Have you ever watered your plant, and what seemed like a thousand tiny black flies came out? All houseplant owners will agree that the most annoying plant pest to exist has to be those little black flies that seem to come out of nowhere.
One day you find a single fly, and suddenly they’re everywhere! These tiny black flies are called fungus gnats. And they are the most common indoor plant pest there is.
While these small flies are relatively harmless, they can be a massive pain in the a$$!
If you’re like the rest of us, you’ll want to get rid of them as soon as possible. The good news is, treating them is easy and can be done with minimal time and energy.
Below are the 13 best ways to treat your plants for fungus gnats, along with some killer prevention methods!
Table of Contents
What Are Fungus Gnats?
Fungus gnats are small black flies living in your potting soil. While they may be hard to see with the naked eye, they have long legs, clear wings, and black bodies. They look like tiny mosquitoes!
The gnats feed on the organic matter, compost, and other decaying plant matter in your houseplant soil and quickly breed and lay eggs in your growing medium or other moist organic debris.
Like all pests, fungus gnats will most likely enter your indoor space through new plants you bring into your home. Pest infestations often happen at nurseries, where thousands of plants are grown (the perfect breeding ground for fungus gnats). Bringing one infected plant into your house means you’re also potentially bringing home a pest!
Potted Exotics Pro Tip: Always inspect your new plants carefully before bringing them into your space. A magnifying glass can help spot some of the smaller pests or newly forming diseases like mealybugs or powdery mildew.
What Is The Difference Between Fungus Gnats And Fruit Flies?
Are you having a fly problem, but you’re not sure what you’re dealing with? Fungus gnats are often mistaken for fruit flies or drain flies. While they are similar, there are a few key differences to help identify them.
Fruit flies are often reddish-brown and have rounded, petite bodies, whereas gnats will have long, dangly legs, a shiny black head, and thin, mosquito-like bodies. Drain flies look like tiny moths with round bodies and thick wings.
The most telltale sign of a fungus gnat infestation is finding the bugs near or in your plants. Even small numbers of flies can be a sign of a quickly approaching infestation.
Fruit flies do not typically lay eggs nor feed in houseplant soil (decaying organic materials such as fruit compost may be a different story). If you see little flies in your soil, you’re almost certainly dealing with fungus gnats!
LECA is a great alternative to a soil medium if you can’t seem to shake a fungus gnat infestation. Fungus gnats have a very hard time laying eggs on the substrate, so LECA-based growing systems are a great choice for some.
The Life Cycle Of Fungus Gnats
There are four life stages: egg, larva, pupa, and adult. Adult female fungus gnats can lay up to 300 eggs in their short lifespan, which only lasts for about a week.
This means that a single gnat can produce large numbers of new flies!
The cycle, from larva to adult, takes about three weeks. Because of this short life cycle, treating the pest effectively can take a lot of work. As soon as you think you’ve gotten every last fly, new ones will emerge from the pupa stage.
Signs and Symptoms Of Fungus Gnats
Identifying if you have a gnat infestation is relatively easy. Check for the most apparent sign; tiny flies flying around your indoor plants, especially when you go and water your plants!
They may be hanging around the leaves or the plant pots, but they won’t shy away from taking a seat on your face from time to time, either.
The gnat larvae can leave a slime trail on your soil. Check the soil with a flashlight. If you see shiny, slimy trails, they are likely from fungus gnat larvae. When the infestation is advanced, you may see a decline in your plant’s health, such as slowed leaf growth, yellowing leaves, or leaf loss.
Are Fungus Gnats Harming Your Plants?
While they look like mosquitoes, adult gnats do not cause harm to your plants, pets, or yourself.
However, the larvae can cause damage to your plant roots, especially on young plants. As the bugs feed on plant material, such as roots, dead leaves, and compost, they create little openings in the roots, inviting bacteria and disease into your plant root system.
Root rot can also occur as a direct result of a fungus gnat problem, as fungus gnats carry nasty root-destroying bacteria.
13 Easy Ways To Treat Flies In Your Houseplants
1. Sticky Gnat Traps
These bright, yellow sticky traps are available in nearly every garden center. They are a great and simple way to reduce the adult population flying around, which will decrease the number of eggs laid since fewer adults will be present.
The yellow color attracts the bugs, and the sticky residue traps them, eventually killing them. You will be surprised how many gnats you can catch with these!
Once the sticky fly traps are filled with gnats, simply replace them with new ones.
Pro Tip: Rotate traps to different locations on a regular basis. This will help mitigate new and expanding clusters of these nasty little buggers.
2. Apple Cider Vinegar
You can easily create a DIY fungus gnat trap with good old apple cider vinegar.
- Combine equal parts apple cider vinegar and water with a few drops of dish soap in a clear cup or jar and mix well.
- Cover the opening with a plastic bag or wrap it and poke a few holes in the plastic wrap.
- Place this jar near your infested plants, and watch the gnats get trapped in the liquid.
- You will need to replace the mixture every couple of days to prevent mold from growing.
The gnats are attracted to the sweet, tangy smell of the vinegar, and the dish soap suffocates them as they try to go for a drink. This trap can work well for fruit flies, too!
3. Beneficial Insects
One of the most effective ways to attack fungus gnats is by using beneficial nematodes.
Nematodes are microscopic bugs that you can buy in powdered form. These tiny insects feed on the larvae of other bugs like gnats, attacking the pest at the larval stage, which is more effective than treating it in later stages.
You can use nematodes to treat a variety of soil pests that lay eggs in your plant’s soil, including weevils, cutworms, white grubs, and more. They are an all-around excellent choice for pest control.
Alternatively, if you don’t mind letting a few ladybugs loose in your indoor space, they are an extremely effective predator of fungus gnats and can quickly mitigate an infestation.
