Fungus gnats, the tiny dark flies hovering around your pothos, breed in the moist organic soil inside the pot, not on the plant itself. Getting rid of them means targeting the larvae living in that soil and cutting off the conditions that let adults keep laying eggs. Pothos is especially prone because it’s often grown in peat-heavy mixes that stay damp between waterings. The good news is that a few straightforward changes to how you water and a targeted soil treatment can break the breeding cycle within a few weeks.
Why Pothos Soil Is a Magnet for Fungus Gnats
The small flies you see are almost certainly fungus gnats in the genus Bradysia. They’re weak fliers that stay close to the soil surface, and they’re drawn to two things: moisture and decomposing organic matter. Research on growing media has found that substrates with higher rates of decomposition and more microbial activity are the most favorable for fungus gnat breeding, with composted bark being particularly attractive to egg-laying females.1Acta Horticulturae. Fungus gnats and growing media Pothos is typically sold in peat-based potting mix, which checks both boxes: it holds moisture well and supports plenty of microbial life as the organic material slowly breaks down.
Female fungus gnats don’t seem to be picky about the exact brand of potting soil. What matters more is the moisture level and the volatile compounds released by wet, biologically active soil.2Journal of Economic Entomology. Egg-Laying Preference of Female Fungus Gnat Bradysia sp. nr. coprophila (Diptera: Sciaridae) on Three Different Soilless Substrates Pothos tolerates a wide range of watering schedules, so many owners water on autopilot and keep the soil perpetually damp. That creates a perfect nursery. Each female can lay dozens of eggs in the top layer of soil, and at room temperature the entire lifecycle from egg to adult takes roughly 25 days.3Southwestern Entomologist. Biological Cycle of the Fungus Gnat Bradysia sp. in a Lily (Lilium spp.) Crop That means a small problem becomes an obvious one within about a month.
The Single Most Effective Step Is Drying the Soil
Before you buy any product, change how you water. Let the top two inches of soil dry out completely between waterings. Pothos handles this easily; its thick, waxy leaves store enough water to tolerate some drought, and a little wilt is far less damaging than a persistent gnat infestation. Dry topsoil does two things at once: it kills young larvae that need moisture to survive, and it makes the pot unattractive to females looking for a place to lay eggs. Since moisture content plays a major role in both egg-laying site selection and larval development, simply shifting to a drier watering schedule disrupts the entire cycle.1Acta Horticulturae. Fungus gnats and growing media
Bottom watering helps too. Set the pot in a tray of water for 15 to 20 minutes and let the soil wick moisture up from below. The deeper roots get hydrated while the top layer stays relatively dry. This alone won’t eliminate an established infestation, but it removes the welcome mat for new egg-laying.
Killing Larvae That Are Already in the Soil
Drying the soil prevents the next generation, but you still have larvae chewing on organic matter (and sometimes on fine roots) right now. The larval stage lasts roughly 17 days at typical indoor temperatures, so you need something that reaches them in the soil.3Southwestern Entomologist. Biological Cycle of the Fungus Gnat Bradysia sp. in a Lily (Lilium spp.) Crop There are a few reliable options.
Bacillus thuringiensis israelensis (Bti)
Bti is a naturally occurring soil bacterium that produces a toxin lethal to fungus gnat larvae when they ingest it. You’ll find it sold as “Mosquito Bits” or “Mosquito Dunks” at most garden centers. Soak the granules in water, then use that water to drench the soil. In greenhouse experiments, exposing fungus gnats to Bti from the egg stage through pupation brought survival down to about 8 percent, compared with 84 percent in untreated controls.4Journal of Economic Entomology. Activity of Bacillus thuringiensis var. israelensis on Bradysia coprophila (Diptera: Sciaridae)
There’s a catch, though. Bti works best on young larvae. Research has shown that it loses effectiveness against second- and third-instar larvae, the older, larger ones. That means you need to apply it before populations build up and before overlapping generations develop in the soil.5PubMed. Effect of Bacillus thuringiensis subsp. israelensis and neonicotinoid insecticides on the fungus gnat Bradysia sp nr. coprophila (Lintner) (Diptera: Sciaridae) In practice, this means applying Bti-treated water every time you water for several consecutive waterings, so each new batch of freshly hatched larvae meets the toxin before they mature. One application and done will not clear an active infestation.
