Steinernema feltiae – Fungus Gnat Killer

Steinernema feltiae
Fungus gnat killed by predatory nematodes, Steinernema feltiae (other critters are springtails)

Back in April I wrote about a ‘fungus gnat invasion‘ in my wooden stacking vermicomposting bin. Well, ever since the first outbreak of gnats I have been trying to stay patient as they’ve proceeded to set up shop is pretty well every indoor bin I have.

My wife has been fairly tolerant of the gnats that have managed to make their way upstairs, but I knew something needed to be done prior to having my mother-in-law come to stay with us for a little while. As mentioned in my creepy pants wrap-up, it’s not that she gets offended by these things – it’s more a matter of trying not to look like a complete slob (difficult enough as it is when you have a vermicomposting operation in your basement!).

As I’ve written before, fungus gnats (Bradysia sp) are unbelievably difficult to get rid of once they’ve become well established in a worm composting system. As bad as fruit flies can be, I actually find them much easier to deal with, since they can be trapped quite easily, and are reliant on certain ‘foods’ to keep them going – once these are exhausted, the population tends to crash. Fungus gnat larvae on the other hand thrive in most vermicomposting systems – especially those that have been active for at least a few months – since they are nice and moist, and offer a wide array of decomposing organic matter to keep them well fed.

In the past I have tested out the predatory mite Hypoaspis miles as a fungus gnat control, but ended up disappointed with the results. The problem with biological controls is that they tend to be somewhat ineffective when used for serious infestations. They tend to be more effective when used as a defense mechanism, or during a light infestation.

Nevertheless, I decided to try out a different biological control this time – predatory nematodes, Steinernema feltiae. I felt a little more optimistic about them simply because each batch contains millions of them, and every time they attack a fungus gnat larvae many more nematodes get produced.

As soon as the nematodes arrived last week I mixed them up with water, and applied them to my indoor bins. Basically they work like little larvae-seeking-missiles – once they track down fungus gnat (or other fly) larvae, they penetrate their body and release a bacterium which does the actual killing. The nematodes then reproduce like crazy and basically burst out of body of the host, before venturing off to find another target.

I knew that even if the nematodes did end up controlling the gnat population, it would likely take some time before I observed an significant results. Still, I’ve been keeping a close eye on the bins to see if I can find any signs of gnat destruction. As picture above demostrates, I have indeed found evidence that the nematodes are working. What’s interesting is that while the nematodes are supposed to attack and kill the gnats while they are in their larval stage, I have been finding a lot of adults that seem to be badly infected, or dead (as in the photo). I suspect that they were invaded shortly before becoming adults and still managed to pupate, only to then die shortly thereafter.

I am still seeing quite a few adults flying around, but the stacking system (which was a serious fungus gnat hub) seems to have far fewer gnats crawling out of it now. I am optimistic that my overall population of gnats in the basement will be greatly reduced within the next couple weeks. I actually still have some of the nematodes (in water) sitting in the fridge – they apparently last for a couple weeks. I will likely apply them again to certain (badly infected) bins, and I also want to conduct some tests.

These nematodes are reportedly effective fruit fly predators as well – something I want to see for myself. I have a nice healthy population of fruit flies in a couple of my outdoor composting bins, so I’m going to lure some of them into small bins containing fruit scraps, then apply the nematodes to one of the bins to see if they have any effect.

As per usual, I will certainly keep everyone posted!

**For Even More Worm Fun, Sign Up for the RWC E-mail List!**
Previous Post

Reader Questions – 07-14-08

Next Post

Harvesting My Vermicompost


    • apple
    • July 8, 2008

    amazing as always, bentley!

    do you have a recommendation as to where to find nematodes?

    • Sherry
    • July 10, 2008

    Hi Bentley!
    Just this morning the gardener on the news program was talking about using nematodes on the lawn to get rid of grubs. He mentioned that they are microscopic and you add water to them, then water the lawn.

    My “sweetie” is VERY tolerant of my worm hobby, and he shrugs off the flies that have emerged from my bin that hve the worms from the farm manure. I know for a fact, that he would not be impressed with a bloom of fungas gnats. I’ve had them in houseplants before and they are a big pest. Having 4 bins in an apartment (!) I am always on the lookout for unwanted pests and not overfeeding to ward off potential problems.
    Like a “apple” I would like to know where you purchased these good bugs in case I have a need for them in the future.

    LOVE your site!

    • Bentley
    • July 13, 2008

    Hi Guys,
    I bought these nematodes from a place called ‘Natural Insect Control’
    The product is referred to as ‘NEMS’:


    • Robert
    • June 16, 2009

    Hello, googled Steinernema feltiae and your page was #1 hit :).