4. Hydrogen Peroxide Solution
You may know hydrogen peroxide as a disinfectant, but its uses are much broader. It can also be very effective for treating insect larvae in your houseplants.
Combine one part hydrogen peroxide with four parts water, and mix the solution well. Use this mixture to water your plants, but ensure that you only use hydrogen peroxide for plants planted in pots with drainage holes.
Without drainage holes, all the water, including hydrogen peroxide, gets collected in the bottom of the pot, which can lead to root burn. A sterile potting mix is the goal, but you don’t want to overdo it.
Drainage holes also allow excess water to run from the soil, which also helps to prevent wet soil and other problems like root rot – something that is a particularly big problem for species like ZZ Plants.
The hydrogen peroxide kills larvae and pupas on contact and will take out any adult flies that come to lay eggs in your plant’s soil.
Don’t worry. Hydrogen peroxide is completely harmless and safe to use. As it evaporates, it turns into oxygen and water molecules, and it will not cause damage to your plant.
Cinnamon is a cheap, effective, and readily available fungicide that you can use to treat fungus gnats and protect your plant from getting infected with any fungus or bacteria.
By sprinkling a thin layer of cinnamon on the surface of the soil, you are eliminating the food source (the fungus that grows on organic materials) for the gnat larvae.
Additionally, the cinnamon creates a barrier and acts as a prevention method to keep adult fungus gnats from getting into the soil in the first place.
6. Pesticides (Pyrethrin)
Pyrethrin is found in most insecticidal soap and pest-repellant sprays. It is organic, safe to use, and affordable.
The pyrethrins penetrate the insect’s nervous systems, making it impossible for them to move, feed or reproduce. However, the spray will only be effective for a short while and only works on direct contact.
Most organic pesticides will contain pyrethrins, so they should be easy to find.
Simply spray down the soil daily for about a week. If you have a bad infestation, you may need to spray daily for up to three weeks until all signs of the pest are gone.
7. Remove The Top Layer Of Soil
Fungus gnats only lay eggs on the top of the soil, so replacing the top inches of soil with fresh soil can help remove many eggs.
Remember that this method does not kill any of the eggs directly, so properly discard the infected soil so that the eggs don’t hatch and continue reproducing elsewhere!
Alternatively, you could totally repot the plant in fresh soil. This is sure to eliminate the fungus gnat eggs!
Make sure to discard the used soil somewhere outside. Otherwise, your home will still be infested!
8. Diatomaceous Earth
Diatomaceous earth is an organic substance made from ground-up prehistoric fossils. These particles are so tiny that they can enter the airways of insects.
When the gnats inhale these particles, it causes the bugs to dehydrate from the inside, killing them quickly.
To use diatomaceous earth, simply sprinkle a generous layer on the top of your soil. Any insect that inhales the substance will suffocate and die.
Diatomaceous earth does not cause any harm to pets, children, or humans, as the particles are so small that they are practically dust.
9. Neem Oil Spray
Neem oil is a wonderful pest repellent and easy to make at home.
Mix a teaspoon of dish soap, a tablespoon of neem oil, and a gallon of warm water. The dish soap acts as an emulsifier since oil and water do not naturally mix. Pour the solution into a spray bottle, and spray your plants with this.
Neem oil (and other essential oils) are also an effective way to prevent other kinds of pest infestations on your indoor plants. Neem oil contains compounds that distort insects’ hormones, making it impossible for them to eat or reproduce.
10. Let Your Soil Dry Out
Fungus gnats like to lay eggs in rich and damp soil, where their larvae will be safe and have plenty of food!
Since the gnats hate dry soil, allowing your house plants to dry out between watering can significantly reduce your gnat infestation!
An easy way to do this is through bottom watering. Bottom watering means placing your plant in a deep water dish and allowing the soil to absorb the moisture. As a result, the soil surface remains dry, while the lower part of the soil, where all the roots are, is nice and saturated!
Additionally, you can prevent overly moist soil by using the right soil mix. A good soil mix will have good drainage, which means it drains off excess water quickly and efficiently.
Compact, dense soils will attract pests like fungus gnats faster than a well-draining soil medium!
11. Mosquito Bits (Mosquito Dunks)
Mosquito bits, or mosquito dunks, are not only effective at treating mosquito infestations!
Mosquito bits contain a beneficial bacteria, bacillus thuringiensis subspecies israelensis, which can help eliminate fungus gnat larvae. The bits are wholly natural and organic, which means they are safe to use and cause no harm to you, your pets, your kids, or your plants.
You can use mosquito bits to top-dress your soil. Simply sprinkle a layer of mosquito bits on top of your houseplant soil, and every time you water, the bacteria get released into the soil and kill the eggs and larvae on contact.
Alternatively, you can mix the bits directly into your soil before planting your plants or mix them with a wooden stick or pencil! This is a great way to prevent soil pests such as fungus gnats, as beneficial bacteria will always be present in the soil.
12. Introduce A Carnivorous Plant To Your Collection
Carnivorous plants, like the Venus Fly Trap or Cape Sundew, are known to feed on insects. Some varieties even love to feed on fungus gnats.
Some great gnat-eating plants include Butterwort, Cape Sundew, and Dewy Pine.
Simply place one of these meat-eaters among your plant collection, and watch it feed on your pest! You will be surprised at how many pesky flies these plants can catch daily.
13. Combine Methods
While all the above mentioned methods can be effective, they will likely not independently eliminate the entire pest.
The best way to treat the gnats at every stage of their lifecycle is to use multiple methods simultaneously!
This will make the treatment more likely to be successful, speed up the progress significantly, and ensure that the pest stays away once you’ve gotten rid of them.