Beneficial Nematodes
If Bti is the most accessible option, beneficial nematodes are arguably the most thorough. Steinernema feltiae is a microscopic parasitic worm that actively hunts fungus gnat larvae in the soil, enters them, and releases bacteria that kill the host within a day or two. In floriculture trials, S. feltiae significantly reduced fungus gnat larvae in potting media, performing on par with conventional chemical insecticides.6Biological Control. Use of Entomopathogenic Nematodes and a New Monitoring Technique for Control of Fungus Gnats, Bradysia coprophila (Diptera: Sciaridae), in Floriculture A follow-up study confirmed that applying roughly 250,000 nematodes per square meter consistently suppressed fungus gnat populations across different host plants.7Biological Control. Application rate and timing, potting medium, and host plant effects on the efficacy of Steinernema feltiae against the fungus gnat, Bradysia coprophila, in floriculture
You can buy S. feltiae online. They arrive refrigerated and need to be mixed with water and applied as a soil drench. Keep the soil moist (not waterlogged) for a few days after application so the nematodes can move through the medium and find larvae. One thing to be aware of: if your home runs warm, above about 25°C (77°F) consistently, S. feltiae’s performance may drop off. At higher temperatures, other nematode species like Heterorhabditis bacteriophora have shown stronger results against fungus gnats.8Biological Control. Effect of entomopathogenic nematode species, split application and potting medium on the control of the fungus gnat, Bradysia difformis (Diptera: Sciaridae), in the greenhouse at alternating cold and warm temperatures For most indoor settings, though, S. feltiae works well.
Hydrogen Peroxide Soil Drench
A diluted hydrogen peroxide drench (roughly one part 3 percent hydrogen peroxide to four parts water) is a popular home remedy. The fizzing action on contact kills larvae and disrupts eggs near the surface. This does work to some degree, but keep in mind that hydrogen peroxide is not selective. Research on soil microbiology has shown that even moderate concentrations reduce both soil bacteria and fungi substantially.9American Chemical Society (ACS Omega). Effect of Hydrogen Peroxide on the Soil Microbial Community Structure and Growth of Malus hupehensis Rehd. Seedlings under Replant Conditions That includes beneficial microbes your pothos roots rely on for nutrient uptake. A one-time drench probably won’t cause lasting harm to a robust houseplant, but making it a regular habit could degrade your soil quality over time. Think of it as a reset button, not a maintenance strategy.
Dealing with the Adults You Can See
The adults are the visible annoyance, but they’re actually the least important part of the problem. Adult fungus gnats live only about two days and don’t bite or feed on your plant.3Southwestern Entomologist. Biological Cycle of the Fungus Gnat Bradysia sp. in a Lily (Lilium spp.) Crop Their sole mission is to mate and lay eggs in soil. Still, trapping adults reduces the number of eggs going into your pots and gives you a visible way to gauge whether your other efforts are working.
Yellow sticky traps are the standard tool. Place them horizontally at or just above soil level, where adults congregate. Greenhouse studies have shown that yellow sticky cards catch significantly more fungus gnats than other trap colors, and traps paired with a small green LED light catch even more.10Insect Science. LIME GREEN LIGHT‐EMITTING DIODE EQUIPPED YELLOW STICKY CARD TRAPS FOR MONITORING WHITEFLIES, APHIDS AND FUNGUS GNATS IN GREENHOUSES You don’t need to rig up LEDs at home, but the takeaway is that bright yellow near the soil surface works. Small stakes that hold a yellow card an inch or two above the pot are cheap and widely available.
Apple cider vinegar traps (a shallow dish with vinegar and a drop of dish soap) catch some adults, but they’re far less effective than sticky cards for fungus gnats specifically. Vinegar traps are better suited to fruit flies, a different insect that’s drawn to fermenting food rather than moist soil. If you’re not sure which bug you have, set out both kinds and see what lands where.
What Doesn’t Work as Well as You’d Think
A few popular recommendations circulate online that the research doesn’t support as well as their reputations suggest.
Diatomaceous earth (DE) is often recommended as a top dressing to kill adult gnats and block egg-laying. The idea is that the abrasive particles damage insect exoskeletons. In lab testing, however, both dry and moist layers of diatomaceous earth generally failed to prevent adult emergence or egg laying. The dry DE absorbed moisture from the growing medium beneath it, expanded, and developed cracks that larvae could pass through and females could use to reach the soil.11Journal of Economic Entomology. Effect of Diatomaceous Earth and Trichoderma harzianum T-22 (Rifai Strain KRL-AG2) on the Fungus Gnat Bradysia sp. nr. coprophila (Diptera: Sciaridae) In a greenhouse pot, where you’re regularly adding water, DE is almost certain to absorb moisture and lose its structure. A thin layer of sand or fine gravel works better as a physical barrier because it doesn’t crack.
Neem oil sprays applied to leaves or the soil surface have a mixed track record against fungus gnats specifically. Research on azadirachtin, the active compound in neem, found that its toxicity to fungus gnat larvae varied depending on temperature and the type of substrate, with effectiveness fluctuating across conditions.12SpringerOpen / Environmental Sciences Europe. A new ecotoxicological test method for genetically modified plants and other stressors in soil with the black fungus gnat Bradysia impatiens (Diptera): current status of test development and dietary effects of azadirachtin on larval development and emergence rate Neem may slow development somewhat, but it’s not as reliably lethal as Bti or nematodes. If you already use neem for other pests, it can be part of the picture, just don’t lean on it alone for a fungus gnat problem.
