Fungus Gnats on Cannabis plants

How to identify and treat the infestation

Female fungus gnat on a leaf

Hey there and welcome (back)!
In this article we’re going to cover fungus gnats… Wasn’t really planned, but since we’re dealing with these suckers right now we figured it was a good time to share the knowledge we’ve acquired while it’s still fresh.

Although these suckers can do real damage to your plants, it’s a pest pretty easy to treat 😉 Let’s get into it

Fungus Gnat Infestation – Table of Content

Knowledge about Fungus Gnats

What are Fungus Gnats

Fungus Gnats are a little fly, similar to fruit flies, that are generally considered like a minor houseplant pest, although they can quickly become an issue.
Attracted to moisture and humid conditions, female gnats lay up to 200 within the top layers of the soil.

Once the larvae hatch from the eggs they feed on organic matter within the soil, and when a plant is around, that organic matter will be the roots, which is the source of the issues related to this infestation.
Totally harmless to humans, an large colony of fungus gnats can do great harm to your plants root system.

The life cycle of Fungus Gnats

The life cycle of fungus gnats is divided in 4 stages, 2 of which are a problem for your root systems, and only 1 easily visible..
From egg to adult, the fungus gnat lives between 30 and 36 days and can do some serious damage if left untreated.

Stage of development Specification
Egg Female lay between 30 and 200 eggs singly, or in clusters, within top layers of moist organic debris and prefer areas where fungus is growing.
The eggs will hatch within 4 to 6 days
Harmless to plants in this stage
Larvae Once hatched, the fungus gnat is in a larvae stage during 12 to 14 days. This is one of the most problematic stages since, to develop, it feeds on organic matter.
Generally that matter is… plant roots. At this stage, you can start seeing issues on your plants, starting with stunned growth.
In this stage they hurt your plants’ root system
Pupa After the 2 weeks of “being a larvae” (4 larvae stages), the gnat enters the pupa form. 
Much more mobile, for 3 to 7 days the crawler stays within the top layers of the soil and keeps feeding on organic matter.
In this stage they hurt your plants’ root system

Once ready the pupa transforms into an adult fungus gnat. Starting here you will start being able to see the gnats, this is also when they stop damaging your roots, their mission is now to spread around..
After about 8 days of adulthood the fungus gnat will naturally die, leaving behind between 30 and 200 eggs ready to repeat the process.
Harmless to plants in this stage, but this is when they spread.

Fungus Gnat egg – Image credit to epic gardening

Close up on fungus gnat larvae in soil

Fungus Gnat larvae – Image credit to bugwook

Fungus Gnat Pupa- Image credit to Matt Bertone

Adult fungus gnat on a ruler

Adult Fungus Gnat – Image credit to the Pest Cemetery

Conditions under which Gnats thrive

As their name points to, Fungus gnats enjoy humid conditions, especially where there’s a lot of decaying vegetation and fungi.
They thrive in medium where fungus is already growing, which is also how they got their name.

Identifying and Killing Fungus Gnats

How do Gnats harm Cannabis

As we started mentioning above, during 2 stages of development of fungus gnats, they feed on organic matter within the soil. 
Part of that organic matter are the roots of plants. The larvae and pupa’s eat through the root system and as you can imagine, the plant doesn’t enjoy that.
As the infestation grows, your plant will not be able cover the root loss, weakening and living a slow death.. 

Here’s a video that shows, up close, a fungus gnat eating through soil and a young plants root system.

Minute 1:25 – Larvae feeding off roots
Credit to Umdis Store

Signs of Fungus Gnat Infestation

Soooo spotting fungus gnats before it’s too late can be tricky… you gotta spot the adults, and kill them, before they lay any eggs… easier said than done.
Generally you don’t spot the first individual so your ground is infected, and the larvae are so small that you can’t see them with you bear eye. As always, your plant will tell you whatsup!

Since fungus gnats larvae and pupa’s feed on the plants root, you will see your plants growth slow down, the leaves will start showing weird damage. In time, she will fully stop growing and, if left untreated, will die..

