Fungus Gnats Management Guidelines--UC IPM (2023)

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Pests in Gardens and Landscapes

Fungus Gnats

Revised8/13

In this Guideline:


  • Identification
  • Damage
  • Life cycle
  • Management
  • About Pest Notes
  • Publication
  • Glossary

Fungus gnat adult caught in a yellow sticky trap.

The shore fly (right) has a more robust body and shorter antennae than a fungus gnat (left).

(Video) HOW TO PREVENT + GET RID OF FUNGUS GNATS | Plant pest management deep dive– everything I know! 🤓

Fungus gnat larvae.

Fungus gnats are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots and can be a problem in greenhouses, nurseries, potted plants and interior plantscapes. Adult fungus gnats may emerge from houseplants indoors and become a nuisance.

IDENTIFICATION

Fungus gnats (Orfelia and Bradysia species), also called darkwinged fungus gnats (Sciaridae), are dark, delicate-looking flies similar in appearance to mosquitoes. Adult fungus gnats have slender legs with segmented antennae that are longer than their head. Their long antennae distinguish them from the more robust shore flies, which are also found in greenhouses, associated with algae and decomposing organic matter, but have short bristle-like antennae. Although a few species are up to 1⁄2 inch long, fungus gnat adults commonly are about 1⁄16 to 1⁄8 inch long. Wings are light gray to clear, and the common Bradysia species have a Y-shaped wing vein.

Because adult fungus gnats are attracted to light, you first might notice these pests flying near windows indoors. However, in comparison with more active species such as the common housefly (Musca domestica), fungus gnats are relatively weak fliers and usually don’t move around much indoors. Fungus gnats often remain near potted plants and run across (or rest on) growing media, foliage, compost, and wet mulch piles.

Females lay tiny eggs in moist organic debris or potting soil. Larvae have a shiny black head and an elongated, whitish-to-clear, legless body. They eat organic mulch, leaf mold, grass clippings, compost, root hairs, and fungi. If conditions are especially moist and fungus gnats are abundant, larvae can leave slime trails on the surface of media that look like trails from small snails or slugs.

DAMAGE

Adult fungus gnats don’t damage plants or bite people; their presence is primarily considered a nuisance. Larvae, however, when present in large numbers, can damage roots and stunt plant growth, particularly in seedlings and young plants. Significant root damage and even plant death have been observed in interior plantscapes and in houseplants when high populations were associated with moist, organically-rich soil. Thus, a houseplant that is wilting may not indicate a lack of water, but rather root damage by fungus gnat larvae or (more commonly) other causes of unhealthy roots. However, too much or too little water, root decay fungi, and improper soil conditions (e.g., poor drainage, or waterlogging) are much more common causes of wilted plants.

Serious fungus gnat damage is more common in greenhouses, nurseries, and sod farms. Although larvae also feed on plant roots outdoors, they don’t usually cause serious damage.

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LIFE CYCLE

Fungus gnats develop through four stages—egg, larva (with four larval stages or instars), pupa, and adult. The tiny eggs and oblong pupae occur in damp organic media where females lay eggs and larvae feed. At 75ºF, eggs hatch in about 3 days, the larvae take approximately 10 days to develop into pupae, and about 4 days later the adults emerge. A generation of fungus gnats (from female to female) can be produced in about 17 days depending upon temperature. The warmer it is, the faster they will develop and the more generations will be produced in a year.

Fungus gnats have many overlapping generations each year. Outdoors, they are most common during winter and spring in interior areas of California, when water is more available and cooler temperatures prevail. They can occur during any time of the year in moist coastal regions and indoors.

MANAGEMENT

Most of the fungus gnat’s life is spent as a larva and pupa in organic matter or soil, so the most effective control methods target these immature stages rather than attempting to directly control the mobile, short-lived adults. Physical and cultural management tactics—primarily the reductions of excess moisture and organic debris—are key to reducing fungus gnat problems. Commercially-available and naturally-occurring biological control agents can also control this pest. Insecticides are considered an important control option in some commercial plant production but generally aren’t recommended for fungus gnat management in and around the home.

Monitoring

Visual inspection for adults usually is adequate for determining whether a problem exists. You will see adults resting on plants, soil, windows, or walls, or you might see them in flight. Besides looking for adults, check plant pots for excessively moist conditions and organic debris where larvae feed. Yellow sticky traps can be used to trap adults. Chunks of raw potato placed in pots with the cut sides down (not the peels) are sometimes used to monitor for larvae.

