How to Control Fungus Gnats in Indoor Plants

The annoying little flies often seen flying around indoor plants are fungus gnats. The adult flies are just a nuisance, but their larvae, which mainly eat fungi growing in decomposing organic matter (such as over-watered plant potting mix!) will also eat the roots of houseplants, and can transmit root diseases.

Fungus gnats are only a minor pest, but when their populations increase to very high numbers, they can cause significant root damage which will weaken plants and stunt their growth.

What Are Fungus Gnats?

Fungus gnats (Bradysia spp.) are tiny, delicate-looking flies around 1.5 – 3.0mm (1⁄16 to 1⁄8”) long, with dark-brown coloured bodies, a single pair of light-grey to clear wings, with Y-shaped wing vein pattern on the ends of their wings.

Due to their small size, these pests just look like tiny ‘fruit flies’ when they land on the sides of pots or surrounding surfaces, and many gardeners mistake them for fruit flies, which they are not.

In flight, fungus gnats move around in a slow, gentle, floating manner, as they’re weak flyers. They usually don’t fly far, and are often seen flying up into the air when pots are watered or when the houseplants are disturbed. They also tend to fly around windows, as they’re drawn to the light outside.

In the title photograph at the top of this article, the extreme close-up of a fungus gnat (magnified over x20) shows they have long, slender legs and look somewhat like mosquitoes, with small dark round heads and long segmented antennae. You wouldn’t see any of this with the naked eye! What you’ll observe is something closer to the photograph below.

On houseplants, fungus gnats can be found running around or resting on the surface of the potting medium, the inside rim of the pot, on the leaves of the plant and nearby surfaces.

Fungus gnats don’t just infest indoor plants, they’re also a major pest of greenhouse production systems, as they thrive in locations with high levels of moisture and organic matter. In greenhouses,they can also be found amongst compost and mulch piles.

Fungus Gnat Life Cycle

The adult fungus gnat females lay their eggs (up to 300) in moist organic matter such as damp potting medium, which hatch in around 3 days to produce fungus gnat larvae (maggots or grubs).

Fungus gnat larvae have an elongated legless body, are clear to white and semi-transparent in colour, with a small, shiny black head, and grow to around 6mm (1/4”) long. They live and feed near the surface of potting medium, and in moist conditions with high pest populations, the larvae can leave fine snail-like slime trails on the surface of the growing medium.

The larvae go through four larval stages or instars, and take approximately 10 days to mature and develop into a pupae.

After around four days as a pupa, the adult fungus gnats emerge from the potting medium to repeat the cycle.

The fungus gnat life cycle takes 18 – 27 days to complete, depending on the temperature. Warmer temperatures lead to faster development and more generations in a given time.

Damage Caused by Fungus Gnats

The larvae of fungus gnats actively feed in the top 3cm (1-1/4”) of the potting medium, just below the surface, where they feed on decaying plant matter, soil fungi, algae.

They also feed on healthy fine roots and root hairs and tunnel into stems of young plants and the crowns of mature plants, causing damage to the the plants, impairing their ability to take up water and nutrients, leading to wilting of leaves and tender shoots, and stunted plant growth.

When fungus gnat larvae feed on plants, they creates wounds which allow entry of certain soil-borne fungi, which may predispose the plants to attack from soil-borne plant pathogens.

Fungus gnat larvae are also capable of directly transmitting certain fungal diseases including Pythium spp., Fusarium spp., and Verticillium spp., from diseased to non-infected plants.

Adult fungus gnats are really only a nuisance, they cause minimal direct plant damage, but do leave unsightly flyspecks (faecal droppings) on the leaves and flowers of houseplants and areas around the pot.

How to Control Fungus Gnats

An integrated pest management (IPM) approach is a scientific and strategic way to control pests using a combination of techniques to limit pest populations and the damage they cause, while eliminating or drastically reducing the use of pesticides to minimise risks to people and the environment.

Any of the following control strategies will be more effective if implemented simultaneously with alongside the other control strategies listed here.

Cultural Controls for Fungus Gnats

These controls are practices which disrupt the environment of the pest, reducing pest establishment, reproduction, dispersal, and survival.

• Allow the top layer of the potting medium dry out to prevent adults laying their eggs on the damp surface.
  
• Don’t overwater houseplants, especially in winter when water demand are much lower, and don’t keep the constantly wet, most plants prefer a continuous wet-dry cycle. Water every week or two, depending on the ambient temperature. Saturated potting mix will decompose and attract fungus gnats.
  
• Cover the watering ports of self watering pots with tape temporarily to reduce fungus gnat access to moist potting mix.
  
• Use pots with drainage holes on the underside, rather than the bottom edges, as the exposed edge drainage holes allow access to the moist potting medium that sits at the bottom of the pot. Cover, but don’t seal with tape temporarily if edge drain holes are exposed.
  
