How To Get Rid Of Fungus Gnats

How to Get Rid of Fungus Gnats
April 27, 2020

When growing cannabis at home, it’s hard to imagine everything running smoothly and problem-free for the entire cycle. There are all sorts of issues you could experience… pests and pathogens can invade the tent, equipment can break, waterings can be forgotten… learning how to deal with issues first-hand is the best way to become a better gardener. 

Fungus gnats are one of those potential problems. They’re annoying little bugs about the same size as a mosquito. If you allow fungus gnats control of the grow, they could damage and even destroy your entire crop. This article will ask, what are fungus gnats? How do I spot them? How do I get rid of fungus gnats larvae and how do I prevent them from coming back?

What Are Fungus Gnats?

Also known as sciarid flies, fungus gnats are a common agricultural pest that target soil-grown plants. They love humid areas, especially ones with standing water. You will often find them around damp compost hills and natural ponds.

Fungus Gnat On A Leaf

What do fungus gnats look like?

At around 2mm long, a fully-grown fungus gnat is no thicker than a dime. They resemble mosquitoes, with long legs and transparent wings. The larvae or maggots are 1/4 inch in length. Sporting a whitish, transparent body, they’re most distinguishable by their shiny, black head.

Despite their tiny size, fungus gnats can cause significant harm to individual plants AND entire gardens.

How long do fungus gnats live?

Fungus gnats reproduce quickly, giving birth in fertile, moist soil. An adult typically lives up to a week and can lay as many as three hundred fungus gnat eggs within its lifetime. In less than a week, these eggs hatch and tiny larvae emerge, feeding continuously. The maggots then enter the pupal stage before becoming fully grown fungus gnats.

They complete their entire fungus gnat life cycle in under a month. This means multiple generations of the pest could be present under the soil before you’ve even spotted the flying gnats.

How do I tell a fungus gnat from a fruit fly?

Fungus gnats and the common fruit fly look a lot alike. There are, however, some identifiable differences, most notably in color and size. Fungus gnats are grayish black and much smaller than the yellowish or pale brown fruit fly.

Fungus gnats make their home in wet, damp soil. Fruit flies, on the other hand, live around ripe or rotting fruit and vegetables. You can also find fruit flies around moist areas such as drains, empty containers… even garbage bins. If your growing area is nice and clean then the flies, if you have them, are most likely fungus gnats.

Fungus gnats and cannabis plants.

Fungus Gnats

Fungus gnats love cannabis plants that give them a place to breed and to eat. The perpetually damp soil of the inexperienced grower is the perfect home for fungus and decaying matter – rich food sources for the fungus gnat. Composts, peats and coco coir contain plenty of organic matter already, but the addition of excess moisture makes them VERY attractive. Rich, damp soil is like an all-inclusive stay at the Ritz-Carlton… presidential suite!

The gnats lay their eggs in the soil and, when they hatch, the larvae embark on a feeding frenzy. They will chew through anything edible, including the roots of your cannabis plant. And, to add insult to injury, they will clog up the soil with digested roots, making it harder for the soil to drain, encouraging the growth of more fungal matter (food) for larvae.

How do I know if I have a fungus gnat infestation?

There are many ways you can tell if you have a fungus gnat infestation, some more obvious than others. Let’s take a look at the main things to concentrate on.

Spotting a fungus gnat

The most obvious sign of an infestation is the sight of fungus gnats in and around your cannabis plants. If you spot them, you need to take action ASAP, whether the plant is showing symptoms of distress or not.

Adult insects are not a threat to the plants. But you can be sure they will have started laying fungus gnat eggs and some of these eggs will have hatched already.

Damage to your cannabis plant

Visible plant stress is another sign of a fungus gnat infestation. You need to look out for droopy leaves, pale or yellow leaves or leaves with brown edges and dark spots. It will appear as if your plant is suffering from nutrient deficiency, but it won’t be down to a pH, nutrient or any recognizable nourishment problem.

The infected plant may also show development issues and withering. Symptoms include small buds and stunted growth. If you don’t get rid of the gnats and their larvae, you should expect reduced yields from weak, dying plants.

How do I get rid of fungus gnats?

Watering Marijuana Plant

Once you’ve spotted the signs of a fungus gnat infestation, you need to take curative action. Here are some tried and tested ways to get rid of fungus gnats from your cannabis grow.

Start watering properly, immediately!

Overwatering is the primary cause of fungus gnat infestations. Check out Kyle Kushman’s guide to feeding and watering before you continue. 

