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Fungus Gnats

Fungus Gnats on Cannabis: How to Identify and Keep Them off Weed

As every marijuana grower knows, pest infestations are challenging and, in some cases, potentially deadly. The propagation of fungus gnats on cannabis plants is a perfect example. 

These insects may severely threaten your weed crops as they attack the root systems, affecting nutrient absorption. As a result, you might have unhealthy plants and a poor harvest without proper treatment and removal.

Fortunately, you can put your mind to ease as these bugs are easy to dispose of if you know what to do. So let’s explore what fungus gnats look like, what attracts them to your plants, and how to know you have an infestation

Learn simple methods to tackle fungus gnat symptoms when they occur and how to implement preventative measures to safeguard your cannabis plants.

What are fungus gnats, and what do they look like?

Fungus gnats comprise six families of insects, namely the Sciaridae, Ditomyiidae, Mycetophilidae, Diadocidiidae, Bolitophilidae, and Keroplatidae.

what is a fungus gnat
This is how a fungus gnat look

Most species aren’t harmful to your crops except those in the Sciaridae family, namely the Orfelia and Bradysia species. They’re tiny flies that look similar to mosquitoes. You notice them buzzing around weed plants close to the soil. 

Also called dark-winged fungus gnats, they’re black, brown, or gray with slender legs. Adults are usually about 1⁄16 to 1⁄8 inches long, although some may reach up to 1⁄2 an inch in size.

a gnat in its larva phase
Gnat larva

They have segmented antennae longer than their head and a pair of light gray to clear wings. Those from the Bradysia species have a Y-shaped wing vein.

The cannabis fungus gnat goes through four development stages:

  • Egg
  • Larva
  • Pupa
  • Adult

Once you have a population of bugs, the problem can get out of hand as they reproduce fast. A female lays about 200 eggs per week in moist organic matter, often at the bottom of your plants. 

Given a temperature of 75ºF, eggs take three days to hatch. Their larvae have shiny black heads and long translucent legless torsos. They develop into pupae beneath the soil’s surface after about ten days.

Fungus gnat larvae
What do fungus gnat larvae look like?

Adult gnats appear approximately four days later, and the females start laying eggs. With a short reproduction phase of around 17 days, many generations appear within a year. 

The infestation problem occurs during warm weather and prevails as long as food and moisture are present. Fungus gnats thrive in temperatures of 75ºF– 80ºF. 

They can’t survive in cold environments. However, they may remain active indoors if they have a warm and wet habitat.

Fungus gnats vs. shore flies

Fungus gnats and shore flies (Scatella stagnalis)—along with mosquitoes and midges—are from the Diptera order. Although both pests thrive in moist areas like greenhouses, there are several differences.

fungus gnats vs shore flies
fungus gnats vs shore flies

Physical appearance

Fungus gnats on weed plants have a dark brown-to-black appearance similar to mosquitoes and have long antennae. They’re weak fliers, usually resting on the top of potting soil. 

Scatella stagnalis look like fruit flies and have short feelers. Their wings are sturdy, allowing them to buzz around actively. 


Fungus gnats on cannabis inflict damage to their hosts. They eat decaying organic matter, algae, and fungi within the soil and, subsequently, the roots.

Shore flies, like cannabis whiteflies, are merely an annoyance and don’t cause harm to your cannabis plants.

Growth cycle

While the eggs of both bugs hatch within three days, gnat larvae take longer to enter the pupal phase.


Use yellow sticky traps to snare adult fungus gnats as they’re fascinated with the hue. Shore flies prefer blue material.

Fungus gnats vs. leaf miners

Leaf miners are also pests but different from pot plant gnats. They’re not bugs but the larvae of the following insect species that live inside plant foliage:

  • Lepidoptera
  • Diptera
  • Hymenoptera
fungus gnats vs leaf miners
fungus gnats vs leaf miners

Physical appearance

The larvae are generally pale yellow, light green, or dark brown compared to gnats, which are gray, black, or brown. Adults are 1/10 inch long and look like tiny house flies. Females lay eggs under the epidermis surface.