Water and Soil Management

Because fungus gnats thrive in moist conditions, especially where there is an abundance of decaying vegetation and fungi, avoid overwatering and provide good drainage. Allow the surface of container soil to dry between waterings. Clean up standing water, and eliminate any plumbing or irrigation system leaks. Moist and decomposing grass clippings, compost, organic fertilizers, and mulches are also favorite breeding spots. Avoid using incompletely-composted organic matter in potting media unless it is pasteurized first, because it will often be infested with fungus gnats. Improve the drainage of the potting mix (e.g., increase the proportion of perlite or sand in the mix). Minimize organic debris around buildings and crops. Avoid fertilizing with excessive amounts of manure, blood meal, or similar organic materials. Screen and caulk leaky windows and doors to help prevent pests from coming indoors.

If you have infested plants, don’t move them to new areas where flies can emerge to infest other pots. In some cases you may wish to toss out severely infested plants.

Purchase and use only pasteurized container mix or potting mix. Commercial growers often treat potting soil with heat or steam before using it; this will kill flies and the algae and microorganisms they feed on. Home gardeners can solarize soil:

  • Moisten it.
  • Place it in a bag of transparent plastic or black plastic.
  • Make the pile no deeper than about 8 inches.
  • Place the bagged soil on a slightly elevated surface, such as a pallet in a sunny location, for about 4 to 6 weeks.

See the Pest Note: Soil Solarization for details. Store pasteurized potting soil off the ground and in closed containers to prevent it from becoming infested before use.

Trapping

In home situations where fungus gnat adults are a nuisance, it may be possible to reduce the problem by using sticky traps available at retail nursery and garden centers. Yellow sticky traps can be cut into smaller squares, attached to wooden skewers or sticks and placed in pots to trap adults. Also, raw potato chunks placed in the soil are very attractive to fungus gnat larvae. These may be used not only to check pots for larvae but also to trap them away from plant roots. After a few days in a pot, remove infested chunks, dispose of them, and replace with fresh ones.

Biological Control

Three commercially available biological control agents can be purchased to control fungus gnats in pots or container media (Table 1). These include Steinernema nematodes, Hypoaspis predatory mites, and the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti). Several Bti products (Mosquito Bits, Gnatrol) are readily available in retail nurseries and garden centers, so these products may be the most convenient for home gardeners to use. Bti does not reproduce or persist indoors, so infestations in potting media might require repeated applications at about five-day intervals to provide control. Nematodes and Hypoaspis mites must be mail-ordered and are live and perishable products, requiring immediate application. Nematodes can provide relatively long-term control of fungus gnat larvae, and they can be self-reproducing after several inoculative applications to establish their populations. Steinernema feltiae is more effective against fungus gnats than other commercially available nematode species. Mix Bti or nematodes with water, and apply as a soil drench, or spray onto media using a hand-pump spray bottle or other spray equipment, following label directions.

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Several natural enemies help to manage fungus gnat populations in outdoor systems, such as landscapes and gardens, and indoors in greenhouses and conservatories, including the predatory hunter flies, Coenosia spp. These flies catch and consume adult fungus gnats in mid-air, and prey on fungus gnat larvae in soil while developing as larvae themselves. Conserve these and other natural enemies by avoiding broad-spectrum insecticide applications.

Table 1. Commercially Available Biological Pesticides and Natural Enemies for Controlling Fungus Gnat Larvae.
Biological Comments
Bacillus thuringiensis subspecies israelensis (Bti) (Gnatrol) A naturally occurring, spore-forming bacterium produced commercially by fermentation. Bti applied at labeled rates provides temporary control and is toxic only to fly larvae, such as mosquitoes, black flies, and fungus gnats. Repeat applications commonly are needed for long-term control. This Bt is a different subspecies from that applied to foliage to control caterpillars. Bt labeled for caterpillars is not effective against fly larvae.
Hypoaspis (=Geolaelaps or Stratiolaelaps) miles A light-brownish predaceous mite adapted to feeding in the upper layers of moist soil. Preys on fungus gnat larvae and pupae, thrips pupae, springtails, and other tiny invertebrates. Commercial mites commonly are shipped in a shaker-type container used to apply them. Recommended rates in commercial nurseries are about 1/2 to several dozen mites per container or square foot of media. Make applications before pests become abundant. Hypoaspis probably won’t perform very well in individual houseplants and probably isn’t a good choice for use in homes.
Steinernema feltiae This nematode is effective when temperatures are between 60° to 90°F and conditions are moist. You can apply it as a soil drench and to media using conventional spray equipment. Nematodes reproduce and actively search for hosts, so under moist conditions they can provide season-long control after several initial applications to establish populations.
These materials are essentially nontoxic to people and are compatible for application in combination. Bt is available from many well-stocked nurseries and garden supply stores. Predaceous mites, Bti, and nematodes, are commercially available through mail order from special suppliers.
Chemical Control

Insecticides are rarely warranted to control these flies in and around homes. However, if you do apply an insecticide for fungus gnats, consider using Bti or Steinernema feltiae nematodes to control the larvae; see the section Biological Control for more information.