Predatory Mites for Ongoing Control
If you have a large plant collection and fungus gnats are a recurring issue, predatory soil mites are worth considering. Stratiolaelaps scimitus (sometimes sold under its older name, Hypoaspis miles) is a tiny soil-dwelling mite that feeds on fungus gnat larvae, among other small soil organisms. These mites reproduce successfully on fungus gnat larvae as their sole food source and can establish a self-sustaining population in your pots.13PubMed. Development and reproduction of Stratiolaelaps scimitus (Acari: Laelapidae) with fungus gnat larvae (Diptera: Sciaridae), potworms (Oligochaeta: Enchytraeidae) or Sancassania aff. sphaerogaster (Acari: Acaridae) as the sole food source They’re invisible to the naked eye, harmless to plants and people, and marketed specifically for greenhouse use against fungus gnats.
The catch is cost and availability. A container of predatory mites runs $25 to $40 and needs to be shipped quickly because they’re living organisms. For a single pothos on a windowsill, this is overkill. For someone with 20 or 30 houseplants sharing a room, it can be the most hands-off long-term solution available. You introduce them once, they eat the larvae, and as long as the soil stays moist enough to support them, they persist. They’ll also eat thrips pupae and other soil-dwelling pests, which is a nice bonus if you grow a mixed collection.
Repotting and Soil Swaps
Sometimes the fastest way to deal with a heavy infestation is to repot. Remove the pothos from its container, shake off as much of the old soil as you can, rinse the roots under lukewarm water, and repot in fresh mix. If you’re repotting anyway, this is a good time to amend the soil to make it less hospitable to gnats going forward. Mixing in perlite, coarse pumice, or orchid bark increases drainage and reduces the amount of fine, moisture-retaining organic matter in the pot. A chunkier mix dries faster and provides fewer pockets of the damp, decomposing material that gnats need to breed.
Since composted bark is highly attractive to egg-laying females, consider using a mix that favors mineral components over organic ones.1Acta Horticulturae. Fungus gnats and growing media Pothos grows well in a blend that’s roughly half perlite or pumice and half peat or coco coir. The mineral components create air pockets that dry quickly, and the reduced organic fraction means fewer microbes and less food for gnat larvae. You can also top the finished pot with a half-inch layer of coarse sand or decorative pebbles. Unlike diatomaceous earth, inorganic top dressings don’t absorb moisture or crack, and they create a barrier that female gnats have trouble navigating when they need to reach the soil to lay eggs.
Preventing Gnats from Coming Back
Once you’ve cleared an infestation, a few habits keep it from returning.
- Water by weight: Lift the pot before watering. If it still feels heavy, wait another day or two. Pothos wilts visibly before it suffers permanent damage, so err on the dry side.
- Quarantine new plants: Any new pothos or houseplant you bring home may carry eggs or larvae in its soil. Keep it isolated from your collection for two to three weeks and monitor for adults on a yellow sticky card before moving it near other plants.
- Empty saucers: Standing water in drip trays wicks back into the soil and keeps the bottom of the pot perpetually wet, exactly where larvae pupate.
- Check other pots: Fungus gnats don’t care about species. If your pothos has them, nearby plants with moist soil probably do too. Treat the whole shelf or room, not just one pot.
Fungal gnat infestations in houseplants are common enough that commercial greenhouses consider them one of their most persistent problems alongside aphids and thrips.14PubMed Central. Prevention and control of pests and diseases The tools used at home are the same ones that work at scale: managing moisture, biological larvicides, and sticky traps. The difference is that a single pothos in a living room is a much simpler system to control than a greenhouse full of crops. Adjust how you water, treat the soil once or twice with Bti or nematodes, and give it a month. The 25-day lifecycle means that if no new eggs hatch, the last generation will emerge and die off on its own without leaving replacements.
When Gnats Keep Coming Back Despite Everything
Occasionally someone does all of this and the gnats persist. The most common reason is a hidden moisture source. Check whether the pot has a drainage hole that’s partially blocked, trapping water at the bottom. Ceramic pots without drainage are notorious for this, because the soil at the base stays saturated even when the top feels dry. Roots sitting in that standing water create an anaerobic zone that’s ideal for gnat larvae and harmful to the plant.
Another overlooked culprit is the potting mix itself. Cheap bagged soil sometimes arrives pre-infested with eggs or larvae. If you’ve repotted into fresh mix and gnats appeared within a week, the mix was likely the source. Baking small batches of potting soil in the oven at around 180°F (82°C) for 30 minutes before use kills eggs and larvae without destroying the soil’s structure. It’s a nuisance, but it guarantees clean soil for plants you’ve just freed from an infestation.
Finally, if you keep your pothos in a bathroom or kitchen where humidity stays high, the soil surface may never dry enough even with reduced watering. Moving the plant to a room with better airflow, or setting up a small fan nearby, speeds up surface drying and makes the pot less inviting. Fungus gnats are weak fliers, so even gentle air movement near the soil discourages adults from landing and laying eggs.