Fungus Gnats on a Cannabis Plant, Diagnosis and Treatment

Fungus gnats are a type of flying pest. Adult fungus gnats will grow to around 5mm-1cm, and can be seen flying around your grow room, especially near the top of the medium in your pots.

The adults will lay eggs in your soil, and they will hatch within 4-6 days. Once hatched, the larvae will start eating the hairs on the roots of your plants and the problems will start to arise quickly

Just one adult female, can lay up to 400 eggs in a week, and a fungus gnat infestation can quickly become a problem.

In this guide we will discuss how to properly identify fungus gnats on your cannabis plant, and the best way to treat them.

If you need urgent help with a fungus gnat problem on your cannabis plants, feel free to ask for help in our cannabis growers forum

Contents

• What are Fungus Gnats
  • Fungus Gnat Eggs
  • Larvae Stage
  • Adult Stage
• The Effects of Fungus Gnats on a Cannabis Plant
• Checking for Fungus Gnats
  • Larvae and Eggs in the Medium
  • Adults on the Soil, and Flying Around
• How to Kill Fungus Gnats
  • Proper watering Technique
  • Sticky Traps
  • Pesticides

What are Fungus Gnats

Fungus gnats are a type of small black fly, that resemble a mosquito.

This picture is of an adult female, and these are what you are most likely to find in your grow room.

The fungus gnat, like most bugs, like the warm moist area at the top of the medium. They will lay their eggs in the top of the soil, and within a few days, these eggs will hatch, and the life cycle of the fungus gnats begins.

Eggs

An adult female, can lay up to 400 eggs in a week! All of these eggs will hatch within 6 days after being laid. Though the eggs are not a problem, what comes out of them will be.

The eggs are going to be very hard to see, but can be found in the top layer of your soil. If you use a microscope and inspect closely you may be able to see the eggs:

Larvae Stage

During the larvae stage, fungus gnats will begin to wreak havoc on your plants root systems. They will not only start to eat the small hairs on the roots, but their droppings will have an affect on the drainage of the medium.

The fungus gnat larvae loves the warm moist medium, their droppings will help them achieve this desired environment. The more fungus gnats droppings there are in the medium, the more moisture will be held in it, and this will give the larvae and the eggs the ideal conditions to thrive.

The larvae will live in the medium, and feast on your plants roots for 7-14 days. At this point is will go into its "Pupa Stage". It will wrap itself in a cocoon, and 7 days later will hatch into an adult.

Adult Stage

An adult fungus gnat will lay between 30 and 200 eggs in one sitting. These eggs will be laid where fungus spores are at the highest amount, as this is what the larvae will eat when they hatch.

The flying adult fungus gnat, can be hardy, and not an easy pest to kill, or fully eradicate, but the larva and pupa are easy to control.

The whole life cycle can take between 3 and 4 weeks, for eggs to hatch and grow to lay more eggs. If you notice you have fungus gnats, by gnats flying around the grow room, the chances are the infestation has been there for a while, and should be treated quickly.

In many cases of bugs on cannabis plants, it can sometimes be easier to spot the effect the bugs are having, rather than the bugs themselves.

The Effects of Fungus Gnats on a Cannabis Plant

An adult female fungus gnat will lay her eggs in the warm, moist layer of your soil. When these eggs hatch, the larvae will start to consume the roots of your cannabis plants.

Not only do they eat the roots, but their dropping will thicken the soil, and affect the drainage of the medium. This will help keep the medium wetter, for longer, and your plants will start to show symptoms of over watering.

Look out for wilting leaves, and a generally weak plant. Having fungus gnats will look like many other cannabis plant deficiencies, the best way to see if you have fungus gnats is by looking at the top layer of your medium.

Checking for Fungus Gnats

To make sure you have a proper diagnosis and can treat your problem correctly, you have to make sure you have fungus gnats in the first place. Sometimes the effects can be mistaken for fruit flies or a different bug, but with similar effects as fungus gnats.

Adult fungus gnats will look like mosquitos, and be small, and black. You will find them flying around the lower section of your plants and around the medium.

This is where they will lay their eggs and find their food. Looking at the top layer of your soil is the best way to find fungus gnats.

Larvae and Eggs in the Medium

By taking a good close look at the top of your soil, you may find these creatures.