If Bti or nematodes aren’t available and high populations are intolerable, pyrethrins or a pyrethroid insecticide may provide temporary, fast-acting control. Spray the surface of potting soil and plant parts where adults typically rest. Do not aerially fog indoors or attempt to spray adult gnats in flight. Be sure the product is labeled for your particular use (e.g., for "house plants") and read and follow the product's directions.

Pyrethrins have low toxicity to people and pets and are the active ingredients in the botanical pyrethrum, which is derived from flowers of certain chrysanthemums. Many products include a petroleum-derived synergist (piperonyl butoxide, or PBO) to increase pyrethrum effectiveness. Pyrethroids (e.g., bifenthrin, permethrin) are synthesized from petroleum to be chemically similar to pyrethrins; they often are more effective and persistent but are more toxic to beneficial insects. When using these products on houseplants or interiorscape containers, if possible move plants outdoors for treatment as a precaution, and wait about a day after applying the chemical before bringing them back inside.

For information on managing fungus gnats in commercial flower, nursery or greenhouse operations, see the UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries and the book Integrated Pest Management for Floriculture and Nurseries.

WARNING ON THE USE OF PESTICIDES

ADAPTED FROM

Dreistadt, S. H. 2001 Pest Note: Fungus Gnats, Shore Flies, Moth Flies and March Flies. Oakland: Univ. Calif. Div. Agric. Nat. Res. Publ. 7448.

REFERENCES

Cloyd, R. A. 2010. Fungus gnat management in greenhouses and nurseries (PDF). Kansas State University Agricultural Experiment Station and Cooperative Extension Service. Publication MF-2937:

Dreistadt, S. H. rev. 1986. Fungus Gnats and March Flies. Oakland: Univ. Calif. Div. Agric. Nat. Res. Publ. 7051.

Dreistadt, S. H., J. K. Clark, and M. L. Flint. 2001. Integrated Pest Management for Floriculture and Nurseries. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3402.

(Video) Identifying Insect Pests in the Home and Garden

Harris, M. A., R. D. Oetting, and W. A. Gardner. 1995. Use of entomopathogenic nematodes and a new monitoring technique for control of fungus gnats, Bradysia coprophila (Dipt.: Sciaridae), in floriculture. Biological Control 5:412-418.

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3392.

Nielsen, G. R. 1997. Fungus Gnats. Department of Plant and Soil Science, University of Vermont Extension. Publication EL 50:

Stapleton, J.J.; C.A. Wilen, and R.H. Molinar. Pest Notes: Soil Solarization. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7441.

Wright, E. M., and R. J. Chambers. 1994. The biology of the predatory mite Hypoaspis miles (Acari: Laelapidae), a potential biological control agent of Bradysia paupera (Dipt.: Sciaridae). Entomophaga 39:225-235.

PUBLICATION INFORMATION

Pest Notes: Fungus Gnats

UC ANR Publication 7448

Authors:
  • J.A. Bethke, UC Cooperative Extension, San Diego Co
  • S. H. Dreistadt, UC Statewide IPM Program, Davis

Produced by University of California Statewide IPM Program

PDF: To display a PDF document, you may need to use a PDF reader.

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FAQs

What is the best insecticide for fungus gnats? ›

Steinernema feltiae is more effective against fungus gnats than other commercially available nematode species. Mix Bti or nematodes with water, and apply as a soil drench, or spray onto media using a hand-pump spray bottle or other spray equipment, following label directions.

How do you treat fungus gnats in a greenhouse? ›

Apply insecticide drenches to the top 1 inch of soil to kill larvae; avoid applying excessive spray volume that may leach or move insecticide too deeply into growing media. Pyrethrins and other adulticides such as aerosols, foggers, or sprays can quickly, but temporarily, reduce adult fungus gnat numbers.

How do you prevent fungus gnats? ›

Keep soil dry: Fungus gnats seek out moist soil, so allowing your houseplants to dry out a bit between waterings can slow down or stop an infestation. Let the top inch or two of soil dry out before watering again, and try to go as long as possible between waterings.

Is there an insecticide for fungus gnats? ›

The systemic insecticide imidacloprid will also kill fungus gnat larvae when applied to the growing medium. This active ingredient is available in a number of houseplant insecticide formulations as granules, slow-release “spikes”, and in combination sprays with a pyrethroid-based insecticide.