Mature fungus gnats look similar to mosquitoes, and they propagate at the base of your marijuana plants if it’s moist.


Unlike gnats on weed plants, which attack the roots of the plants, leaf miners live inside the leaves. You’re unlikely to notice them except for the long, winding marks on the foliage. These are tunnels the maggots create when they chew through them. 

Fungus gnats cause damage that stunts plant growth. While heavy leaf miner infestations may affect leaves, the effects are cosmetic and don’t threaten the host.

Growth cycle

Leaf miners have a longer growth cycle than pot plant gnats. They mature into pupae within 2–3 weeks, dropping from the leaves to the soil and burrowing about 2 inches into the earth. Adult flies emerge after 15 days.

Why do fungus gnats attack your weed?

Fungus gnat larvae survive on decaying matter and thrive in moist conditions. Therefore, if you overwater your soil, you attract these bugs. 

They ultimately attack the roots as they breed at the base of your cannabis plants. A sign that you have an infestation is the presence of pests at the top of your soil. Without treatment, the problem may progress to the point of fungus gnat leaf damage.

Do gnats kill cannabis plants?

Feeding on your marijuana plant’s roots is a secondary effect. The pests initially set up their habitat because of the decaying matter in your moist soil.

As the infestation grows and consumes more parts of the roots, your plant won’t absorb sufficient nutrients. If left unchecked, your crop weakens and ultimately dies. It’s one of the most deadly cannabis seedling problems due to its vulnerability during that growth stage.

Besides damaging the plant, the Sciaridae fungus gnats on cannabis can spread disease because they consume fungal growth, which may contain spores from infected plants or soil. 

Some common infections include black root rot, Fusarium wilt, Pythium blight, Verticillium wilt, and Botrytis blight. These threaten your crop’s health, potentially resulting in their death.

Fungus gnat symptoms

Unlike many threats to your crops, fungus gnat symptoms first appear at the topsoil. That’s where the pests proliferate and feed. If your growing medium is wet, the chances of an infestation are higher.

gnats in the soil
Gnats are visible and appear in the soil

Using the best water for cannabis growing doesn’t eliminate the risk of these bugs. Beginners may overhydrate their cannabis plants, or the growing conditions may be less than optimal, allowing moisture to set in. If this happens, you may even experience gnats in a grow tent.

gnats damage
A damaged weed plant affected by gnats

Common signs of a bug problem include:

Leaf discoloration

If you notice any of these changes to your plant’s leaves, you may have fungus gnats:

  • Brown or burnt edges
  • Pale hue
  • Yellowing new growth
  • Black or gray patches
  • Brown and dark spots
  • Yellowing between the veins
a damaged cannabis leaf

Plant symptoms

It’s not only the leaves that let you know something is wrong, be on the lookout for:

  • Slow root development and plant growth
  • Wilting & drooping
  • Leaves curling
  • Gnats on the surface at the plant’s base
  • Smaller buds

Using the best cannabis seeds for beginners helps as they have excellent genetics. As a result, you get robust plants that are less susceptible to diseases and pests. 

Getting rid of gnats in soil and compost pile

Are you ready to learn how to get rid of gnats on weed plants? The most common cause of infestation is wet topsoil, as the pests thrive in moist conditions. Without a damp habitat, you won’t have a problem. 

The first course of action is to stop overwatering your crops. It should make the fungus gnats on weed plants go away. However, if they persist, try these methods:

Stop watering for a few days

Learning how to water cannabis is vital. Always check to see if your plant needs hydration before adding more. Insert your finger into the soil up to your first knuckle. If it feels dry, add more moisture. If not, repeat the process the next day.