Leaf showing weird damage patterns

Weird damage on leaves

Growth slowed and damaged leaves

Weird damage on leaves

Leaf damage keeps spreading

Plant leaves growing abnormally

New leaf growth looking healthy

Starting to recover avec Nematodes were introduced

How to get rid of Fungus Gnats

Alright let’s get into the reason your here, how to get rid of these suckeeers. We’ll cover organic ways to deal with them, there’s probably non-organic treatments but honestly, you’ll kill your soil & we’re not dealing with spidermites here, you can get rid of them swiftly.
Each level of development of fungus gnats has a weakness, it’s important to tackle each one at the same time in order to wipe out the whole population.

How to Kill Fungus Gnat Eggs

There are basically 2 ways to get those eggs out of there :

  1. Remove the top layer of soil : Since female fungus gnats lay their eggs within the top 4 centimeters of the soil, if you get it out of there, they should be gone.
    The main issue with is is that you might damage your plants root system. Yo might also miss some, making this technique not very effective.
    Removing top soil can be good to contain the situation.
  2. Nematodes : We go in further in depth in regards to nematodes a bit later in the article, but basically these little guys will penetrate the eggs & larvae and kill them from the inside.

How to Kill Fungus Gnat Larvae & Pupa

Larvae & pupas are a bit easier to get to since they are mobile, although i’m not saying it’s easy.

  1. Hydrogen Peroxyde : Using a mix of 1/4 Peroxyde and 3/4 of water spray intensely the infected medium. 
    On contact, the hydrogen peroxyde will kill any larvae and pupa’s, but the eggs will stay unharmed.
  2. Removing the layer of soil : Just like eggs, pupa’s and larvae are found within the top soil of the medium. If you remove it you should get ride of the bulk (or all) of the individuals.
  3. Laying potato slices on the oil : Larvae & pupa are attracted to potatoes. Lay slices of tomatoes on the top layer, leave them there for about 4 hours. Once the time has passed, pick up the potato slices, you should see a bunch of crawler under there (larvae & pupas)  
    Remove all the soil surrounding the slices to deal an important blow to the nest, although the eggs will still be chillin’ in the soil..
  4. Nematodes : We cover this further down in the article, but in short nematodes are a natural predator of fungus gnat larva.
    Bonus benefit? They keep your soil alive and help keeping your soil nutrient rich without needing to spice up your water 😉 
using water sprayer to hydrate the soil

Spraying hydrogen peroxyde mix on soil (you’ll hear a poppin’ sound)

2 potatoe slices on medium to attract the larvae and pupa

Slices to potato on top soil to attract larvea & pupas

Soil full of larvae and pupa removed

Removing top soil after leaving potato slices a couple hours

Dozens of nematodes in and around a fungus gnat larvae

Spraying soil in order to introduce the Nematodes (also sticky band visible)

How to Kill Fungus Gnat Adults

Here we are to the visible, annoying, part of the infestation. On top of that, these suckers are to be killed in order to avoid the spread of the infestation to your other plants around. Here’s a couple ways to deal with them (let us know if you’ve got more 😉 )

  1. Sticky traps 
  2. Beer&water (yes, beer :p)
  3. Glue on the side of pots
  4. Manual killing
Sticky Traps

Like any flying pests sticky traps are pretty useful to treat the infestation. The main downside to these is that they’re really sticky, making it hard to maneuver within your grow room when they’re laid out.
To increase its effectiveness lay them out across your pots, that way even the young adults, that really just bounce around, can get caught up in them too 😉

Sticky trap layed out across the pots

Sticky trap freshly set up across the grow room
(adults already stuck on it)

hundreds of adults stuck on trap

Removed about 2 weeks later, hundreds of dead adults on there

Beer & Water

You heard us right, for some reason adults are attracted to beer… soooo pour them their final drink.
You don’t need much, here’s what we did :

  1. Put about 1.5cm of beer at the bottom of a glass
  2. Added a bit of hand soap to break the water tension
  3. Added some water
  4. Put down the glass between all the pots

Within a couple hours the first casualties were visible, and a day or two later there were dozens in there making a little black ball (visible on the image on the right and side)

Sticky trap freshly set up across the grow room
(adults already stuck on it)

Blue on edge of pots

Sooo found this out by “mistake”, while laying out the sticky traps we got glue everywhere.
Turns out it as a pretty good thing, as you can see on the image on the right those little suckers tried jumping out around the cup.