• Cover the drainage holes at the bottom of pots with geotextile fabric to prevent fungus gnats gaining access and laying their eggs there. A geotextile fabric such as Marix light geotextile fabric weed mat can be fastened around the base of the pot with an elastic band or a piece of string. Alternately, a small piece of geotextile fabric can be placed inside the bottom of an empty pot before it’s filled with potting medium and planted up, this will stop the potting medium falling through the drain holes and keep the fungus gnats out.

Physical Controls for Fungus Gnats

These controls block pests out, make the environment unsuitable for them, or kill them directly, and include strategies such as the use of physical removal, barriers and traps.

• Cover the surface of the potting medium with with pebbles to prevent adults laying their eggs on the damp surface.
  
• Set up yellow sticky traps near houseplants to catch adult fungus gnats, they’re attracted to the colour yellow and fly into the traps and get stuck there. Be warned, a trap covered in hundreds of dead gnats can look rather gruesome after a few weeks! Luckily these are cheap and disposable!
  
• Set up apple-cider vinegar traps in shallow containers, mix equal parts apple cider vinegar and water so it’s at least 6mm (1/4”) deep and add a few drops of liquid soap. Sit the container beside plants or on potting medium surface if space permits. Empty and refill every few days. These would work better in a yellow container, and I’ve only found them marginally effective, but may be worth trying.
  
• Place cut raw potatoes on the potting medium surface, with the cut sides facing down, fungus gnat larvae are attracted to this trap. After a few days, dispose of infested potatoes in a plastic bag tied shut, throw into the trash, and replace.
  
• Use a handheld vacuum cleaner with a long nozzle to prod and disturb the resting fungus gnats and then vacuum them up when they fly up into the air, it’s not difficult as they move slowly. NOTE: Don’t vacuum near the potting medium surface as it will be sucked up, and avoid sucking up and tearing the plant leaves!

Biological Controls for Fungus Gnats

Biological control agents, the natural enemies of pests, can be use to keep their populations in check.

There are several biological controls which can be used to control fungus gnats:

• Entomopathogenic nematode (Steinernema feltiae) are microscopic worm-like organisms for control of fungus gnats in indoor plants, nurseries, mushroom growing and hydroponic systems.
  
• Bacterial insecticide Bti (Bacillus thuringiensis subsp. israelensis), a naturally occurring soil bacteria sold under the trade name Vectobac and Gnatrol, used for control of mosquito larvae and fungus gnat larvae.

Predatory mites (Hypoaspis spp.) and predatory beetles (Dalotia coriaria) are used to control fungus gnats in greenhouse environments, but these biological controls are not suitable for indoor use, as they will not survive inside in the long term.

Entomopathogenic nematodes such as Steinernema felitae are delivered in a cooled container (containing a freezer block) during transportation and must be kept in the fridge (not the freezer) until use, and must be used within two weeks. To use, the white powder is mixed in water for at least 20 minutes to allow the nematodes time to activate, then applied as a soil drench with a small watering can.

Bt-i (Bacillus thuringiensis subsp. israelensis) is much easier biological control to store and transport, it just needs to be kept in a cool, dark place away from the UV rays of the sun.

How to Use Bt-i to Control Fungus Gnats

In the steps below I’ve documented the treatment of my houseplants with Bt-i (Bacillus thuringiensis subsp. israelensis).

I’ve found that a single treatment was all that was required to eliminate a mild infestation of fungus gnats.

Here are a few valuable tips when using Bt-i on houseplants:

• When mixing the Bt-i biological control, I diluted it with rainwater, as the chloramine in tap water can’t be that good for soil bacteria, it’s there to kill bacteria. Maybe unnecessary, but it can’t hurt!
  
• Make sure that plants are reasonably watered and the potting medium is not completely dry before application – it’s important to remember that the mixed Bt-i liquid is a potting medium treatment, a way of delivering beneficial bacteria, and not a replacement for routine watering! Trying to do a full watering with the product is wasteful and will use lots of the product for no additional benefit. Don’t use it all at once, as repeat applications may be required!

Pictured below is a 50g bag of Bt-i, which can make up between 10-25 litres, that can treat a lot of houseplants! I have over 45 houseplants, most in large pots, and I have plenty left over after treating them all.

Bt-i can also be used as a preventative at lower application rates. From the label at the back of the packaging:

Application Instruction

Application is most effective when fungus gnats first appear. Larger infestations may require 2-3 repeated applications, 7-10 days apart. Water solution into the soil.

Mixing instructions

• Light infestations or as a preventative: 1 gram per litre
• Medium infestations or as a preventative: 2 gram per litre
• Heavy infestations or as a preventative: 5 gram per litre

Store in a dark place to prevent degradation form UV exposure.