Besides creating a breeding environment for fungus gnats, overwatering also prevents your plants from absorbing cannabis nutrients from the ground.

Use yellow sticky traps 

Placing these traps around your growing space lures adult cannabis gnats from your plants because the hue captivates them. They then get stuck on the glue-covered cards. 

yellow sticky traps
a small gnat catched in a yellow sticky trap

Count the trapped bugs daily to check your progress. A lower number shows that the technique works.

Place raw potato chunks

Placing raw potato chunks in the soil works the same way as the yellow sticky traps as they also attract fungus gnats. After a few days, remove the infested pieces and insert new ones. 

Kill the larvae

By eradicating the younger bugs, you reduce the population growth. There are several ways to kill maggots in the soil, including:

Food-grade diatomaceous earth

Apply food-grade diatomaceous earth over exposed areas of the soil. The powdery organic matter comes from fossilized shells and is deadly to insects. It penetrates the gnats’ exoskeleton and drains their bodily fluids, causing dehydration and death. 

Diatomaceous earth prevents future infestations of the cannabis fungus gnat and accelerates eradication. Although it’s potent, it isn’t harmful to humans and animals. 

Essentria IC3 insecticide

The solution is made from various horticultural oils and poses no risk to humans or animals. Like other cannabis bug killer products, Essentria is very effective if you regularly treat affected plants. Applying it every other day is ideal.

Spray the soil evenly using a mister and ensure you follow the directions. Stop using the solution a few days before harvesting.


SM-90 is an organic solution made from natural plant oils, making it 100% safe for humans. Its advantage lies in eradicating larvae without damaging the roots.

Neem oil

This organic and environmentally-friendly option acts fast against fungus gnats on weed plants. It also interferes with their feeding and proliferation. The concentrated azadirachtin in the oil is safe for greenhouses, outdoor and indoor gardens, and hydroponic systems.

Follow the manufacturer’s dilution directions and drench the soil at the plant’s base, where the gnats thrive. Spray the upper parts as well to ward off the adults.

Pyrethrin sprays

Pyrethrin is effective in combating cannabis fungus gnats and their larvae. Apply a thin mist solution to the topsoil and the plant’s surfaces. Ensure you don’t leave them dripping wet.

Pause watering and let the soil dry to at least two inches deep. Repeat the treatment every three to four weeks until the pest infestation ends.

If you discover fungus gnats in your compost pile, many of the treatments are similar to soil. 

Ensure your compost pile isn’t wet

Avoiding excess moisture is vital. If your pile is moist, allow it to dry, then cover it with plastic or tarp. The protective layer messes with the bug’s life cycle, preventing the females from laying eggs and decelerating the reproduction process. 

Covering it with carbon material, such as shredded paper, cardboard, or brown leaves, also works well. Ideally, use a bin with a lid to prevent the occurrence of an infestation. 

Eliminate the larvae in compost

When you have larvae in your compost pile, sprinkle food-grade diatomaceous earth. It’s deadly against fungus gnats but safe for people.

For extreme situations, use a product with Bacillus thuringiensis. A dry formulation works better than a liquid. Crush the pieces and spread the powder over your compost. Add water to activate the treatment and destroy the larvae.

Avoid adding kitchen waste to compost

Marijuana plants need nutrients in various ratios, which kitchen scraps can’t provide. If you add them and notice fungus gnat symptoms, bury the matter in the middle of the pile. The pests can’t reach those and only feed on food at the top layer.

Maintain your compost pile

Turn over your compost pile regularly, mixing everything and allowing it to dry. Maintaining it well increases the temperature in the stack, making the conditions too hot for any fungus gnats living in it.

How to prevent fungus gnats

Although cannabis gnats can potentially be a severe problem, they’re preventable. Getting rid of the pests is challenging once they set up their home. Breaking their life cycle isn’t easy due to their fast reproduction rate.

