Fungus Gnats and the Cannabis Plant.

By Alex Robles

     Before we start talking about fungus gnats I need to mention how important it is to learn how to water your plants.  I know it seems like a stupid thing to bring up because it sounds so basic, but many plants have met their death by over watering.  Most of those victims have been seedlings and young plants since their root system is still developing. However, too much or too little water or bad soil conditions (e.g., poor drainage, or waterlogging) are much more common causes of wilted plants.  

    Fungus gnats are named that because they feed and thrive on a fungal diet, and constantly wet conditions really help fungal growth.  They’ll pop up where there’s a constantly moist environment or a lot of decaying vegetation that’ll make fungi.

What are they?

  • Fungus gnats, the most common are the Bradysia species, they’re black or dark in color,and look like tiny mosquitoes. Adult fungus gnats have these skinny, bearly visible legs.  
  • They look alot like Shore Flies, which are also found in greenhouses, an easy way to tell the difference is by looking at the antenna.  Fungus gnats have long segmented antenna and the Shore Flies have short feather like antenna.
  • Fungus gnat adults are about 1⁄16 to 1⁄8 inch long and their wings are light gray to clear.
  • They adults aren’t very strong fliers and will usually stay close to a potted plants or by the lights indoor because there also attracted to light.  You can usually spot them running across the top of your growing media or hanging out on your plant as well.
  • The females will lay tiny eggs in groups of 3 to 10 on wet potting soil and one female can lay up to 1,000 eggs in her lifetime.
  • The adults live about one week.
  • The larvae have a shiny black head with long, whitish-to-clear, legless body, like maggots. They live on organic mulch, leaf mold, grass clippings, compost, root hairs, and fungi. When the soil is really wet the larvae will sometimes leave tiny slime trails like snails or slugs.
  • Most of the fungus gnats life is spent as either a larva or a pupa and all of root damage is done during the larvae stage.
  • Fungus gnats develop through four stages—egg, larva (with four larval stages or instars), pupa, and adult. The almost microscopic eggs are oblong and females love to lay them in damp organic media where the hatching larvae can feed.  
  • At 75ºF the eggs will hatch in about 3 days the larvae will hatch.  After about 10 to 14 days the full grown larvae is about ¼ inch long.   
  • When it’s full grown the larvae will stop feeding and spin a very thin cocoon in the soil.
  • The pupa stage lasts about 4 to 7 days.  After that, a full grown winged adult pops out of the cocoon and is ready to fly, mate and the female is able to lay eggs within a few hours of hatching.
  • A new generation of fungus gnats (from female to female) could be made every 17 days depending on the conditions. The warmer it is, the faster they’ll mature and lay eggs.

What does damage look like

  • Fungus gnats infest soil, potting mix and other sources of organic decomposition. Their larvae mostly feed on fungi and organic matter in soil, but will chew roots and can be a problem in greenhouses, nurseries, potted plants.
  • The adults don’t damage plants or bite people, their mostly considered a nuisance. Larvae, however, if not controlled, can damage roots and stunt plant growth, particularly in seedlings and young plants. So a plant that’s wilting may not be from lack of water, but root damage by fungus gnat larvae or other root zone problems.
  • Severely injured plants slow down, go off color, and may drop their leaves and cause it to die without any visible injury showing on the above-ground parts
  • Another thing a large infestation of larvae will start to do is clog up or slow down the soils ability to drain water with their droppings. If the soil can’t drain, it just stays wetter longer which creates more fungal growth, which supplies the gnats with more food.
  • The larvae will feed on roots of plants by chewing or stripping the roots.  Root hairs are eaten off, as well as the small feeding roots. The chewed roots will have small brown scars on the surface. In real bad cases, all that is left is the center part of the root.

How to fight them

  • Avoid over watering and make sure soil has good drainage.  You can do this by adding more perlite or sand to your mix.  Let the surface of container soil dry out between waterings.
  • Clean up standing water and take care of any plumbing or irrigation leaks.  
  • Don’t use incompletely-composted organic matter in potting mixtures unless it is pasteurized (heat treated) first, because it will often come with fungus gnats.
  • If your using manure, blood meal, or similar organic materials to fertilize, don’t go too heavy.  As they break down they’ll attract adult gnats.
  • Peat moss is also living material that will break down and attract pest so it may also come with gnats.
  • When fighting fungus gnats, I would focus on the larvae.  Don’t spend too much time or money trying to control the mobile, short-lived adults.  
  • You can cut down on the number of fliers by using sticky traps.  Yellow sticky traps can be cut into smaller squares, attached to wooden skewers or sticks and placed in pots to trap adults.
  • I read that raw potato chunks placed in the soil are very attractive to fungus gnat larvae. It could be used not only to check pots for larvae but also to trap them away from plant roots. After a few days in a pot, remove and dispose of the infested chunks and replace it with fresh ones.

Biological Predators

  • Some biological controls for fungus gnats include the Steinernema nematodes, Hypoaspis predatory mites, and the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti).
  • Bti has to be ingested by the pest. Once it does that the bacterium produces crystalline toxins which disrupt the pest’s digestive system, which stops continued development and killing the pest larvae before they reach adulthood.  Bti does not reproduce or really thrive indoors, so heavy infestations in potting soil mix might need repeated applications at about five-day intervals for good control. Several Bti products (Aza Max) are easy for find online or a retail nurseries and garden centers.  
  • Nematodes and Hypoaspis mites must be mail-ordered and are alive and perishable and should be useds as soon as possible. Nematodes can provide relatively long-term control of fungus gnat larvae, and they can be self-reproducing. Steinernema feltiae is more effective against fungus gnats than other commercially available nematode species. Mix Bti or nematodes with water, and apply as a soil drench, or use them as a foliar spray.  Remember to the directions on the label.
  • I read about predatory hunter flies, Coenosia spp. These flies catch and eat adult fungus gnats in mid-air, and prey on fungus gnat larvae in soil while developing as larvae themselves.

Chemical Attack

  • Always read the labels and know what your using and why.  Read and follow all safety precaution on the label, DON’T BE STUPID.    
  • Pyrethrins have a low toxicity to people and pets and are the active ingredients in the botanical pyrethrum, which is made from certain chrysanthemums. A lot of products include a petroleum-derived synergist or enhancer (piperonyl butoxide, or PBO) to increase pyrethrum effectiveness.
  • Pyrethroids (e.g., bifenthrin, permethrin) are made from petroleum to be chemically similar to pyrethrins; usually they’re more effective and last longer but are more toxic to beneficial insects.  
  •  Remember, we want to conserve all natural enemies by avoiding broad-spectrum insecticide applications.


Well I hope this advice makes it easier to win the fight against the fungus gnats.  I don’t have any advice for fighting them in a hydroponic system. I’m a soil guy.

Yellow Sticky Trap.


Grow Learn Teach






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