Nothing ruins the joy of gardening faster than seeing a swarm settling around your plants or realizing these persistent pests have taken over your outdoor yard.
Dealing with a fungus gnat infestation can be incredibly frustrating for any gardener. Many people first reach for common kitchen remedies like baking soda, cinnamon, vinegar, or hydrogen peroxide.
And others prefer organic options like neem oil or BTI, there is another powerful biological ally that expert gardeners swear by: beneficial nematodes.
If you are tired of trying different plants that repel gnats and want a more aggressive approach to stop the cycle, you might wonder if these microscopic hunters are the answer.
So, do nematodes really kill fungus gnats? The short answer is yes, beneficial nematodes (specifically Steinernema feltiae) are highly effective at killing fungus gnat larvae in the soil, which are harmful to plants, especially when the plant itself has sensitive roots.
These microscopic worms act as natural hunters in your potting mix. Once applied, they seek out gnat larvae, enter their bodies, and release specialized bacteria that kill the pests from the inside, often eliminating them within just 24 to 48 hours.
While they are incredibly fast at clearing out the “nursery” in the soil, they don’t affect flying adults, so it’s best to use them alongside other methods to fully clear the air.
How Do Nematodes Kill Fungus Gnats?
According to entomological research from the University of Connecticut (UConn) IPM Program, beneficial nematodes operate as high-speed biological hunters.
Their effectiveness depends on which stage of the gnat’s life cycle they are targeting.
The Larval Stage (The Killing Zone)
In the soil, nematodes use advanced biological cues like CO₂ emissions and root secretions to track down fungus gnat larvae.
Based on data from UConn’s Integrated Pest Management, here is how the process works:
1) The nematodes enter the larva through natural openings or by penetrating the body wall.
2) Once inside, they release a specific symbiotic bacterium that poisons the insect’s “blood” (septicemia). This typically kills the gnat larva within 1 to 2 days.
3) The nematodes then use the dead larva as a breeding ground, multiplying and eventually sending out a new army of “infective juveniles” to hunt more larvae in your soil.
The Adult Stage (The Limitation)
It is important to understand that nematodes are soil-dwelling organisms. As noted by experts, nematodes do not kill adult fungus gnats.
Since adults are flying insects that live above the soil surface, they never come into contact with the nematodes.
However, by aggressively destroying the larvae and pupae, nematodes prevent the next generation of adults from ever emerging.
To manage the current flying population, UC IPM suggests combining nematodes with tools like yellow sticky traps or netting to achieve complete control.
What Kind of Nematodes are Best to Fight Fungus Gnats?
According to research from the UConn IPM and Colorado State University Extension, the specific species you need is Steinernema feltiae.
This species is the “clear champion” for fungus gnat control because it is naturally adapted to hunt small fly larvae in potting mixes and greenhouse environments.
Unlike other species, S. feltiae remains active in cooler soils down to about 50°F, making it reliable for indoor plants and early-season gardening.
It is biologically programmed to seek out sciarid (fungus gnat) larvae specifically.
Other Species to Watch Out For
While you may see other nematodes for sale, they are often less effective for this specific problem:
Usually geared toward surface pests like caterpillars and fleas, not soil-dwelling flies. These are better suited for larger pests like grubs and weevils.
You might encounter “multi-species” or “3-way” blends. These are fine to use as long as Steinernema feltiae is one of the species listed on the label.
As UC IPM experts point out, ensure this specific species is present to guarantee you are actually targeting the fungus gnat larvae effectively.
How to Use Nematodes to Kill Fungus Gnats?
Start by purchasing a high-quality Steinernema feltiae product specifically labeled for fungus gnat control (such as NemAttack™ or Scanmask™). Because these are living creatures:
- Refrigerate immediately: Store unopened packages at 40 or 45°F.
- Use quickly: Apply them within 2 or 4 weeks of purchase and never leave them in a hot car or direct sunlight.
Prepare the Mixture
Mix the nematodes into clean, lukewarm water (50–75°F) according to the label instructions. Most products require roughly 1 or 2 million nematodes per gallon of water.
Here’s a pro tip→ Use gentle agitation to keep them dispersed and avoid highly chlorinated water, which can be lethal to the nematodes.
Apply to Moist Soil
Nematodes need a film of water to move. Lightly water your plants so the soil is moist but not soaking wet before you begin.
- The Drench: Pour or spray the mixture evenly over the soil surface so it soaks in 1–2 inches deep.
- Equipment Check: If using a garden sprayer or watering can, remove any fine filters or screens to prevent the microscopic worms from getting stuck.
Time it Right
Timing is everything for survival:
- Avoid the Sun: Apply in the early morning, evening, or on overcast days, as UV rays can kill nematodes.
- Temperature: Ensure the soil temperature is between 50°F and 90°F. Avoid application if the soil is bone-dry or excessively hot (above 95°F).
Post-Application Care
Keep the soil consistently moist for at least 3 or 4 days after application. This allows the nematodes to swim through the soil pores to hunt down the larvae. If the soil surface dries out completely, the nematodes will die before they can do their job.
Repeat the Process
For heavy infestations, a second application is often recommended 7 or 10 days later. This “staggered” approach ensures you catch any larvae that were still in the egg stage during the first treatment.
Limitations and Cuations
- Nematodes only attack the gnat larvae living in the soil. They do not kill the flying adults or the eggs, so you will still need sticky traps to catch the ones flying around.
- These are living creatures that “swim” through the soil. If the soil dries out, the nematodes will die. You must keep the soil damp for several days after applying them.
- Direct sunlight (UV rays) and very hot soil (above 95°F) can kill nematodes. It is best to apply them in the evening, morning, or on a cloudy day.
- Some strong pesticides or bug sprays can kill your beneficial nematodes. If you have recently treated your soil with chemicals, check the label before adding nematodes.
- Because they are alive, they have a very short shelf life. You must keep them in the fridge, never freeze them, and use the whole pack as soon as you open it.
- For a big gnat problem, one treatment might not be enough. You should plan to apply them again after 7 to 10 days to catch any new larvae that have hatched.
- The good news is that these nematodes are harmless to people, pets, and your plants. They only focus on tiny soil-dwelling bugs.
How Long Do Nematodes Take to Work on Fungus Gnats?
The (UConn) IPM research also shows that these tiny hunters work fast, infecting and killing gnat larvae in the soil within just 1 to 2 days.
However, because the gnats flying around today have already hatched, you won’t see them disappear instantly; experts at Arbico Organics explain that it usually takes about 1 to 2 weeks to see a big drop in the number of adult flies.
To get the best results, it is a good idea to do a second treatment 7 to 10 days later to catch any new “babies” that hatch from eggs.
Once they are settled in warm, damp soil, these nematodes can keep working and reproducing for up to 14 days, giving your plants long-lasting protection as confirmed by UC IPM.