How often use neem oil for fungus gnats? ›

Neem oil spray

Spray the soil of the houseplant with diluted neem oil to take down the baby gnat larvae. Wet the top 5-10cm of potting mix using neem oil every week for two months or until the flying adults disappear. Make sure you don't overwater your plant while applying neem oil spray to your houseplants.

Will hydrogen peroxide eradicate fungus gnats? ›

It is readily available, easy to work with, and won't harm your house plants. The best news? Hydrogen peroxide reportedly kills fungus gnat eggs, larvae, pupae, and adult flies on contact.

Why do fungus gnats keep coming back? ›

Gnats, especially fungus gnats, are attracted to damp soil and that's where they lay their eggs. If you're overwatering your plants, these bugs will find their way there and start laying eggs. They can lay up to 800 eggs in one breeding cycle so this can become a major problem very quickly.

How do you monitor fungus gnats? ›

To monitor for fungus gnat larvae, place potato slices on the soil surface of potted plants. If there are fungus gnat larvae in the soil, they will come to the surface to feed on the potato tissue. Check the slices for maggots after three to four days.

How do you disinfect soil from fungus gnats? ›

Mix two tablespoons of dish soap in a quart of water. Slowly and thoroughly drench the top of the soil with the soapy solution. Do this the next three times you water your houseplant collection. The soap should kill the young fungus gnats.

What kills fungus gnats in soil? ›

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.

Can fungus gnats live in Coco coir? ›

These insects love damp organic matter, which happens to be great for your plants too. The more “good stuff” in your soil, the more likely that you are to attract fungus gnats. Soils that hold moisture especially well, including those with coconut coir, are more likely to see fungus gnats.

How do you treat soil for fungus gnats before planting? ›

You open a fresh bag of potting soil to fill up your plant's pot when you see them – fungus gnats. They can get into your potting soil and wreak havoc.
...
  1. Sterilize with Heat.
  2. Use Hydrogen Peroxide or a Fungicide.
  3. Tightly Seal the Soil in Another Bag After Opening.

What are fungus gnats most attracted to? ›

Fungus Gnats - Fungus gnats are attracted to decaying organic matter in the soil that is moist. In the home, fungus gnats are usually found in potted plants that are overwatered.

Does neem work on fungus gnats? ›

Neem oil is an all-natural, non-toxic insecticide that kills many pests on contact, including fungus gnats. To deal with this particular threat, a neem soil soak or neem cakes are perhaps the best remedies out there.

How do you get rid of fungus gnats once and for all? ›

Eradicate your fungus gnat infestation for good with the following method: Put up a bunch of sticky traps, top-dress the soil with sand and do nematode soaks every 10 to 14 days until the problem is resolved. Also, stop overwatering your plants and eliminate any unnecessary sources of moisture. That's it.

Can I put neem oil directly on soil? ›

Use as soil drench: To prevent root rot, pour the solution on to the soil around the plant to soak completely. Repeat after 2 weeks. Use to maintain good plant health: Once a month spray all plants in your garden with the Neem Oil solution. This will help to repel any pests and ensure good health.

Can fungus gnat eggs survive in dry soil? ›

Neither fungus gnats nor their larvae can survive in dry soil, so let your soil dry out completely between watering as often as possible. This will help the topsoil (where the gnats lay their eggs) stay dry while keeping your plants hydrated and happy.

Will fungus gnats go to vinegar? ›

You can make your own organic traps to kill the adult fungus gnats. You can fill the bottom of a deep bowl with apple cider vinegar or red wine, then add several drops of liquid dish soap. Put it near your infested house plants. The gnats love it, and will fall into it and die.

How do you use 3% hydrogen peroxide for fungus gnats? ›

Mix one part 3% hydrogen peroxide with four parts water. Allow the top layer of your soil to dry, and then water your plants with this solution as you normally would. The soil will fizz for a few minutes after application; this is normal. Contact with hydrogen peroxide will kill fungus gnat larvae on contact.

What kills gnats permanently? ›

Here's how it works: Bleach is a most effective method of removal for gnats that tend to congregate around drains. Whether it's your sink, bathtub, or shower drain, pouring a half cup of bleach down the drain should wipe them out.

How long does it take to get rid of a fungus gnat infestation? ›

Apply a layer of DE to the top of the soil, or mix it into soil mix if you are repotting a plant. Did It Work? DE was very effective in killing off both larvae and gnats, with good results within a week. After around three weeks, the population was all but gone.

