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1. Dealing with Fungus Gnats on Cannabis Plants

There are, in all honesty, not many things more frustrating than pouring your heart and soul (and cash) into a cannabis crop, only to realize you’ve got cannabis fungus gnats flying around feeding off of your precious plants.

Not only are these pesky little critters annoying, but they can wreak havoc on your crop and severely deplete the yield capability and final potency if not dealt with quickly.

Let’s take a deep dive into the world of fungus gnats, and find out just exactly how to identify, eliminate, and hopefully prevent any chance of the annoying little bast***s taking hold in the first place.

Fungus Gnats Biology

Fungus Gnats on cannabis, also referred to as sciarid flies, are small, 2 to 3.5 mm in length, flying insects that closely resemble tiny mosquitos (although they are not related). They live for an average of 4 weeks, but this can be affected by the environmental conditions, as fungus gnats prefer and develop faster in warm and humid conditions.

They progress through a few distinct stages during their life cycle, starting out as an egg, then 4 larval periods, before developing into a pupa, and finally the flying adult stage. Fully developed fungus gnats live for around 10 days, with females capable of laying up to 200 eggs. It’s thanks to this quick lifecycle and the large egg-laying abilities that allow for such fast infestations to take hold and quickly cause issues for cannabis crops. The larvae feature an elongated, off-white body with a small black dot on the head.

They stay in the larvae stage for around a week, and are actually slightly larger in size when compared to fully developed adults – around 5 to 6 mm – and it is during this stage that they are at their most destructive. Although adult gnats are annoying to have buzzing around your crop, they are actually pretty harmless – apart from the fact that the females are capable of of quickly becoming sexually mature and laying a sh**load of eggs that quickly mature into the more damaging larvae, and repeat the cycle. 

Growers, especially beginners, usually don’t realize they have fungus gnat issues until they see the adults flying around the crop, at which stage the infestation has already been taking place for quite some time, and root damage is probably already an issue. 

How Much Damage Do Fungus Gnats Cause?

Unlike a bunch of the other common cannabis pests, it’s not the fully developed form that we have to worry about. Female cannabis fungus gnats lay their eggs in any moist and humid areas, usually in the top layer of soil (or coco-coir) near the roots, which means that it is actually the larvae that cause most of the damage.

The larvae feed on the roots of cannabis plants, as well as any organic material found in the grow medium such as, you guessed it, beneficial fungus. This damage to the root ball can quickly lead to stunted plant growth as well as a generally unhealthy appearance, with leaves often turning yellow or wilting, and sometimes displaying some weird, dark spots. It can also open the plant up to a wide range of secondary issues, as the root damage allows soil bourne disease and pathogenic fungi to find an easier way in.

Fungus gnat damage can also make the plant display signs that can be easily confused with nutrient deficiencies. If you have been correctly dosing your nutrient solution, and ensuring the pH levels are in the right range (5.8 to 6.2 for hydro and coco, or 6.5 to 7.5 for soil) and you are still seeing signs that look like nutrient deficiencies, then it could be a fungus gnat infestation.

They can also cause immature plants (seedlings) to wilt to the point that they fall over and lay down onto the growing medium. If this happens, it can be close to impossible to save them. In this case, we recommend clearing out and sterilizing your grow space and starting again with new plants and media.

Causes of cannabis fungus gnats

If a severe infestation is left unchecked for long enough, it can even lead to the death of the plant – or at least, that’s what will happen if they’re not treated. Fortunately, for the vast majority of fungus gnat infestations, this won’t be the outcome. Although they can be a huge annoyance, they are rarely a life-or-death issue for your crop. 

Infestation Identification

Fungus gnat infestations are pretty easy to identify. The first sign that most growers will see is the adults jumping around the top soil or coco layer, or flying around the crop, often in large numbers. With every cannabis crop, you should be always on the lookout for issues and be inspecting your plants daily. The maggots are harder to spot (as they live in the soil or coco), but that doesn’t mean they’re any less of an issue.

Fungus Gnats Vs Fruit Flies

Fungus gnats look pretty similar to fruit flies, so they can be often confused. The main difference is that fungus gnats have longer antennae, and their bodies and wings are darker in appearance. Fruit flies’ antennae are much shorter and their bodies and wings are more of a yellow-brown color.

If you do suspect a fungus gnat infestation, then there are a few things that you can implement to help with detection.

Yellow Sticky Traps

Place some yellow sticky traps near the base of your plants. These will attract and trap any adults, which will give you a better idea of how many gnats are actually in the grow room. They won’t totally remove the infestation, but they will help you work out exactly what’s going on.

Potato Wedges or Disks

To help with larval detection and identification, we recommend slicing up some potatoes and placing them on top of the growing medium. Disks seem to work best for this. Cut 5 mm disks and let them sit for between 24 to 48 hrs. The maggots will be attracted to the potato and congregate in the area underneath, making them much easier to find, meaning you don’t have to go digging which hurts the roots.

Management And Prevention

Fungus Gnat on leaf

As with all cannabis pest issues, prevention is the best form of attack. Make sure you’re starting off with clean equipment, and a growing medium that has been correctly sanitized. Maintaining a high standard of cleanliness in and around your grow room or tent can be slightly tricky, but it is 100% imperative to ensure healthy plant development. 

