How to Get Rid of Fungus Gnats

Fungus gnats alone aren't too bad, but the fungus gnat larvae are downright destructive. Epic Gardening Founder Kevin Espiritu explains the control methods for these dastardly pests!

Written by Kevin Espiritu Horticulture review by Sarah Jay Last updated: October 2, 2024 | 8 min read

An overhead macro image of a dark-winged fungus gnat on the surface of soil. The underbelly of the fly is light orange, and the dorsal region is iridescent black, with hints of blue and turquoise.

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I’ve dealt with fungus gnats in my indoor and outdoor garden. While they didn’t do a lot of damage indoors, they decimated my outdoor sprouts during a particularly rainy spring season. They may not have the strength in numbers, but they damage plants quickly.

Controlling fungus gnat populations is key. They go for potted plants and feed on organic material (plant roots, and root hairs too). Overly moist soil is one of the attractants for these creepers. But there are other things that bring them in.

Adult gnats need to be controlled to keep larvae at bay because their feeding isn’t the end of the problems. They can also infect plants as they spread fungal diseases.

What Are Fungus Gnats?

A macro shot of a Mycetophilidae fungus gnat on a blade of grass. The body of the fly is light tan on its undersides. Its wings are black, and its dorsal region has stripes and blotches of black on darker tan. — A Mycetophilidae fungus gnat has a pale color compared to other species.

If you live in a temperate or tropical area, you’ve most likely seen insect pests called fungus gnats before. They’re small flies that infest soil potting soil, container media, and other organically decomposed materials.

Fungus gnat larvae feed on fungi and plant roots in organic matter. They’re a problem in commercial greenhouses, indoor houseplants, and potted plants. Adult fungus gnats emerge when warmer temperatures occur in spring and summer. I’ve personally dealt with the indoor version of fungus gnats.

Adult fungus gnats are weak fliers, giving them a similar appearance to that of mosquitos. They have long legs and antennae longer than their head. These antennae distinguish them from one of their look-alikes, robust short flies. However, short flies are strong fliers.

There are a few species that grow up to a ½ inch long, while most adult fungus gnats are 1/16 to ⅛ of an inch long. Their wings are clear and grayish, and the Bradysia species has a characteristic y-shaped wing vein. The larvae grow a bit larger than the adults.

Fungus gnat larvae have a black, shiny head with a white to clear body that can reach almost a quarter inch in length. They’re hard to locate because they tend to remain under the soil’s surface, out of view. They may, however, make a shiny trail on the soil surface of heavily infested plants.

Life Cycle

A macro shot of a fungus gnat larva, likely from the Keroplatidae family. The larva is mostly tan, with darker brown stripes on its back. Its head is black. There is mucus covering the larva's body and strings of it spread around the area. Presumably, the gnat is being photographed on a leaf surface, as evidenced by pores on a brown surface. — Fungus gnat larva, likely Keroplatidae family.

The life cycle of the fungus gnat is made up of four stages: egg, larvae, pupae, and adult. Adult fungus gnats lay about three hundred eggs in the moist, rich soil at the base of the plant during their short lifespan. They live for about a week as adults, making the most of their time!

About four to six days after the eggs are laid, the gnat larvae emerge. These larvae are extremely tiny at first. During the roughly two-week period in which fungus gnat larvae grow, they rapidly increase in size by eating their way through organic matter in the soil.

Once they’ve achieved maximum growth, they undergo a three to four-day pupal phase. The pupae are hidden under the soil’s surface. When the fungus gnat adult emerges, they start laying eggs immediately.

Because of this quick life cycle, multiple generations are born in quick succession as larvae feed. Subsequently, overlapping generations of adults emerge from the soil surface.

Many stages of the life cycle may be present at any given time. Not only do you need to get rid of the annoying gnats themselves, but you need to deal with the larvae and find a way to sterilize the eggs before they hatch.

Identifying the Damage

An up-close shot of a dark-winged fungus gnat larvae feeding on plant matter. The larva is ridged and light yellow with red eyes. Strings of mucus surround it as it feeds on dead leaf tissue. — Dark-winged fungus gnat larvae feed on plant matter, generally at the root level.

Fungus gnat adults don’t do much direct damage to plants. It’s the fungus gnat larvae that are most damaging. An infestation causes damaged roots that stunt growth, especially in young plants.

In houseplants that have too much moisture in their potting soil, high populations of adults and larvae kill plants. Before that, it’s difficult to tell from symptoms whether or not the issue is feeding larvae, overwatering, fungi infections, or generally unhealthy roots.

These don’t cause much damage outdoors – though they did for me in that one freak spring when it wouldn’t stop raining. More often, they infest houseplants or potted plants kept indoors or in a greenhouse.

