How to Use Beneficial Nematodes

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Another day, another way of defending ourselves from the onslaught of fungus gnats.

I’m pretty lucky in the whole fungus gnat department.

There was a time last spring when I couldn’t eat ANYTHING without one of the little buggers trying to get into my mouth (what part of vegan don’t they understand??).

Don’t even get me started on wine. Fungus gnats are crazy big drinkers.

But recently, they’ve been quiet.

Either their New Year’s resolution was to keep to themselves a bit more, or my almost exclusively bottom-watering regime has paid off.

I’m leaning towards the latter. If you want to read more about the benefit of bottom watering I have an article all about it here.

The problem with top watering is that the top of the soil gets wet, which attracts fungus gnats. Gnats aren’t too much of a problem (unless you have a full-on infestation) but they’re extremely irritating.

Beneficial nematodes have been used to get rid of pests for yonks. I first heard about them when I watched a Betsy Begonia video, and thought I’d write a post based on my research, to provide a guide for those of you with a gnat problem.

What are beneficial nematodes?

Is everybody ready for a really long and gross word? Great. Beneficial nematodes used in plant care are two species of entomopathogenic nematodes commonly known as heterorhabditis and steinernema. Steinernema feltiae are the ones I see being sold most often.

As well as fungus gnats, they’re used to control ants, fleas, moths, beetles, and weevils.

Imagine tiny weeny white worms. That’s pretty much what they look like.

How do beneficial nematodes reduce fungus gnat populations?

Prepare yourselves, because this bit’s gross.

The beneficial nematodes enter the fungus gnat through any available orifice (or failing that, right through the side of their body) and poison it, by releasing bacteria from their gut.

At first, this may seem mean and disturbing (and it is), but the beneficial nematodes aren’t just doing this for a laugh. The purpose of killing the host is to provide the nematodes with something to feed on.

How long does it take for beneficial nematodes to kill fungus gnats?

It takes 24-48 hours for the nematode bacteria to cause the host fatal blood poisoning. Pretty speedy then. However, it can take a couple of weeks for your gnat problem to be under control.

The length of time it takes for the nematodes to kill pests varies due to factors such as the size of the pest, the size of the infestation, and the environment they’re in. Nematodes are sensitive to ultraviolet light and extreme temperatures.

It’s recommended that two doses of beneficial nematodes are given, a week or so apart, to make sure all gnats are eradicated.

Pros and cons of using beneficial nematodes to get rid of fungus gnats


  • Beneficial nematodes don’t pose any risk to humans, pets and plants. They’re predators that only prey on other bugs.
  • Beneficial nematodes are naturally occurring, although they have been bred commercially. They’re a pretty holistic and non-toxic method, as terrifying as their methods are.
  • Fungus gnats can’t build up a resistance to them, just like penguins can’t develop a resistance to leopard seals
  • Beneficial nematodes don’t pose a threat to beneficial insects such as ladybirds, earthworms, and bees. How do they know?
  • They can last for up to 18 months in the soil. Pretty impressive, although it’s unlikely that you’ll have enough pests to sustain them for that long. If you have a vegetable garden that’s prone to pests though, beneficial nematodes would be a cost-effective, non-toxic solution.

Disadvantages of using beneficial nematodes

Whilst there are many disadvantages to using beneficial nematodes outside, such as:

  • You need to time application right so that the pests are in the right stage of their lifecycle
  • They need to be kept above 54F/12C
  • They can be difficult to store.

These don’t really affect those of us using beneficial nematodes specifically on house plants, where the pretty constant environmental conditions provide an artificial environment for pests, so there will probably always be larvae in the soil.

The storage issue is probably more for people with massive gardens that need to bulk buy. We can just keep our little packet in the fridge.

The only issue I can really find with using beneficial nematodes to get rid of fungus gnats is that they die in dry soil, so keep your cacti and succulents away from your other plants so that they don’t get re-infested once they dry out.

Since fungus gnats also don’t like dry soil that shouldn’t really be a problem after the initial application.

How to use beneficial nematodes on house plants

Your packet will come with instructions, but it’s…kinda like applying fertiliser. The nematodes usually come on a little sponge. You run water through the sponge and into a watering vessel. Then water your plants with the nematodes, then again with clean water to really work them into the soil.

I’d have thought they’d wash out, but I am not a nematode expert. Just follow the instructions.

Where can I get beneficial nematodes?

Your local garden centre probably, but Amazon sells them too.

You buy them by the MILLION. How creepy is that??

Would I use beneficial nematodes?

100%. I used to be super weirded out by the thought of intentionally releasing thousands of bugs into my home, but it’s preferable to releasing toxic chemicals.

I do use predatory mites to keep other pests, such as thrips and spider mites, at bay.

Caroline Cocker

Caroline is the founder and writer (and plant keeper) of Planet Houseplant

6 thoughts on “How to Use Beneficial Nematodes”

  1. Fun read
    However after initially stating that you’ve solved your gnat problem by bottom watering as the gnats favour wet soil, you later stated “Since fungus gnats also don’t like moist soil … ”
    Typo perhaps?

  2. You’ve got some great stuff Caroline, very helpful and good to read You should send a video to gardener’s world 🙂

  3. I’ve used diatomaceous earth and it didn’t work as well as I’d hope. I’ve done a lot of research on beneficial nematodes and that seems to be the winner. I applied my first round this evening. I’m praying I get better results that I did with the DE.

  4. Let me know how it goes! Diatomaceous earth needs to be dry to work, so it’s more of a faff than beneficial nematodes. Good luck!

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