Everything you need to know about growing cannabis at home

Fungus Gnats & Cannabis Plants: Identify, Treat, and Prevent

by | Jun 20, 2022 | Cannabis, Environmental Threats, Troubleshooting

Fungus gnats are a common pest that can be found in many indoor plant environments. They feed on decaying organic matter and the fungus growing on the surface of plants. They often enter homes through open doors and windows, or they may lay eggs near damp areas such as leaky pipes, dishwashers, or standing water on potted plants. Once inside, fungus gnats will infest any houseplants they come across in your home. This article discusses identification, prevention, treatment, and control methods for this pesky insect invader on cannabis plants

What are fungus gnats?

Fungus gnats are a common problem for indoor growers. They can affect cannabis plants in particular, and they’re difficult to identify because they look just like other types of flies. These tiny pests feed on fungus that grows naturally in the damp soil, which is why you’ll often see them around your house or office when there’s a damp area with decaying organic material.

The larvae live in the top layer of soil where their food source is abundant. The fungus gnat adults lay eggs directly onto moist organic matter, plant material, or low-lying plant saps near your houseplants. After hatching from their eggs, larvae will begin to feed on fungus growing in the soil. They need moist conditions to thrive, so they usually only become a problem when you overwater your plants or if there are high humidity levels near your houseplants. Adult gnats are often seen flying around wet areas indoors because that attracts them to their food source and it gives larvae the ideal conditions to grow and develop.

How do I identify fungus gnats on cannabis plants?

Fungus gnats, also called “sciarid flies,” are about the size of fruit flies, but they look slightly different. They have very long legs and delicate wings that resemble small bristles sticking out from their body. Fungus gnats are dark in color with clear or smoky-gray wings, although some species can be yellowish as well. The larvae (babies) are even smaller, about half the size of an adult fungus gnat, and they’re very thin.

The easiest way to identify fungus gnats is to look for their larvae. You’ll see them crawling on the soil surface or flying around your plants when you first notice fungus gnats in your house. They move very quickly and they’re incredibly small, so make sure you know what to look for before trying to identify a fungus gnat infestation.

The Lifecycle of Fungus gnats

The lifecycle of fungus gnats consists of four different stages: eggs, larvae, pupae, and adults.

Fungus gnats have a very quick life cycle because their entire lifecycle can be finished in as little as two weeks. The adult fungus gnat lays her eggs on the soil or damp areas near your plants. Once the eggs hatch from their tiny white eggs, larvae begin to feed on fungus. They feed on decaying organic matter in the soil of your houseplants, and they develop quickly because their environment is ideal for growth.

After larvae molt several times into larger nymphs (instars), they eventually turn white or transparent as they prepare to enter the pupae stage. Pupae are immobile cocoons that fungus gnats enter when they’re ready to develop into adults. About five days after pupation begins, the adult fungus gnat emerges from its cocoon and is fully grown.

Fungus gnat symptoms

If you have fungus gnats, you will know because you will see them flying around your plants. You’ll also notice that the soil has turned dark and is damp to the touch or it may even be slimy. When you lift up a plant, gnats are sometimes seen crawling on top of the soil, but they’re easy to miss because they look like tiny specks moving quickly.

Fungus gnats can easily infest your houseplants and destroy them if you don’t take action. They feed on fungus growing in the soil, so it’s common to see more of these plant pests after overwatering or when humidity levels are high near your plants.

How do I control fungus gnats on cannabis plants with predators?

If you already have a fungus gnat infestation, then it could be time to take control of fungus gnats and fight back with predators. There are many different types of insects and bugs that eat fungus gnats that offer fungus gnat control. You can purchase them at your local gardening center or online through Amazon. Ladybugs, lacewings, nematodes, and hoverflies are all types of predators that will feed on fungus gnats.

The predators, or biological control agents, should usually have the fungus gnat population in control by about the fourth day of their release. It’s important to know that predators cannot be used as a preventative measure, or they will eat too many fungus gnats and cause an infestation themselves. They must only be released after you’ve already noticed fungus gnat activity in your indoor garden space.

How can I treat fungus gnats on my cannabis plants?

