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How to Control Fungus Gnats in Indoor Plants

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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.

Fungus gnats are small flies that are pests of indoor plants

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.

Fungus gnat life 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.

Pots with drainage holes underneath rather than on the edges are better at keeping out fungus gnats from the moist potting medium layer that stays damp at the bottom of the pot. Pots with flat bases with the holes underneath work best.
Taping shut the water inlet port temporarily in self watering pots keeps out fungus gnats. The blue tape used here is masking tape, it peels off easily and doesn’t leave a glue residue.

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.

Sticky traps are cheap, and very effective at catching adult fungus gnats, reducing the breeding population. Remember to treat the larvae in the potting medium too though for effective control!

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:

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:

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.

An effective biological control for fungus gnats is the soil bacteria Bt-i (Bacillus thuringiensis subsp. israelensis)

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

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.

Pest control products containing the soil bacteria Bt-i (Bacillus thuringiensis subsp. israelensis) look like a fine brown powder

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

A teaspoon, holding around 5g of powder, can make a litre of potent mix for heavy infestations

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

For light infestations, one teaspoon is enough to make 5 litres of fungus gnat treatment

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

Stir well, then water all indoor plants, even the ones not showing signs of pests, so all plants are protected

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!

Peace lily, a perfect indoor plant for beginners, is much happier without fungus gnats eating away at its roots!

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.

Neem oil insecticide and pyrethrum insecticide are effective chemical controls of fungus gnats.

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.

Apply pyrethrum insecticide with a cotton tip inside the rim of the pot to control roving adult fungus gnats.

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!

A small jumping spider, hunter and predator of fungus gnats!

References:

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