Mixes that contain a lot of coco coir or peat moss are prime environments. Sod farms are another area where fungus gnat adults can easily proliferate.

Fungus Gnats as Disease Vectors

Potato plants with (right) and without (left) verticillium fungal infection, which can be spread by fungus gnats. The two plants have been pulled from the ground and are in a semi-mature stage of growth with small tubers forming at the root level. The plants on the left are much more profuse with better root development. Those on the right have few tubers and sparse roots. Both plants lay on a truck bed. — Potato plants with (right) and without (left) verticillium fungal infection, which can be spread by fungus gnats.

Yes, fungus gnats damage plants, but it’s important to kill fungus gnat larvae and adults because they may carry diseases. Those plants in the process of recovering from fungus gnat problems still face the risk of fungal infiltration.

The larvae spread fungus spores dropped by adult gnats to your plant’s roots, potentially causing a number of common plant diseases. The most common ones include black root rot, Pythium blight, Verticillium wilt, Botrytis blight, and Fusarium wilt.

As a precaution, treat any affected plant and those in the area with a copper fungicide a day or two after pesticide application. This ensures that if your plant had contact with potentially dangerous disease fungi spores, they won’t contract the disease.

A root drench is more effective than spraying the plant’s foliage, as that’s where damage would be done.

Types of Fungus Gnats

An up close image of a fungus gnat larva, likely from the family Sciaridae. The larva is on the soil surface, which is rich and red clay with flecks of dark black humus. The larva itself is whitish yellow with a black head. — Fungus gnat larvae, like this one from the family Sciaridae, cause damage to plants.

There are six different families of insects that make up the broader category of fungus gnats. These include the Sciaridae, Mycetophilidae, Ditomyiidae, Bolitophilidae, Diadocidiidae, and Keroplatidae.

Most fungus gnat species are not harmful to our gardens, but the dangerous ones are in the Sciaridae family, especially those in the Orfelia and Bradysia species.

While adults don’t cause any lasting damage, the fungus gnat larvae of that family will move on to plant roots once their preferred foodstuff is gone. The larvae chew holes in the roots and cause yellowing, wilting, and eventually plant death in large numbers.

Not only do they damage roots, but the Sciaridae can spread disease. Since their preferred food is fungal growth, they easily pick up spores from infected plants or soil and spread diseases.

9 Ways to Get Rid of Fungus Gnats

So, fungus gnats have attacked your African violets? Let’s talk about modes of controlling adults and larvae too. We recommend an integrated pest management strategy to cover all your bases and protect your plants for good.

Soak Your Soil

A shot of a gardener (out of frame) watering their green lettuce plants with a teal plastic watering can. The plants are planted in a wooden raised bed, set in a large garden. As the water pours into the garden, the sunlight shines through illuminating the water stream. — Combine some hydrogen peroxide or neem oil with water to remove fungus gnats.

Interestingly enough, a common household item is one of the top recommendations that I have for controlling these pests. Hydrogen peroxide (the standard 3% topical variety) can be used as a soil drench.

Mix one part peroxide with four parts water, and pour it through the soil at the root zone until it begins to come out of the base of the pot. The peroxide kills fungus gnat larvae on contact.

Neem oil is also an effective soil soak to combat fungus gnat larvae. Dilute the oil with water per the manufacturer’s directions and directly drench the soil at the roots of the plant. You can also spray the upper portion of the plant to keep adult gnats at bay.

Concentrated azadirachtin naturally occurs in neem oil. It’s safe in hydroponic use as well as in greenhouses, gardens, and indoors. Use it per the manufacturer’s directions in the same way you would use neem oil.

Organic Insecticides

A male gardener's hand with a stainless steel spray bottle that has a plastic sprayer, coming from the left, spraying a mustard plant with organic pesticide. In the background there is a mature dill or asparagus plant. — Use organic insecticides to help you control fungus gnats in your garden.

Pyrethrin sprays are effective against fungus gnats and their larvae. To use them, lightly mist all plant surfaces and the top of the soil. You don’t want the plants dripping wet: a thin mist is enough.

If there are fungus gnat larvae in the soil, spray the soil directly where adults typically rest to thoroughly moisten the top, then avoid overwatering. Let the soil dry to at least a two-inch depth before you water again.

These sprays can kill beneficial insects, so opt for other control methods where possible to protect your garden ecosystem.

Predatory Insects

An up close, overhead image of Sciaridae fungus gnat on a light green leaf surface. The fly's wings are opened as if it was just in flight. The body of this fly is totally black and shiny. The wings are grey and transparent. The fly faces the direction of the camera. — A Sciaridae fungus gnat on a leaf surface.