This is what Bt-i looks like, it’s just a brown granular powder containing dormant bacteria.

A teaspoon holds approximately 5g and can make a litre of liquid for treating heavy infestations.

For heavy infestations, place a teaspoon for each litre of water in the watering can.

Stir the contents of the watering can to ensure they are thoroughly mixed through.

Water houseplants with the Bt-i liquid, and the fungus gnats will disappear in no time. Once the fungus gnat larvae ingest the insecticidal bacteria, it can take up to 12 hours before they are eliminated.

In my experience Bt-i (Bacillus thuringiensis subsp. israelensis). is very effective in controlling fungus gnats. Here’s my happy, fungus gnat-free, 15-year old peace lily as proof!

Organically Acceptable and Safe Chemical Controls for Fungus Gnats

Insecticides can be used to control fungus gnats, and in integrated pest management (IPM), chemical controls are used as a last resort.

It’s a bad idea to rely solely on insecticides, as insects may develop resistance to them. Due to genetic variation from one generation to another, all it takes is one single insect that survives a chemical treatment to produce a resistant population!

The two insecticides used for controlling fungus gnats in houseplants are safe and organic-certified.

• Pyrethrum is a natural, plant-based insecticide derived from the pyrethrum daisy Tanacetum cinerariifolium (syn. Chrysanthemum cinerariifolium), and works by paralysing the insect’s nervous system, eventually killing it. It has a fast knockdown effect, and works very quickly after insects are sprayed or make contact with it. It remains effective for around 24 hours and is degraded by UV light from the sun.
  
  Many formulations of pyrethrum contain a synthetic synergist such as piperonyl butoxide which make the insecticide even more effective, but the inclusion of these synthetic synergists disqualifies organic certification. Note synthetic pyrethrins are not the same as natural pyrethrum, are extremely toxic, and should be avoided!
  
• Neem oil is another natural plant-based insecticide derived from the neem tree (Azadirachta indica). It contains the active ingredient azadirachtin, which acts as an anti-feedant, stopping insects from eating, which eliminates them after around three days, and it also disrupts the life cycle of insects, preventing them from maturing into adults and breeding.
  
  The application rate is 30mL per 10L water (1 tablespoon per gallon), and it’s applied as a soil drench with a watering can, using approximately 1 litre of mixture per 8 litres of potting medium (a 1:8 ratio of neem-water mixture to potting medium by volume).

Here are a few handy tips when using insecticides:

When applying spray insecticides to houseplants, don’t spray inside the house, take pots outside to do any spraying, that should be common sense – the fine water droplets containing insecticide will be suspended in the air, and breathed in over time!

There isn’t much point spraying pyrethrum for this pest, as the adult fungus gnats are very easy to eliminate by other means such as yellow sticky traps and vacuuming. It’s more important to treat the larvae living in the potting medium!

An easy way to eliminate adult fungus gnats with pyrethrum is not by spraying it, but by painting it on the inside rim of the pot with a cotton tip (Q-tip). Adult fungus gnats often run circles horizontally around the inside of the pot above the potting medium and will walk through the pyrethrum. There is no need to apply the pyrethrum to the surface of the potting medium. If the pyrethrum comes in a spray container, spray some into a lid from an empty bottle, or some other shallow container, dip the cotton tip into it, and apply to the pot.

By implementing a combination of the fungus gnat controls outlined in this article, it should be quite easy to eradicate this pest.

Incidentally, if you spot any small jumping spiders lurking around your indoor plants, they’re present because they hunt and eat the flying adult fungus gnats! Sometimes nature’s pest controls make their way indoors!

One More Method to Control Fungus Gnats

After this article was written, I discovered one more method of controlling fungus gnats in indoor plants using a portable USB rechargeable bug zapper. This is a totally safe, environmentally friendly method, and probably one of the easiest! See the article here – The Easiest Way to Get Rid of Fungus Gnats from Houseplants with a Bug Zapper

More articles on Garden Pests, Diseases and Problems

References:

• Government of New South Wales, Department of Primary Industry, PRIMEFACT 1006, May 2010, Fungus gnat management in greenhouse crops
• University of California, Agriculture and Natural Resources, Statewide IPM Program – Pest Notes: Fungus Gnats, UC ANR Publication 7448 by J.A. Bethke, UC Cooperative Extension, San Diego Co and S. H. Dreistadt, UC Statewide IPM Program, Davis
• Planet Natural Research Centre – Fungus Gnat
• The Old Farmer’s Almanac – Fungus Gnats: How to Identify and Get Rid of Fungus Gnats by Christopher Burnett
• Cloyd RA. Ecology of Fungus Gnats (Bradysia spp.) in Greenhouse Production Systems Associated with Disease-Interactions and Alternative Management Strategies. Insects. 2015 Apr;6(2):325-332. DOI: 10.3390/insects6020325.