The ideal solution is to close the door on the pests when you set up your grow space. In addition, we recommend adopting the following preventative measures:

Clean your grow room

After a pest attack, thoroughly clean your growing space before you start cultivating again. That minimizes the risk of an existing colony of fungus gnats on cannabis plants multiplying and infesting new crops. If you notice bugs around other houseplants, dispose of them.

Implement a watering schedule

Since the bugs thrive in moist substrates, a proper routine for watering outdoor marijuana plants is vital. Set up a program so you don’t create a breeding environment for the pests. Without a hospitable habitat, they won’t survive. 

Pay attention to how often and how much you hydrate your weed plants. Always allow the soil to dry before you add more moisture. 

To ensure adequate hydration, do a quick test by inserting your finger into the earth up to about an inch. If the medium isn’t damp, water it. Wait a day if the matter is still wet and check again. Ensure there’s adequate drainage to avoid pools of liquid on the soil.

Indoors, use a fan to speed up the drying process. The breeze also prevents fungus gnats from flying around and laying eggs.

Use uninfected soil

When starting to grow cannabis, avoid using substrates previously affected by gnats on weed plants. To be sure, observe if there are signs of larvae or insects flying around. 

Using the best soil for growing weed is an excellent way to give your plants a perfect start. In addition, you may sterilize the medium by baking it in the oven before use to kill maggots and eliminate other threats.

Solarize soil before using

When buying soil, choose only potting mix or pasteurized container mix. Besides cooking the medium, you may expose it to the sun to let the heat kill the pests:

  • Moisten the soil but don’t soak it.
  • Put it in a transparent or black plastic bag.
  • Keep the pile no deeper than 8 inches.
  • Place the bagged soil in a sunny location for about 4 to 6 weeks. Elevate it slightly by putting it on a pallet.

Add mulch

Mulching adds a layer of protection as it prevents adult cannabis fungus gnats from laying eggs on the soil’s surface. Any other covering offers the same benefit.

Grow from seeds

Although many cultivators grow from cannabis clones, seeds are a safer option because there’s a higher risk of introducing pests when using the former. 

If you prefer using clones, be sure to get them from a trusted supplier and inspect for signs of cannabis fungus gnats and other bugs. Then, to be safe, quarantine and observe them for three to four weeks to see if there are new adults.

Use biological control agents

Using biological control agents helps to control fungus gnats in pots or containers. Some commercial products include:

Steinernema feltiae nematodes

Steinernema feltiae nematodes are most effective in controlling larvae of several species, including the cannabis fungus gnat and root aphids. These worms safeguard the upper portion of the soil up to three inches, then search for and infiltrate their prey. 

As they reproduce inside their victim, more nematodes seek other targets. After spraying the soil, Steinernema feltiae provides long-term protection against new infestations.

If your weed plants are outdoors, only apply nematodes in cooler temperatures. They’ll die in hot weather before they perform their duty.

Hypoaspis predatory mites

These predatory mites feed in the top layers of moist soil, preying on larvae and pupae, thrips, and other small invertebrates. Apply several dozen bugs in each container or per square foot of substrate.

It’s best to use this treatment before the infestation gets out of control. Avoid using it inside homes.

BT bacteria

Another pest treatment that eradicates larvae in the soil contains the Bacillus thuringiensis (“BT” or “BTi”) bacteria. The pathogen creates a toxin that prevents larvae from eating.

The BT bacteria is ideal as it destroys the maggots but doesn’t damage your cannabis plants and their root system. It’s also not dangerous to humans. As BTi doesn’t reproduce, repeat applications every five days to provide control.

Say goodbye to gnats!

Pest infestations can be a grower’s nightmare. Fungus gnats are no different. They do damage below the surface, posing a potentially lethal threat to seedlings. In addition, if the bugs attack your marijuana plants during flowering, you could have a lower yield.