It’s a good video, right? So now you know how to cycle wet to dry and how to promote a strong, healthy root system. Watering correctly will naturally deter pests like fungus gnats because you’ll no longer be creating a nice home for them.

Yellow sticky cards: A fungus gnat trap

Fungus gnats love yellow and some careful placing of yellow sticky cards can draw them away from your plant, trapping them in the process.

You can monitor the effectiveness of your treatments with a regular body count. As the number of trapped gnats decreases, you’ll know your actions are having an effect.

Dry the top layer of soil

Directing airflow over the topsoil (with a strategically-placed fan) is a fungus gnat double-whammy. It will dry the very top layer of soil, making it inhospitable for the larvae, and will blow away the adult gnats as they try to occupy the pot. This step will not be necessary once Kyle has taught you how to water, of course.

Destroy the fungus gnats larvae

This measure involves eliminating as many larvae in the soil as possible. One cannot employ the following steps with wet soil so please make sure the soil is fairly (but never completely) dry.

Neem Oil Fungus Gnats

Neem oil is safe to use and it kills gnats almost immediately. The problem is that it really stinks, and will do so for a few days at least. If your plant is in full flower and close to harvest, don’t spray it on the buds. It will alter the flavor of the cured herb and will be a disappointing end to your grow.

Neem oil is a great product to have on hand. Apart from fungus gnats, it’s also a great way to treat aphids, caterpillars and other garden pests.

Food-Grade Diatomaceous Earth

Diatomaceous earth is an organic insect killer made from fossilized shells. It pierces the exoskeleton of insects on a microscopic level. Once punctured, the pests lose body fluids and eventually die.

Despite this lethal-sounding description, the product is safe for humans and pets. To apply, spread it over the soil using a powder duster.


Like diatomaceous earth, SM-90 is safe to humans even when ingested. The product kills the larvae in the soil while leaving the roots unharmed. Mix 1 part of SM-90 to 5 parts water and spray it on the topsoil no more than once a week.

Essentria IC3 Insecticide

Although marketed as a “bed-bug killer,” Essentria IC3 insecticide is great at killing fungus gnat larvae. It’s safe for humans as it’s an organic mix of various horticultural oils.

Use a one-hand pressure sprayer to make sure the soil is evenly doused. It can be applied every other day and combined with other products, too.

BT Bacteria

BT Bacteria stands for Bacillus thuringienesis var. israelensis. It’s a type of bacteria that produces chemicals that are harmless to the plant but really bad for the gnats and their larvae. It basically stops the maggots from being able to eat.

There is a variation of BT Bacteria that will kill caterpillars but leave your gnats unaffected. Make sure you get the one for Mosquito Bits or Dunks. Crush the pellets into powder and mix well with water before application.

Fungus Gnats Hydrogen Peroxide

Although not as potent as the solutions made especially for killing pests, Hydrogen Peroxide still does the job quite well. The suggested ratio of Hydrogen Peroxide to water is 1:4 when dealing with 3% pure H2O2. Choosing a food grade brand is highly recommended.

How do I prevent fungus gnats?

Once you’ve got rid of the fungus gnats, you need to make sure you keep them away.

The VERY FIRST thing you need to do is stop overwatering the plants. Follow Kyle’s watering system and make sure the soil is never too wet. This may sound obvious but double-check the drainage holes in your pots – if water cannot drain it will stand and the problems will return.

Be vigilant and check everything you bring into your growing environment. Never bring in soil or clones that are showing any signs of flying bugs or larvae. Some growers even go as far as cooking the new soil to kill any larvae and eggs before planting. This doesn’t work as well with clones 🙂

Keep the tent clean and your eyes keen

Prevention is the best cure and most problems can be prevented with common sense. Ask yourself: is my grow tent as clean as it can be? Am I watering my cannabis plants properly? Am I maintaining optimum levels of relative humidity and temperature? Am I introducing pathogens via my clothes or my clones? Am I checking my plants often enough?

High Potency Cannabis

Address these questions, make sure you’re not just growing your plants, but tending to them, too. Look out for signs of stress, look at the leaves… are they sagging, or are they praying high to the sky? Look at the soil… is it draining okay? Are you cycling properly?

They say you learn something new every day and this is SO true for cannabis growers. Preventing and/or curing problems like fungus gnats will give you confidence, add to your experience and will make you a better grower

“Anyone who keeps learning stays young.”

 – Henry Ford