteria that kill them, then feed and reproduce inside their bodies. It's pretty gruesome, but effective. They arrive alive in the mail, so use them right away. Keep the soil moist and the temperature between 60 and 90°Fahrenheit (16–32°C), or they won't work. Whichever you buy, check that the label allows indoor potted plants, then follow it.

Catching larvae early makes a fungus gnat problem much easier to control. A few larvae in one pot make for a far smaller job than a houseful of flies a month from now – and a potato slice is a cheap way to see whether your houseplant soil is hiding them.

So keep a potato around this winter. Set a fresh slice down whenever the flies show up again, and go easier on the watering while days are short. Fungus gnats have a much harder time of it once the top of the soil dries out between drinks!

Fungus Gnat Control Essentials