Do fungus gnats mean root rot? ›

If you've got fungus gnats, you most likely also have pythium root rot, as gnats can carry it.

What is the life cycle of fungus gnats? ›

Life Cycle

Fungus gnats develop through four stages: egg, larvae (four larval stages or instars), pupa, and adult. They produce many generations in a year. Adult females deposit 30 to 200 whitish-yellow eggs singly or in clusters in crevices or cracks on the surface of growing media, and in moist, organic debris.

What eats fungus gnats? ›

Use natural predators. Nematodes (Steinernema) and predatory mites (Hypoaspis) feed on fungus gnats and their larvae and can be applied to the potting mix as eggs that will hatch and begin to feed. They will gradually attack the problem until there is nothing left for the predators to feed on.

Can you microwave soil to get rid of fungus gnats? ›

The fastest way to sterilize soil with heat is to use a microwave that effectively kills gnats. With this method, you can sterilize a few pounds of soil in a few minutes. You also do not have to monitor the sterilized soil as carefully as with some other heat methods.

What are the 4 methods of soil sterilization? ›

The labor involved

The four most common methods for sterilizing soil are: Oven sterilization. Solarization. Steam sterilization.

How do you sanitize potting soil? ›

Disease-free Potting Mix

To ensure that your medium is clear of bacteria and illnesses, disinfect it with hydrogen peroxide. To do this, just sprinkle some hydrogen peroxide in your organic potting mix.

How long do fungus gnats live in soil? ›

What are they and how can we get rid of them? Fungus gnats are interesting little insects. The adults, which are what you see flying around and being a nuisance, have not been found to do much of anything but lay eggs in soil. They do not feed on the plants and do not bite, and they only live for about eight days.

Do fungus gnats feed on mold? ›

Many bugs feed on mold and decomposing matter. Booklice, for example, often inhabit moldy books. Fungus gnats and mold mites are two other pests closely associated with mold. Cockroaches, camel crickets , slugs, and millipedes are attracted to mold, too, though not because they feed on it.

What are disadvantages of coconut coir? ›

The most common problem with coir is it can have an extremely high salt content, especially in lower grades. Coir high in salts should be leached before use. Coir has a lower cation-exchange capacity and it is high in phosphorous and potassium.

Should I throw away potting soil with gnats? ›

Discard the soil immediately, and replace this top layer with sand, which is free of organic matter and dries out quickly—an unappealing medium for fungus-gnat eggs. Let the soil below dry out completely before the next watering. This should eliminate all fungus-gnat larvae present and prevent new eggs from being laid.

What is the fastest way to get rid of gnats? ›

In a spray bottle, mix one cup of water, a tablespoon of vinegar and a bit of dish soap. Spray this mixture directly at the gnats whenever you see them fly by. You can also use vinegar to get rid of gnats in plants.

Does UV light attract fungus gnats? ›

Traps using UV light bulbs attract fungus gnats, fruit flies, and phorid flies, making them suitable for indoor use.

What smell do fungus gnats like? ›

All gnats are attracted to fruity smells which can be placed such as gardens (fruit, vegetable, and flower), garbage cans and body washes/perfumes.

What is the fastest way to get rid of gnats in your house? ›

The most popular option is a vinegar trap, which is simple and cost-effective to create. Simply place a few tablespoons of apple cider vinegar, a few drops of dish soap, and a tablespoon of sugar in a bowl and stir. Set your bowl in an area where gnats are prevalent, such as your kitchen or bathroom.

Why do I have so many fungus gnats? ›

Fungus gnats infest homes when there is enough moisture in the property for fungi to grow. Water-related issues from leaky pipes in your basement to a malfunctioning appliance. However, the most common cause of fungus gnat infestations is over-watered indoor plants.

How do you get rid of gnats once and for all? ›

Pour a few tablespoons of apple cider vinegar into a bowl or jar, then stir in a few drops of dish soap. The bugs will be attracted to the sweet smell, and the sticky soap will prevent them from being able to fly away. Some people also like to mix in a little sugar, as well, to really amp up the sweetness.

What kills gnats instant? ›

Lure and kill gnats with a mixture of apple cider vinegar, water, sugar, and dish soap. (Alternatively, achieve the same result simply by combining red wine and dish soap.) Pour diluted bleach down the sink or tub drain, if you find gnats hovering near plumbing fixtures.

Do fungus gnats mean mold? ›

Many bugs feed on mold and decomposing matter. Booklice, for example, often inhabit moldy books. Fungus gnats and mold mites are two other pests closely associated with mold. Cockroaches, camel crickets , slugs, and millipedes are attracted to mold, too, though not because they feed on it.

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