    I’ve had problems with these buggers for a long time, I don’t know if it’s fungus gnats but these little fellows like to put larvae in my flower pots and I have tried every trick in the book…
    Killing them each time I see one…
    Use water with some vinegar and soap (it does attract them but they reproduce faster than they are dying) and change soil – I have not tried putting sand on top, apparently this will suffocate the larvae and make the plants less interesting for the flies to lay their eggs in.
    There was a time I only found one fly in total, they seemed to have died (after I had located what flower had been infected to the max and sprayed it almost to death with green soap (I was desperate))!

    I had once planted a garlic, it grew nicely until they actually made a LARVAE NEST inside the actual garlic! These larvae seem to love garlic, lol. It was a horrible experience seeing all the larvae which looked more like translucent worms with something black in them, crawling around the garlic root and eating it from the inside. Ugh… *shivers*

    But now they are back, more than ever and they rapidly increased when I bought three new plants from a flower shop (eggs or larvae probably came with all of them – you could think they actually try to defeat these buggers in the store but it doesn’t look like it!)

    I ordered a bag of nematodes (Steinernema feltiae) and put it into water and watered almost all plants (one bag wasn’t enough – I probably used more than I should have in each pot).
    The results will be very interesting, I can’t wait to see these f*****s gone o_O.

    • Eric
    • October 21, 2009

    About 10 years ago Arizona Botanical sold me beneficial nematodes for yard ussse. One small package of nematodes seems to have populated my Arkansas yard forever. The first year we had a few less brown beetles in yard and the annual army worm crawl up the front wall of house did not happen. Since the first year, yard has less ants, fewer & smaller insects. Cicadas leave less shells. Yard is quieter day & night.We do not find winged termites in yard.
    We still have northern fence lizards, box turtles, anoles, toads & snakes, so I have no reason to fear the nematodes in yard.

    • James
    • November 10, 2009

    Umm, they do not harm worms? Seeing Eric’s comment… less this less that.. but what about worms?

    • Bentley
    • November 10, 2009

    Hi James,
    The opposite appears to be the case actually – there is at least one scientific study to indicate that composting worms can end up killing off these nematodes over time.
    These nematodes are specialized for attacking insect hosts – not all invertebrates (or other creatures in general)

    • Bryan
    • November 15, 2009

    Great advice here. I just bought some nematodes. I’m wondering if I should go ahead and mix them up in water, or if I should just cut up the little sponge they come on and place that around the bin. I’m worried about adding too much extra liquid (the package directions say to rinse out the sponge in something like a number gallons, which would be ridiculous).

    • Bentley
    • November 20, 2009

    Hey Brian – sorry for the delay getting back to your here. That is a good question. Given my frugal nature (and how expensive I remember the nematodes being – haha) my gut feeling is to recommend doing the water dilution. BUT, if there is absolutely no other place you will be using the nematodes (your garden, other worm bins etc) then perhaps there really isn’t much point in doing so. I think they will keep for a period of time in the fridge, but again if you don’t have any other potential application for them, it will be kind of a waste. At least by adding the sponge to the bin (not sure I would bother to cut it up) you will end up with LOTS of them in there. If you don’t add water – at least make sure to mist the area where they are added. The moisture will help them move around and find their hosts.

    • Bryan
    • November 20, 2009

    Thanks! I ended up diluting them in a fraction of the water called for on the package–maybe 2 cups instead of a gallon or more. I don’t have a garden, so I wanted to use them all in the bin. I soaked some dry newspaper in the nematode mixture and put that in there, and just poured the rest on and left the bin uncovered. So far so good. I don’t know if I’m going to be able to capture photographically evidence of their operations, like you have, but I’m hoping I’ll notice a discernible reduction in the gnat population.

    • Mike
    • January 17, 2010

    I have a serious fungus gnat infestation in my indoor tote bin. At least I think they’re gnats – no red eyes that I can see. I emailed Natural Insect Control (NIC) before x-mas for some info on costs for an order of NEMS (Soil Larva Parasite) shipped to Alberta. Haven’t heard back – do they shutdown over the winter?

    • Bryan
    • January 17, 2010

    Mike–I had a pretty serious infestation as well. I managed to get it partly under control with a combination of constant sweeping up of the gnats (my bin’s in my garage), greater concentration of bedding (apparently they don’t like to burrow), and the nematodes.

    I’ve since discovered that keeping the lid off and keeping the bin in a place with some light keeps the gnats under control. And keeping the bin elevated off the ground, like on a pair of 2x4s. They don’t like the light and they want a pretty damp environment. And they like to collect beneath my bin.

