Does Neem Oil Kill Fungus Gnats? (A Natural Solution)

Summertime at last! For many of us, this is a favorite season, filled with barbecues and pool parties, and pleasant afternoons spent in the garden. After holing up indoors for so long during the inclement winter season and rainy early spring, you may be anticipating making the most of your outdoor space.

And then the gnats arrive. There is little more frustrating than a horde of tiny insects determinedly buzzing around your head, getting in your food and drink, and even occasionally, flying into your eyes or mouth. What many people find surprising is that these pesky critters may also be damaging your plants.

If your potted plants are suddenly wilting, have yellowed leaves, stunted growth, or are generally looking unhealthy, it could be a problem with fungus gnats. This can occur in garden beds, also, but is much more common in potted plants.

Fungus gnats resemble fruit flies, but they are uniquely different. Their bodies are longer, like mini mosquitos, as opposed to fruit flies, which have a round body. And while fruit flies can be a pesky problem in your house, swarming around garbage cans and food or drink, fungus gnats can potentially kill your plants, if left untreated.

It’s not the gnat themselves that do the damage – it’s the larvae. Fungus gnats lay eggs in moist soil, and within 4 – 6 days, the larvae emerge and burrow down further, in search of fungus to eat. The larvae resemble tiny clear or pale white worms with shiny black heads, starting out miniscule, but eventually measuring up to 1/4 inch long.

Once they run out of fungal matter in the soil to eat, they start munching on the roots of your plants.

A single mature fungus gnat can lay over 200 eggs in its short life span of about one week. If you think about each one of those eggs developing into larvae in 4 – 6 days, eating their way through the soil for 2 weeks before a quick rest as pupae, then emerging as adult gnats 3 – 4 days later and laying 200+ more eggs… you can see why this might become a big problem, rather quickly.

The problem with fungus gnats is not just that the larvae may chew on your plant’s roots. Fungus gnats are also notorious for spreading fungal disease.

When fungus gnats lay eggs on the soil’s surface, they often transfer fungus spores – picked up from potentially unhealthy plants – to the larvae. The larvae may then carry the fungal disease beneath the surface, infecting your plant at the root level.

Some diseases that fungus gnats may spread are:

  • Black root rot
  • Pythium blight
  • Verticillium wilt
  • Botrytis blight
  • Fusarium wilt

Fortunately, there are effective and natural measures you can take to eradicate fungus gnats, including the use of one of my favorite organic pesticides: neem oil.

Does Neem Oil Kill Fungus Gnats?

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Neem oil is an excellent choice for managing a fungus gnat problem. Made from the pressed seeds of the neem tree, neem oil is 100% organic, safe to use around people and pets, and will not harm beneficial insects or birds, if used according to the label directions.

It has been shown to be an effective pesticide for over 170 types of insects, including gnats, spider mites, white flies, thrips, aphids, and Japanese beetles, to name just a few.

Neem oil’s active ingredient, azadirachtin, disrupts an insect’s biological functions when they consume it, rendering them unable to eat, breed, or lay eggs. The oil also works on contact by smothering small, soft bodied insects, like gnats, by clogging their airways with oil.

Does Neem Oil Kill Fungus Gnat Larvae?

When used as a soil drench, repeat applications of neem oil solution can kill fungus gnat larvae. The solution must penetrate the soil deeply enough to saturate the organic matter on which the larvae feed.

Does Neem Oil Prevent Fungus Gnats?

Neem oil has a strong, garlicky/sulfuric odor and taste which many insects find unappealing. There is anecdotal evidence that regular applications of neem oil solution will help repel fungus gnats.

The best preventative measure you can take, however, is to ensure that you are not overwatering your plants. Overwatering contributes to the development of fungus (which may not be readily visible as much of it occurs beneath the soil surface), and fungus gnats are there to eat the fungus. Getting rid of their food source goes a long way toward preventing the problem.