Always, and we mean ALWAYS, fully sanitize and sterilize your grow area between crops. Its also important to keep the area as clean and organised as possible during the duration of each grow. That means removing or mopping up any runoff and dead plant material, as these can quickly attact a wide range of pests and boost the chances of mold issues occurring.

There is a range of inferior soil and coco blends on the market, but as long as you stick to the better cannabis-specific brands then you should have no problems. Look for stuff from brands such as Canna, Atami, House and Garden, Botanicare, Advanced Nutrients, or Plant Magic to be sure you are getting the highest quality. Alternatively, head to your local hydroponics supplier and have a chat. They should stock a wide variety of great options, as well as some local brands that are usually a little more wallet-friendly than the stuff from the big dogs

If you are ever unsure about your soil or coco, then it’s always best to sterilize it before using it. To do this, simply spread it out evenly on some oven trays, cover it with aluminum foil, and bake it at a low temperature (around 90 degrees Celcius or 200 degrees Fahrenheit) for 45 mins. Just keep in mind that this will also kill beneficial bacteria.

There are a bunch of options out there to help with prevention and infestation eradication. We like to go down the non-toxic route, as this is usually the most effective and least harmful approach – but if all else fails, then it may be time to resort to chemical warfare. This shouldn’t be necessary in almost all cases, though.

When used correctly, insecticides and fungicides can work wonders. Just make sure you are following the instructions for your specific problem, and remember that over-application can be very damaging to both plants. But, before you get the big guns firing, try out these options.

Neem Oil

Neem Oil is one of the best natural solutions for fungus gnats, as it works by disrupting the normal growth patterns of these pests. There are also a number of different preventative Neem products available. When battling fungus gnats, give the top layer of the growing medium a good old spray every day or two.

Biological Control Agents

Biological Control Agents might sound a little intimidating, but they are actually totally natural products that prey on the pests they have been developed to target. The most common examples of these are predatory mites, beetles, and nematodes. They attack the problem at the source and feed directly off the maggots. There’s also Bacillus Thuringiensis Israelensis, a form of beneficial bacteria that are known to work wonders in the battle against fungus gnats.


Another organic pesticide solution is SM-90. This is a concentrated formula that uses natural oils and extracts to kill off pests without harming beneficial bugs – or us. It’s considered extremely safe, so you can use it without having to worry too much about any nasty side effects. You can apply SM-90 directly to the medium, or as a foliar spray.

Diatomaceous Earth

Made from the pulverized remains of microscopic algae, diatomaceous earth can be used to great effect against fungus gnats. Spread a light layer over the top of the soil or coco, and the tiny razor-sharp particles act like glass shards to the maggots, cutting them up and slowly dehydrating them. Reapply after watering, and only use a food-grade certified option on cannabis crops.

Water Control

Fungus gnats need a pretty moist environment to survive, and one common mistake that new growers make is overwatering. Keep an eye on the moisture levels of your growing medium, and make sure you aren’t over-watering plants or allowing them to sit in water from the runoff. This is usually enough to keep fungus gnats at bay, as they don’t like dry conditions. You can let the plants dry out for a few days if you are battling the little buggers.

Creating a wet/dry cycle is not only good for getting rid of the gnats but also beneficial for overall plant development and health. Ensuring short periods where the medium is allowed to dry out encourages the root mass to go searching for water and nutrition, boosting the overall size.

You can also consider covering the top layer of the medium with some perlite or sand, but keep in mind that this will slow the rate of evaporation and keep the soil moister for longer.

Fungus Gnats And Flowering Cannabis Crops

While fungus gnats are more often found attacking younger plants still either in the seedling or vegetative growth stages, they do sometimes start to become an issue during flowering.

In this case, you want to be extra careful with how much moisture is present in the soil. Make sure you are giving the correct amount of water, and allow it to dry out between each watering. This should be enough to keep any fungus gnat invasion at bay during the later stages of growth.

If you do find they have become an issue, later on, remember that you want to keep non-organic sprays from the budding sites. There are some organic solutions that can be applied during flowering, such as Neem oil and diatomaceous earth, but it’s still best to try and keep the buds protected from any sort of spray defenses. Be sure to check the labels first and make sure the product is safe to use on flowering plants.


While fungus gnats can be a real pain, they are anything but the most serious of pests – and they are relatively easy to get rid of. All it takes is a little bit of knowledge, some basic preventative measures, and a sprinkling of luck to make sure that your plants don’t fall victim to these little rogues. 

Take all of the necessary precautions, and you should be able to keep your grow room pest free!

Picture of Sam North

Sam North

Sam North is a content writer with a passion for everything cannabis. After working multiple seasons on weed farms absorbing the ins and outs of cannabis cultivation and culture, he decided to transition into a role that would allow him to work from anywhere, anytime. Sam now writes for multiple weed publications. He has extensive experience with a wide range of canna-agriculture styles, from smaller artisanal farms to large-scale commercial operations, and is here to share his knowledge to give you all the best chance of cultivation success.

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