If your fungus gnats are still actively infesting plants, then you can use organic treatments to kill the larvae during the pupal stage. A houseplant insecticide spray is a good way to treat adult flies before they lay eggs to stop their life cycle or if their population isn’t as large as it could be. However, some products may only work on contact and not actually stop fungus gnats from breeding. You can also make your own spray at home with a little dish soap and vinegar, but be sure to test it on one plant first before applying the mixture to all of them because some houseplants are sensitive to these ingredients. There are other sprays you can make at home for fungus gnats and these include garlic oil, neem oil, and insecticidal soap. Just be sure to follow the instructions carefully on each product you use because some can burn or harm plants if they’re not used properly.

Another home remedy you can do to control fungus gnats is to place yellow sticky traps around your houseplants. The fungus gnats will fly to the light and get stuck on the tape instead of moving freely throughout your plants or flying around your home. You should change out the tape every day so that you can continue trapping adult fungus gnats while making sure larvae aren’t able to escape from underneath it.

Hydrogen peroxide and water are other methods to control fungus gnats. To use this method you simply need to make a mix of one part hydrogen peroxide and four parts water in a spray bottle and liberally spray your plant and soil. If you notice fungus gnat larvae mating then you can apply this mixture directly onto them because it will kill the larvae instantly without hurting your plants.
There are other organic treatments you could try as well, such as neem oil or BT, or Bacillus thuringiensis. Neem oil is made from the neem tree, and it’s a natural insecticide that can be used on houseplants without making them toxic to people or pets. It should be noted, that if your plant is flowering then neem oil should not be used as it can make the buds taste bitter and have a harsher smoke. BT is actually a bacterium found in soil everywhere, but when fungus gnat larvae eat it, they choke on their own saliva and dies.
If you have a fungus gnat infestation, then it’s important to act quickly before the population grows too large or they begin breeding more often than normal. Once their numbers increase, controlling them becomes much harder because that means there are many larvae feeding on your houseplants at once. You’ll need to uproot the plants and destroy the soil, then scrub down any surfaces to kill off larvae hiding there.

Preventing fungus gnats on cannabis plants

The first step to controlling fungus gnats is to prevent them from reproducing. You can do this by removing all wet, moist areas around your houseplants and making sure the soil stays dry at all times. If you don’t have any damp places inside your home where fungus could grow then it’s unlikely that fungus gnats will be a problem. The more water standing on top of the moist soil, the more fungus gnats can breed and the longer they’ll live. It’s also important to be sure that your houseplants are getting enough sunlight because fungus gnats love shaded areas where their larvae can feed undetected.

If you keep a clean home free of clutter or food sources like crumbs on counters or sink drains, then fungus gnats won’t have an ideal place to breed and you’ll be less likely to have a problem with them. You should also avoid watering your plants from the top because that will only encourage fungus gnat larvae while draining water through the bottom of pots helps get rid of adult fungus gnats on contact.

You can also grow certain indoor plants to help control the fungus gnat population in your home. Marigolds, cilantro, petunias, geraniums, basil, thyme, mints (peppermint or spearmint) are all known to repel fungus gnats. If these plants are kept on windowsills or on the floor, they can help keep fungus gnat populations down because they produce a strong odor that bothers the pests and it spreads throughout your indoor space.


Fungus gnats can be a big problem for many crops, including cannabis. They are especially problematic during indoor grows.

To identify fungus gnats, you should be looking for a fly about the size of a fruit fly with much longer legs and clear or smoky grey wings. The larvae are about half the size of adults and even thinner. There are several predators you can purchase to help control a fungus gnat infestation, including ladybugs and nematodes. If you do not want to introduce more bugs into your environment, there are several DIY sprays you can make for yourself at home. If these prove ineffective, you can always use sticky yellow traps or store-bought neem oil or fungicide.

To prevent fungus gnats, avoid having standing water or constant moisture, for example from a leaky pipe, in your home. There are also companion plants that you can have around cannabis plants that are naturally preventative of fungus gnats. Should you have any further questions, please check our other articles.

To learn more about growing cannabis at home consider checking out our guides on: Getting Started, Troubleshooting, and Optimizing your Grow or search our Knowledge Base.

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