If possible, depending on the plant variety, it’s best to let the soil of your potted plants dry out a bit between waterings. Test the first inch or two of the soil before you reach for the watering can.

How to Mix Neem Oil for Fungus Gnats?

To use neem oil as a soil drench, be sure you’re starting with 100% pure, cold-pressed neem oil. The cold-pressed process is important, as heat processing can denature azadirachtin, making it less effective.

Only mix up enough solution for your immediate needs, as the azadirachtin will deteriorate quickly once mixed with other components.

Combine 1 gallon of lukewarm water with 2 Tablespoons of neem oil and add 1 teaspoon of liquid dish soap (the soap acts as an emulsifier, enabling the oil and water to mix more thoroughly. Shake well.

How to Apply Neem Oil to Kill Fungus Gnats?

Transfer mixture to a spray bottle and spray directly on the soil, wetting it thoroughly. There’s no need to spray the leaves of your plants as fungus gnats congregate on the soil surface. By using a spray application before drenching the soil, the mist might catch some of the gnats before they fly away.

Apply as a soil drench to kill the larvae by pouring 2 – 3 cups of the solution at the base of your plant. Adjust the amount depending on the size of the pot.

How Often to Spray Neem Oil for Fungus Gnats?

Repeat spray applications on the soil’s surface every 7 days. If your plants are outdoors, and it rains heavily, you can repeat the spray more frequently.

Once every 2 – 3 weeks, repeat the soil drench method. Azadirachtin stays active in the soil for up to 21 days.

How Long Does Neem Oil Take to Kill Fungus Gnats?

When using the two methods above simultaneously, you may see an absence of fungus gnats within a few weeks, but it’s important to keep up the applications for 9 weeks, to fully ensure you’ve killed all the larvae and broken the cycle of reproduction.

Neem Oil vs Hydrogen Peroxide for Fungus Gnats

Although hydrogen peroxide soaks have  been touted as an effective method for getting rid of fungus gnats – using a 1:4 diluted ratio – personally, I don’t recommend this for the following reason.

It will sterilize everything.

Healthy soil is alive with all sorts of micro-organisms and beneficial bacteria, which help give your plants what they need to grow. Hydrogen peroxide soaks will kill the good stuff as well as the bad, leaving you with “dead” soil.

If you use this method to treat your fungus gnat problem, you really need to add worm castings or compost afterward to replenish the soil, or you may notice an absence of new leaf growth.

Are There Any More Alternative Ways to Get Rid of Fungus Gnats?

Some alternative methods for combating fungus gnats on your plants are:

  • Sticky traps – there are plenty of sticky traps on the market for both indoor and outdoor use, and they’re extremely affordable. Many have picks to stab into the soil, or alternatively, hang nearby. Be aware, if you use sticky traps in your outside space, the traps may catch insects other than gnats.
  • Top dress the soil – A 1/4” thick top dressing over the soil will help to prevent gnats from laying their eggs. Try using sand, pebbles, or pumice.
  • Nematode soaks – You can purchase microscopic beneficial insects that specifically dine on the gnat larvae. Some varieties come in pouches that you add water to and drench the soil with the mixture. The nematodes released into the soil will find and eat the larvae without harming your plants.
  • Carnivorous plants – Certain types of carnivorous plants called flypaper plants can get your indoor fungus gnat problem under control. Flypaper plants are covered in a natural, sticky substance that will trap any insect who lands on their leaves. This is best for indoor use only. Look for Drosera, Pinguicula, and Drosophyllum plants in your local nursery. As a bonus, many carnivorous plants have a very unique and beautiful appearance.
  • Strategic landscaping – For outdoor fungus gnat prevention, try adding some strongly scented plant varieties to your space, such as citronella, lavender, geraniums, lemon thyme, and Mexican marigolds. It’s reported that gnats can’t stand the smell of any of the above.

Check out our related Neem oil article for more information:

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