Use our guide on how to get rid of gnats in a grow room the next time you face an onslaught. Preferably, set up preventative measures to minimize the risk of an infestation. Then, be vigilant and monitor your cannabis crops to detect the pests as soon as they appear.

Growing marijuana may have its challenges, but with knowledge and proper care, a bountiful harvest awaits.

Frequently asked questions about fungus gnats on weed

Review the following questions and answers to learn more about fungus gnats on cannabis. 

Will plants recover from fungus gnats?

Yes, with early detection and treatment, your cannabis plants may recover. However, if left untreated, fungus gnats can damage your crop’s root system, causing it to struggle to survive. In extreme cases, the attack can be lethal.

Weak seedlings are more vulnerable to cannabis gnats as their roots are still developing. Prevent infestations from occurring by not overwatering your soil and keeping it fungus-free. Also, place yellow sticky traps to detect a problem if it happens. Then, take immediate action.

How do I get rid of fungus gnats during the flowering stage?

An assault by fungus gnats at any growth stage may affect your marijuana plants and reduce their yield quality. Damaged roots can’t absorb the water and nutrients your crops need for healthy development.

You risk getting smaller buds if an infestation occurs when your weed plants bloom. If you notice fungus gnats during flowering, try these effective treatments and choose the ones that work for you:

  • Yellow sticky traps
  • SM-90
  • Neem oil 
  • Food-grade diatomaceous earth 
  • Essentria IC3 insecticide 
  • Dry the top layer of soil

How long does it take to get rid of fungus gnats?

It’s challenging to eradicate cannabis fungus gnats once you have them. How long it takes to solve the problem depends on how early you detect them and initiate treatment.

It could take a few days or weeks before you succeed. The more severe the infestation, the longer the process.

Keep using the yellow sticky traps as a way to monitor your progress. If the number of gnats on the cards drops, you know it’s working. Don’t rush to conclude the infestation is over until you have hard evidence like an absence of fungus gnat damage on leaves. 

Are fungus gnats attracted to UV light?

Generally, no. Although fungus gnats can see UV, it doesn’t attract them. So why do growers use bug zappers with ultra-violet rays? 

The wavelength frequencies disrupt the insects’ ability to maneuver in the air and fly in a straight line. As a result, the cannabis gnats become disorientated, causing them to head to the electrical device. 

Some zappers emit pheromones that appeal to the bugs. Installing one in your grow area may help reduce the gnat population, but it won’t be as effective as other treatments.

Will using good soil help prevent fungus gnats?

Besides providing sufficient nutrients to your marijuana plants, what’s most critical is the soil’s texture, drainage, and water retention ability. 

If the medium can remove moisture effectively, it can help reduce the risk of fungus gnats on cannabis plants.

Are there cannabis plants that resist fungus gnats?

Many marijuana strains are resistant to pests and diseases due to their genetics. It’s best to cultivate the right cannabis seeds to minimize the risk of fungus gnat issues. You get hardy plants, which, with proper care, offer abundant yields.

Can I use household products to control fungus gnats?

Hydrogen peroxide is an excellent household product for controlling bugs. Preferably use 3% food grade and mix one part with four parts water.

Use it as a soil drench. Pour it on the root area until it seeps out the pot’s base. The solution is potent as it kills the maggots upon contact.

Don’t use hydrogen peroxide in conjunction with the beneficial nematodes, as it’s also lethal to them.

AUTHORED BY: Douglas Kester Mr. Kester came to i49 with a wealth of experience. He’s worked in the cannabis industry for more than ten years. As a growing expert at i49, Douglas finds it hard to choose a favorite strain. Instead, he regards each one as unique and full of potential. Douglas finds it rewarding to experiment with specific cultivars and cross-breed to discover a new one. He strongly believes in sharing the benefits of marijuana with as many people as possible to avoid any misconceptions about the herb. Mr. Kester creatively produces information based on what he’s learned and his experience obtained by implementing what he knows. i49 is proud to have Douglas as part of the team.

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