How to Control Fungus Gnats

How to Control Fungus Gnats

Chris
5 minute read

While pests are a natural part of owning house plants, some pests can cause a lot of damage to plants if left to their own devices. However, today we want to talk about a particular pest that is more of an annoyance than harmful. Let’s talk about fungus gnats! Those pesky little bugs resembling fruit flies that you’ve likely seen flying all around your tropical plants. Anyone growing plants has experienced these fungus gnats as they are extremely common.  In order to better control them, we need to first understand their growth cycles.

a photo of a fungus gnats larva a photo os an adult fungus gnat
Left: a fungus gnat larva Right: an adult fungus gnat

Fungus gnats and their growth cycles

Profile: Tiny flies, grey to black in colour, often mistaken as fruit flies.

Damage: As mentioned, fungus gnats are more of a nuisance than they are a threat. They reproduce quickly (each female can lay up to 300 eggs) and constantly having them buzzing around your home can get rather annoying. Their larvae tend to feed on small roots, so they are a concern when growing seedlings, but not so much a concern to already established plants.

Signs: Discolouration of leaves in younger plants. Tiny insects fly up into the air when you water your plants.

Susceptible: Any plants growing in organic matter are susceptible to fungus gnats but it is worth mentioning that they love breeding in damp soil.

Life cycle: A fungus gnats’ lifespan only lasts about a week. You would think that would make them easy to eradicate, but a single fungus gnat can lay up to 300 eggs. After a few days, the larvae will hatch and feed on decaying organic matter and plant roots. Unless you’re growing seedlings, a small infestation will probably not affect your plants at all. After a few more days, the larvae will become adults, which repeat the cycle.


 

Hot to control fungus gnats

The key to controlling a fungus gnat infestation is disrupting their life cycle. Not only do you need to consider the adult flies, but you need to also think about the eggs and larvae in your soil. 

a fungus gnat caught by a drosera carnivorous plant fungus gnats caught on yellow sticky tape
Left: Drosera (sundew) carnivorous plant, Right: yellow sticky traps


Controlling adult fungus gnats:

 Yellow sticky traps - These work wonderfully and can easily be found at garden centres, hardware stores, and on Amazon.

Homemade vinegar and soap traps - This can be easily put together with products you already have at home. Pour some vinegar into a container of your choice and add 1 – 2 small drops of dish soap. For extra security, cover the top of the container with plastic wrap and poke holes big enough for fungus gnats to fly into.

Carnivorous plants - A great option for those who are interested in weird/unique plants and is a great natural solution to fungus gnat control. Plants such as Butterworts or Sundews are great at capturing fungus gnats which they consume for nutrients and energy. Bonus: more plants!

mosquito dunk pellets diatomaceous earth
Left: mosquito dunks, Right: diatomaceous earth


Controlling fungus gnat larvae:

Cinnamon - A natural fungicide that helps to rid of the fungus in which fungus gnat larvae eat. Bonus: it smells nice!

Mosquito dunks - These are inexpensive and usually come in pellet form. They can be broken into pieces as needed and mixed in with water that you will use to water your affected plants. This is the most effective way to immediately rid fungus gnat larvae. These can easily be found at home hardware centres and Amazon, especially in the spring and summer seasons.

Hydrogen peroxide - A great solution if you already have some hydrogen peroxide laying around. Mix 1 part hydrogen peroxide (3% or higher is most effective) and 3 parts water. Gently mix the solution before pouring it into a spray bottle. Wet the entire surface area of the soil of affected plants. The larvae will die upon contact with hydrogen peroxide. Don’t worry – this is safe for plants! 

Diatomaceous earth - A white chalk-like powder made of sharp fossilized remains of microscopic sea creatures made of silica. Use with precaution as the fine dust is an irritant on the lungs if inhaled. To use, lightly and carefully dust the surface area of the soil or any affected areas. When the adults or larvae come in contact with the diatomaceous earth, the tiny and sharp edges will cut into their bodies causing dehydration and ultimately death.


 

Preventing fungus gnats

  • Avoid letting your soil stay damp for too long, though it can be unavoidable when growing plants that like consistently moist soil, such as ferns and prayer plants.

  • Neem oil is an amazing and natural pest repellent. It causes nausea, deterring pests to go back and eat neem covered areas which causes them to starve. Neem also causes fungus to become sterile, making them unable to reproduce.

  • Horticultural oils or insecticides.

  • Citronella sprays.

  • Using a top dressing to that adult fungus gnats cannot get to the soil to lay eggs.

  • Diatomaceous earth.

  • Consider switching to soilless growing methods such as semi-hydro.

  • Cinnamon


 

Tip: Growing your house plants outdoors in the summer can help control fungus gnats, due to warm temperatures drying out the soil faster as well as exposure to natural predatory bugs. When bringing your plants back indoors for the fall and winter season, pay close attention to soil dampness and alter your watering routine accordingly. It’s very common to see an increase of fungus gnats in the cooler months, as the lack of warmth and sunlight causes soil to stay moist for longer periods of time after watering.

 

Like any other pest, practising diligence and implementing preventative measures as a precaution is always a great idea. Frequently inspecting your plants to catch any early signs of a possible infestation is also important. The sooner you catch on, the easier controlling and maintaining a pesky fungus gnat outbreak will be. Happy growing!

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