How to Treat and Kill Fungus Gnat Larvae

One of the most frustrating pests in many indoor gardens is the dreaded fungus gnat. These little buggers can absolutely destroy your garden if you're not vigilant — and they can do it quickly.

How Fungus Gnats Destroy Plants​

The damage done to a new root system by fungus gnats.

The damage done to a new root system by fungus gnats. source

The primary way that fungus gnats affect your garden is through their larvae. They will lay eggs in your growing medium. Once they hatch, the larvae will attach to the roots of your plants and drain them of nutrients. 

Although the larvae are the main negative actors, the adult fungus gnats — small flies that buzz around the bottom of your plants' stems — can carry disease and lay tons of eggs.​ The small adult flies can be easy to miss...but missing them can spell ruin for your garden.

It's not enough to simply inspect your root zone and growing media if you see adult fungus gnats buzzing around. Their eggs and larvae are so small that even close up they're difficult to spot. The only thing you will notice are off-white roots that look damaged.​

The larvae attack plant roots in a variety of ways:

  • Destroying root tissue and disruption water and nutrient uptake
  • Infecting plants with fungus disease that they carry on them

You may have wondered why these pests were called 'fungus gnats'...well, in addition to destroying plant roots, they also carry fungal spores on their body. This wouldn't be so bad, except for that they also damage plant roots, making them more susceptible to infection.

When Fungus Gnats Strike

You're most likely to get fungus gnat problems around autumn time. As the weather cools, they seek out warmer the nice, warm temps of your indoor garden. Once they get into your garden and start laying eggs, they can remain a constant issues for many months unless treated.

If you have plant problems and suspect it might be the work of fungus gnats, be on the lookout for:

  • Yellow leaves that otherwise look normal
  • Extremely slow growth

If you spot either of these symptoms, or the adult flies buzzing around your garden, you need to react immediately and treat your entire crop — not just the afflicted plants. It's very hard to know if larvae are on plants that look fine, so you should cover your bases and treat everything.

Plants recovering from fungus gnat problems still face the risk of disease problems - remember that these bugs can spread fungus spores to damaged roots. As a precaution, these plants should be given a treatment with a fungicide a day or two after pesticide application.

A root drench is more effective than spraying the top growth. Follow a similar procedure to your use of pesticides, drenching the entire root zone with fungicide solution, with irrigation pumps off for at least several hours. Left over fungicide in the root zone will not interfere with nutrients, so it's not necessary to drain and replace the fertilizer mix.​

sticky traps for fungus gnats

An ounce of prevention is worth a pound of cure.

Preventing Fungus Gnats

Catching fungus gnat infestations early is key to not losing your entire garden. The best way to do this is to use sticky cards that are either bright yellow or orange (the flies are naturally attracted to those colors). 

Place these in different areas of your garden and keep a vigilant eye. As soon as you see fungus gnats getting stuck to the paper, you need to move into treatment phase as soon as possible.​

Treating Fungus Gnats

You might be tempted to use a strong pesticide manufactured for outdoor use and blast your growing medium and root zone with it to destroy fungus gnat larvae and eggs. Don't do this, as this is overkill for an indoor garden and is likely to do just as much damage to your plants as it is to the fungus gnats.

There are a few recommended products for killing fungus gnats and their larvae:​

  • Food-grade Hydrogen Peroxide (H2O2) will kill larvae on contact. Mix it in a 4:1 ratio and drench the root zone, making sure to cover all growing media and bottom of the stem. Repeat if you continue to see adult gnats appearing on your sticky traps.
  • Beneficial Nematodes will attack fungus gnat larvae as well. These round worms will break through the larvae and infect them with a bacteria that destroys them completely. If you use these, you'll also get rid of many other annoying garden bugs (over 230 different types of pests).
  • You can use Neem oil as a drench to kill off the larvae.

Whichever treatment you choose, be sure to monitor very carefully for a few weeks. The lifespan of this pest is about a month long, meaning you may need to do repeat treatments for at least that long to be sure you've killed all of the eggs and larvae as they're continually laid.

Fungus gnat larvae and the adult flies can destroy your garden. Learn how to treat and prevent fungus gnats in a few simple steps.
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