    I’m sure Bentley will have more serious advice, but from one worm farmer to another, I thought I’d share. I also just bought my nematodes at my local garden center.

    • Mike
    • January 17, 2010


    I’m an apartment dweller so they’re starting to get annoying. I was surprised my your comment about them not liking light as I find a couple of dozen of them on the window just above my bin each morning. On the theme of a slight silver lining for this cloud of gnats, the window is an old style single pane that frosts over during cold spells. The gnats end up freezing to the glass and die.

    I’ve tried keeping a good layer of dry bedding on top but it hasn’t helped.

    I haven’t looked locally for nematodes but I’m in a small town. Unless Walmart or Canadian Tire carries them and switches to their spring stock I’m probably looking at buying them off the web.


    • Mike
    • January 18, 2010

    Update – I emailed Natural Insect Control (NIC) again yesterday and got a reply this morning. If their shipping costs aren’t outrageous I’ll order some and report back on how they work in my bin.

    • Bryan
    • January 18, 2010

    Good luck! They’re persistent buggers. Hopefully some good ol’ fashion biological warfare will rectify things.

    • Mike
    • January 18, 2010
    • Bentley
    • January 19, 2010

    Hi Mike – sorry for delay responding. Also sorry to hear about your lack of luck with NIC. I am a lot closer to them myself, so the shipping wasn’t TOO bad, but it was still enough (when added to the expensive pricing) to make me decide that I probably wouldn’t be buying any more parasitic nematodes! I’m surprised there wouldn’t be any suppliers out in BC. You’d think all those…uhhh…growers out there would need these things to help keep fungus gnats under control!

    • Mike
    • January 19, 2010

    Hi Bently

    Actually there is and their prices are about the same. I’m currently waiting on a shipping cost from them.

  1. I never did order from the Bug Factory as their cost was only slightly better @ $51 delivered. With the warmer weather I’ve had the bins outside so gnats are mostly absent from my apartment, at least for now.

    Looking ahead I’m wondering if anyone might have tried NemaGlobe Grub Busters available from Canadian Tire?

    NemaGlobe Grub Busters

    It looks like it’s meant to be applied to lawns (additional sprayer required but probably not necessary for a worm bin) and I have no idea what nematodes are used.

    I have also noticed that the flies\gnats I see often are walking together joined end to end. I always assumed these were just a mating pair of fungus gnats (and squished them gleefully) but then read that they might be Minute Black Scavenger Flies I don’t know if they pose additional problems over fungus gnats but are just as annoying.

  2. Did a little Googling and found that “Grub Busters Nematodes are an efficient strain of Steinernema glaserie for control of major turf and garden pests such as white grubs” (link). I’ve emailed to inquire if their product will work for gnats too.

    • Bentley
    • June 7, 2010

    Hiya Mike,
    I’ll have to check out the “Grub Busters” product. I know Bt strains are specific to different types of insect larvae (ie you can’t just use mosquito dunks to kill off caterpillars etc) – not sure if the same is true of nematodes, or if they simply attack any sort of grub.
    Being able to buy it at Canadian Tire would certainly make things easier!!
    Thanks for sharing that!

    Interesting re: the scavenger flies – they DO look quite similar, don’t they?

    • Bentley
    • June 7, 2010

    Oops – just saw your second comment, Mike
    Keep me posted!


  3. Bently

    Received a reply from and as you guessed the “Grub Busters” nematodes will not work on fungus gnats. They’ll have another product “Leatherjacket Busters” that will be effective against fungus gnats. Can’t find a link on their website and I’ve requested more info.

    Unfortunately for me it looks like their Grub Busters product is only carried by CT store in Ontario and Quebec and it’s likely that their other nematode products will be the same.

    • Bentley
    • June 10, 2010

    Thanks for the info, Mike!
    I will have to see if I can track down the other product you mentioned.

    • Melody Silverberg
    • May 30, 2011

    I believe I have a huge infestation of the minute black scavenger flies. Will the parasitic nematodes kill them as well? I keep my composter inside, and this infestation is getting really out of hand. Otherwise my composter seems very healthy.

    • Wjason777
    • June 10, 2013

    hey Bentley do these nematodes work against BSFL or do you know any that does?

    • Kelly
    • January 19, 2015

    I have a question about the Steinernema feltiae. If I feel like I am being invaded by gnats now will I feel the same about Steinernema feltiae coming into flower pots? Will they crawl outside my pots?

Leave a Reply

Your email address will not be published.

Get Your Free Vermicomposting Guide!

* Join the Red Worm Composting E-Mail List Today *