Plant Pest - Fungus Gnats

Common Name: Mushroom fly or Fungus Gnat
Scientific Name: Sciaridae

Fungus Gnats are small black flies that tend to infest moist potting soil, other forms of consistently moist potting substrates, such as coco peat, and other forms of decomposing plant matter. Their larvae (tiny worm like creatures) are the main culprits in damaging houseplant roots, as they feed off fungus and organic matter in the potting soil and will sometimes attack the roots of young plants, which will lead to rot and ultimately decline in your houseplants.

The Adults main purpose is reproduction, and can become an issue when watering houseplants or plants in a greenhouse. When disturbed they will swarm and often become a nuisance. Adults are the first to be noticed when an infestation has arisen, as you will find them flying near potted plants, running over the soil or other forms of decomposing matter, such as leaf litter on the top soil. 

The larvae are much harder to spot and often will only be visible when re-potting your houseplants. They are generally a few millimeters in length, and almost transparent. They will often be found in the top 2cm of potting soil, as this is where the adults breed and lay their eggs.

Fungus Gnat Pest Identification

Identification: Adult

• Small black fly usually around 3 mm in length.
• Usually present flying around moist soil.

Identification: Larvae

• Small white to clear worm-like creature with a black head approx. 6mm in length.
• Usually found in moist potting soil.

How do Fungus Gnats spread?

• The adults are small winged creatures and are most likely to come indoors during Autumn and Winter whilst seeking out warmer more favourable conditions, as the outdoor temperatures drop.

Conditions the enhance or hinder the proliferation of Fungus Gnats?

Enhance:

• Moist organic rich potting soil - especially when overly damp.
• Warm moist conditions.
• Decaying organic matter, like leaf litter on top soil in pot.
• Potting mediums that are heavy in Peat or Coco Coir can encourage them as these stay wet for longer.

Hinder:

• Dry Potting Soil
• Cool conditions of lower than 20°C

Fungus Gnat Pest Treatment Plans

Small manageable infestations

• Quarantine your infected plant.
• Allow your Potting soil to dry out completely. 
• Dispose of the top 2cm of potting mix and replace with fresh soil.
• Take a cup of water and add a few drops of liquid soap, and with a Spray Bottle sprits the soil every few days. This should kill the larvae.
• Top with a layer of stones or mulch, to prevent the Adults from being able to penetrate the top soil.

For tips on homemade pesticides check out: 8 Homemade Pesticide Sprays for indoor plants.

Large infestations

• Quarantine your infected plant .
• Do a Hydrogen Peroxide drench:  Use 1 part 10 Vol Hydrogen peroxide to 4 Parts water, water your plant with this solution, saturating the soil. 
• Use a neem based organic Pesticide such as Bioneem and water, when needed, with this solution until the infestation has subsided.
• Re-pot your plant with fresh potting soil, sprits roots and inside of the planter with neem oil.

Fungus Gnat Preventative Measures

• Avoid allowing your potting soil to stay overly moist.
• Place a layer of stones or mulch on top of the potting soil, this prevents adults from laying their eggs.
• Let your potting soil dry out between each watering, or if it is a plant that needs to stay moist, let at least the top 4cm dry.
• Place sticky pads around plants that need to stay moist.
• If you note some adults, leave bowls of water out, they will be attracted to the water and get trapped.
• Consider growing Carnivorous Plants near plants attracting Fungus Gnats. eg: Sundews, Butterwort and Venus Fly Traps.
• Inspect plants regularly.

For more on Identifying and Treating Pest with Household items, Click here.

Wild Card: 

• If you can find Mosquito bits that contain Bacillus thurigiensis var. israelensis, sprinkle these over the potting soil. This Bacteria will kill Gnat Larvae.
• Use sticky traps to find adult Gnats

For more on Plant Care Troubleshooting, Checkout our 2-Step Plant Health Guide.

Shop Pesticides in our Plant Care Section here.