A predatory mite of the Hypoaspis miles species preys upon adults and larvae. It also attacks other insects like thrips and springtails, which can be a nuisance in the garden or greenhouse. These are best employed outdoors or in greenhouses as single plant specimens are harder to treat.

There are many biological control agents to choose from when it comes to getting rid of fungus gnats. Apply insecticides in conjunction with good cultural practices and prevention methods and you’re set!

Prevent Them

Comparing the gnat to the center of this flower, you can see how tiny it is.

Do not overwater. Waterlogged houseplant soil is a prime breeding ground for these insects. Don’t allow standing water to remain in saucers below houseplants for long. Repair any irrigation system leaks that crop up, and keep your spare soils away from your potted greenhouse plants.

Pasteurize the compost in your soil to eliminate any larvae that may be feeding on the organic materials there. Use commercial blends that have been treated with heat if you can’t or don’t want to do it yourself. Move any infected plant matter and soils away from others while you treat the problem.

Baccillus Thuringiensis

One particular form of bacteria will destroy fungus gnat larvae: Bacillus thuringiensis var. israelensis. These bacteria are not in most commercial BT sprays but are available as active ingredients in commercial products. Use them to fertilize your growing media, and you should see a decline in your fungus gnat problems.

Another way to get this bacteria in your soil is by sprinkling Mosquito Bits or Mosquito Dunks over the surface and watering them in. You can use these both indoors and outdoors. They aren’t just for mosquito killing!

As they break down, they release Bacillus thuringiensis var. israelensis into your soil where it gets to work killing larvae. Mosquito dunks are larger than bits and take longer to take effect.

Monitor New Plants

An overhead POV shot of a male gardener's hands inspecting grass plugs from a pallet of them that sits in front of him. He has removed one of the plugs from a small terracotta colored pot to inspect the roots. As he does this, soil spills from the root mass and onto the white wooden table below. — Inspect the roots of your plants for larvae or eggs before buying them.

Inspect each plant you purchase before buying it. Check the base, gently looking through the growing medium to find signs of the clear or whitish fungus gnat larvae with a shiny black head. Avoid those with adults flying around them.

To be doubly sure they are pest-free, keep them quarantined from other plants for at least three to four weeks. This gives you plenty of time to spot newly-emerged weak fliers, as well as establish temporary control methods before they can spread.

Beneficial Nematodes

A gardener wearing white gloves carefully repots a green plant into a clean white pot. With precision, she uses a trowel to transfer rich, dark soil into the awaiting container, ensuring the plant's healthy growth. — Focus on soil health to eliminate problems.

Beneficial nematode species can also play a major part in eradicating the fungus gnat larvae. You won’t be able to see these microscopic soil-dwellers, but they will take out the larvae along with hundreds of other pests.

For people treating an indoor plant, add these nematodes to your soil with a pot popper. Larger garden or full yard coverage can be achieved using commercial formulations, which disperse the nematodes evenly with water from a hose.

Be aware that you cannot use beneficial nematodes at the same time as your hydrogen peroxide soil drench. This kills the nematodes. Wait for at least a couple of weeks after the infestation and then add nematodes back into the soil.

If you’re working outdoors, apply nematodes in cooler temperatures. Hot weather will kill them before they can do their job. Remember repeated applications are necessary, twice per year, annually.

Sticky Traps

A wide shot of a productive outdoor row garden full of greens. In the front there are two rows of lettuce, one red and one green. Behind this are rows of tatsoi, cabbage, and mustard. In each of the rows, there are yellow sticky traps placed in the ground with stakes. — Yellow sticky traps are commonly used in gardens and greenhouses to control gnats.

Use yellow sticky traps to find adult gnats, or use raw potato slices. Since it only takes one gnat to lay potentially hundreds of eggs, you want to keep the adults at bay!

For an indoor plant, yellow sticky cards (or sticky traps) work well. Outdoors, you can use double-sided yellow sticky cards or traps. Alternatively, trap them with a shallow container filled with apple cider vinegar.

Mulch

Finally, mulching has a good effect against fungus gnat infestation. The mulch keeps adults away from the growing medium. This prevents them from laying their eggs. Use a thick layer of stone chip mulch over your potting mix.

Key Takeaways

Fungus gnats aren’t a big problem outdoors, but exceptionally rainy and wet times may prove beneficial to them. If you keep an eye on your plants, both indoors and outdoors, and do what you need to keep them away, your plants will do well.

Fungus gnats are only hard to control if they really have a foothold in your garden. Use prevention techniques to make it hard for them to successfully multiply. Using organic techniques is one of the most useful ways to successfully remove and control them.

How to Get Rid of Fungus Gnats

What Are Fungus Gnats?

Life Cycle