On the side where there was glue you can clearly see dozens of little adult gnats stuck.

Sticky trap freshly set up across the grow room
(adults already stuck on it)

Manual Killing

This is the good ol’ way.. not super efficient against the smaller individuals, and you definitely won’t be able to fully treat the problem this way, but the flyers that you can easily identify you might as well clap em into another dimension. 
Whether male or female, getting rid of these suckers as quickly as possible will reduce the risk of them laying more eggs, repeating the whole cycle.

We’ve spent a good amount of time killing these suckers manually, especially due to the fact that they spread to our other plants…

Fungus Gnat Predators – Our Nematode friends

These little guys are your best friends, for fungus gnats but also a bunch of others pests, counting ants and caterpillars. On top of that, they keep your soil alive and help return nutrients, that are then available for your babies 🙂
You can buy them online or at a garden store easily, it will be a bag with what looks like yellowish powder; that power is the little nematodes.

Introducing nematodes into your soil
  1. Place the nematodes in a water sprayer
  2. Shake the container in order to get these little worms floating in the water
  3. Spray the soil as if you were watering them. Make sure you cover the whole surface of the soil. 
  4. Wait, the little guys are making their way through the soil and searching for their pray.
Nematodes delivered in a plastic bag

Bag of 5 million nematodes, enough to cover between 10 and 30m²

Pouring nematode "powder" into water sprayer

Pouring some of the nematodes into a water sprayer

Nematodes in water sprayer, before mixing

Nematodes stuck together in the water, shake to even them out within the mix.

Watering plants with nematode mix

Spraying soil in order to introduce the Nematodes (also sticky band visible)

Make sure you :

  1. Cover all the pots in your grow room and apartment, basically all the soil you have around. With these predators introduced, adult females will be searching safe spots to lay their eggs (you can ask our citrus and palm tree… 🙁 )
  2. Keep measures in place to trap & kill the remaining adults. Nematodes are only effective against eggs and larva, the remaining pupa’s will develop into adults.

If you see adults show up days, or up to a week or 2, after introducing the nematodes don’t worry. It doesn’t mean they’re not “working”, just that nature takes time and you probably had a large number of individuals.

How do nematodes kill Fungus Gnats?

To put it simply, nematodes are able to go through fungus gnat eggs and larvae skin and penetrate into them. From there they feed on its organs and grows.
As the nematodes grow the larvae weakens until it dies. The nematodes grow, reproduce and once and too packed within the host, break out of the skin and return to the soil in order to find a new place to feed on.

If you want to check out how it all happens check out this video, you will see the whole process under the scope, with explanations; preeetty informative

Credit to Bannana_bin, check out her insta for more 😉

Preventing Fungus Gnat

The best way to avoid a pest problem… prevention, prevention, prevention. Here’s a couple things we were able to pick up while dealing with the issue, hope it helps out 😉 

A soil that drains well

Since fungus gnats develop in humid, fungus rich, mediums, the last thing you want is bad drainage. 
If, after watering your top soil stays humid for days, you have drainage issue.

Lots of air mouvement

Have some fans blowing within your grow room, at the base and on top of your plants. This will help drying out the soil between waterings.
Be careful not to have the air blowing too hard on your plants.

Add layer of sand over the soil

Since fungus gnat are attracted by humid soil to lay their eggs adding a soil of sand should keep the females from laying there.
Add at least 4cm of sand.

 Keep the grow room clean

As always, having a clean grow room will help keeping unfriendly visitors away (and help you spot them faster)
In our case the infestation started in the cutting’s propagation box. The humidity levels were nice and high, soil was kept humid, a perfect condition for a mother to sneak in a lay her eggs.

Alright folks, that’s it for this article
If you have additional questions, or info to be added, don’t hesitate to get in touch! Whether through the contact form in the top bar or our instagram

Until next time,
Be safe and grow easy

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