Cannabis Pest COntrol: Fungus Gnats


The majority of cannabis growers cultivate their crops in soil so fungus gnats are marijuana pests that aren’t entirely new to them, especially among those who maintain indoor grow spaces. The chances of running into a fungus gnat infestation outdoors may be pretty slim, but it is a common issue among indoor growers. And there’s one primary reason why—moisture.
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Fungus Gnats Pest COntrol - Portland, Oregon & Washington

fungus gnats

The majority of cannabis growers cultivate their crops in soil so fungus gnats are marijuana pests that aren’t entirely new to them, especially among those who maintain indoor grow spaces. The chances of running into a fungus gnat infestation outdoors may be pretty slim, but it is a common issue among indoor growers. And there’s one primary reason why—moisture.

Fungus gnat larvae need wet conditions to thrive. It isn’t uncommon for indoor growers to overwater their cannabis plants or maintain incorrect humidity levels inside grow rooms and wet soil is the perfect home for fungus and decaying organic matter. After fungus has grown or overwatered matter begins to decay in the topsoil, fungus gnat lays their eggs in the top layer of moist soil.


fungus gnats

Fungus gnats are small flies that infest soil, potting mix and other container media. They also thrive in other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in the soil, but will also attack plants when their primary food source runs out. In the case of cannabis, fungus gnats can eat root hairs or chew directly on the roots.

emale fungus gnats can lay up to 200 eggs each week

Adult fungus gnats measure about 2 mm and are black, gray or dark brown. They have long legs and can pretty much look like fruit flies at first glance. The best way to tell is by size and color, as fruit flies are slightly bigger and typically yellow, orange or light brown in color.

fungus gnats larvae

Female fungus gnats can lay up to 200 eggs each week. These eggs hatch into larvae that look like tiny maggots which only live in the top 2-3 inches of soil. Some eggs may also be found in the lower sections of cannabis plants, usually along the stems and leaves. Larvae or maggots have black heads and clear bodies, but are impossible to see with the naked eye because they are too small. Still, both maggots and adult fungus gnats are equally harmful bugs on marijuana plants. Besides feeding on cannabis roots, the fecal matter fungus gnats leave behind can cause soil to suck up more moisture which can lead to increased pest and mold problems.


fungus gnat infestation.

Of course, you first need to verify that fungus gnats are indeed attacking your cannabis plants. As mentioned earlier, fruit flies are often mistaken for fungus gnats, so make sure the ones you see are dark brown, gray or black in color and not yellow, orange or light brown. Also keep in mind that fungus gnats are a lot smaller than fruit flies.

Among the best foolproof ways to check for fungus gnats is using yellow sticky cards which you can purchase at any gardening shop in your neighborhood. Don’t go for any other color because gnats are particularly attracted to yellow. I suggest you place a couple of cards in the soil next to your cannabis plants. Just leave them there and observe if they’re covered with gnats after a couple of days. If they are, then you probably have an infestation.

potato wedges fungus gnats
If you’re pressed for time, however, and have this strong gut feeling that fungus gnats are beginning to devour your cannabis babies, a quicker way to find out is to use something that’s already in your kitchen—a potato. Slice a potato into wedges and stick them into the soil or potting mixture.
How to get rid of fungus gnats

Cannabis plants can die from a fungus gnat problem, particularly if your plants are quite young or if the infestation is so bad. 

There are telltale signs, however, that can prevent this from happening.


Time is of essence so you need to address a fungus gnat infestation the moment you detect these nasty cannabis pests in your grow space.

 While it may be easy to trap adult fungus gnats using yellow sticky cards, it’s the larvae that actually needs more of your attention since they’re a bit harder to deal with.

how to get rid of spider mites
hydrogen peroxide for fungus gnat
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I recommend you don’t delay this any further, as early treatment and control is essential at this point.


To control and eliminate a fungus gnat issue in cannabis grows, I normally use BTi (Bacillus Thuringiensis), a bacteria contained in pest treatments that effectively kills the fungus gnat larvae. It works by preventing the larvae from eating and, in effect, kills them off effectively. This also will have no harmful side effects to you or your plants.

Depending on the level of your infestation, I may also recommend introducing some beneficial nematodes and insects such as rove beetles to chow down on fungus gnats at whatever growth stage they are in. These predatory bugs and nematodes will feed on the larvae in the soil and end the life cycle of fungus gnats in your grow room.
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At Ant & Garden Pest Control in Beaverton, Oregon, I only use top-of-the-line equipment and safe, eco-friendly products combined with the latest techniques to ensure we get rid of fungus gnats and all unwanted marijuana bugs in cannabis plants. 

These are just some of the commercial grows from where I’ve eradicated all types of cannabis bugs through the years.

Give me a call today to discuss how I can help provide you with an effective and long-lasting organic pest control treatment plan specifically designed for your cannabis grow space!

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Ryan Smith of ANT & GARDEN PEST CONTROL in Beaverton is the most trusted and reliable pest control guy in Oregon, and for good reason! I care about helping my clients as though they’re members of my own family. Whether you’re in need of urgent treatment for an existing ant pest problem or are looking for the best prevention methods for your home or business, I am committed to bringing you only the best ant and pest control Beaverton for your complete peace of mind.

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Emma BakerGreat company! I was about to kill all my plants and shut down because of frustration with two-spotted spider mites and broad mites. So happy Ryan verified this through a Dino lite microscope at 220 x magnification. Extremely satisfied in his service, my mites died, no eggs spared.
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Paul EvansAnt & Garden Pest Control deserves 10 stars saved me from bankruptcy. I had root aphids in my indoor garden there’s nobody that knows how to kill them and has posted it on the internet. Ryan has dealt with them before and knew exactly which olcc approved products to buy and how to prevent reinfestation. I just harvested my first pest free crop in years! I am now going with his maintenance sprays 1 time a week Ryan has all the best lab grade equipment and knowledge. Easier to let him focus on my pests my time can be spent growing bumper crops of exceptional quality. This harvest I yielded 50% more than normal and sold everything I grew for 30% more than last time. Not only are my pests gone but Ryan’s service made me money.
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Michael WrightThe commercial-grade soil baggage compost I used for my plants has root aphids. I had to reach out to Ryan of Ant & Garden Pest Control for assistance. The last three months have been quite pleasing as the destructive aphids were gone. Ryan was reliable, providing a constant level of quality with strong results. You can’t beat the peace of mind he offers. I had my first harvest smoothly with his maintenance spraying. We yielded over 2 grams a watt, nearly double our personal best.
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