These white worms, with black heads, are the larvae of fungus gnats. They will vary in size and can grow up to 5 mm in length. They are small, so hard to spot, but are a sure sign that you have a fungus gnat problem.

Don't get these confused for spring tails though. Springtails are also small and white, but they will jump around when they are disturbed.

If you use a magnifying glass you may also be able to see some eggs. The eggs will be round like a ball, and amber in colour. ( See the image above)

Adults on the Soil, and Flying Around

One adult fungus gnat can get into your grow room and lay 400 eggs in a week. It will then be 3-4 weeks before you see another adult flying around your grow room.

By this time, those adults would also have laid a few hundred eggs each, and the problem starts to get out of control.

You can find adult fungus gnats crawling, and flying around the top of your soil. They will live around there, and lay eggs as much as possible. It is a good idea to cover the top of the medium with sticky traps. as this will catch a few of the gnats as they try to fly off.

This will not only stop the gnats from flying around to lay more eggs, but give you a good look at the bug in question, to ensure it is a fungus gnat before treating the problem.

How to Kill Fungus Gnats

If diagnosed and treated quickly, fungus gnats are a relatively easy pest to control.

There is one main reason why these bugs have chosen to live in your grow room, your medium is being kept too wet for too long. Fungus gnats love to live in warm moist areas, where fungal spores are found.

You can treat fungus gnats without using any pesticides at all, but you can if the infestation has become badly out of control.

Let the Top of the Medium Dry Out

Eggs, larvae, and pupa, will all live in the top inches of the soil. They will need moisture and fungal spores to survive. By simply allowing the top 2 inches of your medium to dry out between waterings, you will make it harder for the fungus gnats to live there.

At the very least, this will massively reduce the amount of bugs in the grow room. Most cases of bugs and pests can be put down to bad waterting practise. It is vital, especially when growing with soil or organically, to allow the medium to dry out a little between waterings.

Just by directing the breeze from a fan, over the top of the soil, will help it dry out quicker, this will massively reduce the amount of gnats in the medium

Sticky Traps

Only flying female adults will be able to lay eggs. To prevent more eggs being laid, you can catch these adults with sticky traps. There are 2 ways you can do this:

1: Flat Sticky Traps

Yellow sticky traps are designed specifically to catch flying insects. These flat square ones can be placed an inch or so over the top of your medium.

When a fungus gnat hatches from its pupa, it will attempt to fly off, looking for a place to lay eggs. Having sticky traps above the medium will catch these flying gnats before they get anywhere! Every single one you catch can potentially stop 400 more eggs being laid.

2: String Sticky Traps

These sticky traps come in a small tube, and are hung from the top of the grow room. They are designed to catch big flying insects, as they fly around the grow room.

Though not as effective as square sticky traps, if they are placed above the medium, they can still be used to good effect. Remember every single gnat you catch, will reduce the future population.

Pesticides for Killing Fungus Gnats

There is no real need to use harsh chemical pesticides to kill fungus gnats on your cannabis plants. It can be done simply by letting the medium dry out, and using sticky traps.

In some cases though, the infestation may not have been caught early, and using some pesticides to reduce the number faster, is prefered.

Fungus gnats will not attach to the plant itself, but the roots and the contents of the medium. So, you will need to treat just the medium with pesticides, which can affect the microbial balance, and also cause problems with your plants eating.

Neem oil pesticides will be your best option. That is harmless to the plant when used in small doses, but will stop the fungus gnats from breeding and eating

There are other homemade pesticides you can use, that will have little harm on your plants and the environment. Fungus gnats are not a massive problem, so there isn't a need to use harsh chemicals.

Here are some homemade pesticide recipes you can make yourself. Consider using these, along with a proper watering technique, and you will see a massive reduction in numbers straight away. And the fungus gnat issue you are having will be over in a week.

Final Thoughts

It is never fun to find bugs on your plants or in your grow room. But it is important to remember these problems are easily fixable with proper diagnosis and treatment. If you think you have fungus gnats, and need more help to get rid of them, sign up to the forum and ask for help.

Make sure the problem doesn't happen again by simply letting the top of the soil dry out in between waterings, and keeping a good IPM regime, prevention is better than cure.

Thanks for reading! You can find more information on anything related to growing in our cannabis grow guides. All of our guides are written by experienced growers, and members of our cannabis growers forum. If you have any questions, sign up. It is free, and we are always happy to help!

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