Getting rid of gnats in houseplants with beneficial nematodes (awesome little critters that eat gnat larvae)

I recently wrote a definitive guide to getting rid of gnats. I thought I might provide you with an update. 

The tactics I tried (specifically, a combination of covering the pots with aluminum foil, dusting the soil with cinnamon, a homemade trap using white wine, and lots of yellow sticky traps worked like a charm. Well, mostly. Every now and then at night a gnat would still fly into my face as if to say "ha ha, you missed me!"

Given that one little fungus gnat can lay up to two to three hundred eggs in its lifetime, it stands to reason that I'll still miss some. And so it was time to bring in the heavy artillery. 

David at Gardener's Supply was nice enough to leave a comment on my post talking about something they sell called Gnat Guard, or "beneficial nematodes" (if Gardener's Supply Company is out of stock, you can buy the same thing from Amazon sellers called Scanmask). I had no idea what a nematode was, but since the word "tode" is in there, for some reason I thought of little frogs. And as annoying as a home overrun with gnats are, I figured it'd be a lot more annoying to be overrun with frogs. 

Well, of course had I not been asleep in biology class all those years ago, I would know that a nematode is a tiny roundworm. Now, roundworms get a bad rap out there because there are some nasty ones out there that get into people's tummies and into dogs' tummies and whatnot. 


But something I never knew was that there are good roundworms too. Specifically, little microscopic roundworms that, if put into soil, get in there and start munching away at insect larvae (including gnat larvae), but do zero harm to the plants or to you, your pets, and your kids. Beneficial nematodes have been used by farmers for years as an effective means of pest control without using chemicals or pesticides. 

(Having said that, my hats off to the marketing folks at Gardener's Supply Company for calling them "beneficial nematodes" and not "parasitic roundworms").

So, since I was still being overrun by these gnats, I wrote a little note to the good folks at Gardener's Supply Company asking if they could send a sample. I got a nice note back from their PR department saying that a sample was on the way, and giving these instructions:

** You may apply GNAT GUARD anytime soil temps are between 35-85 degrees F.
** Ships UPS (2nd Day) only- allow 1-3 weeks for arrival.
** OPEN IMMEDIATELY upon arrival. Product arrives in a mini-cooler with a
   cool pack inside to keep it from overheating. Product itself (sponge)
   is OK if warm, room temp, or cool to the touch. Store in refrigerator
   if you can't apply right away (be sure mini-cooler is open).

Lo and behold, yesterday I get this package in the mail: 

Now what I do remember from biology class is that insects have six legs, and roundworms have none. But I figured Gardener's Supply Company use the same yellow sticker when mailing out things like ladybugs. I opened the box, and thankfully nothing flew into my face.

This is the "cooler", using a miniature version of the ice packs some companies use when sending fresh fruit or perishable food. It was all very neatly packaged, I have never seen a cooler cooler (sorry). 

Inside the cooler was this small zip-lock bag with a sponge inside:

The instructions said this:

Gnat Guard

A biological control to rid houseplants and seedlings of fungus gnats and over 100 other insect pests. Gnat Guard nematodes target insect pupa and larvae living in moist soils in the root zone.

Active Ingredient: Steinernema spp.

Contents: 1 million covers 2,000 sq. ft.

Application: (see back label for release information.) For houseplants: 2 tablespoons of concentrate per 8-inch diameter plant. Apply 1 to 2 times per year. For greenhouse plants: apply 1 to 2 times every 3 months.

Storage: These nematodes are alive and should be released as soon as possible. Inside the package is a moist sponge which contains the nematodes. (You can see them with a magifying glass.) Refrigerate at 40 to 50 degrees F to temporarily store the nematodes for a maximum of 2 weeks. The sponge must remain damp. Add a few drops of water if the sponge appears to be drying out. 

Release: Remove the sponge from the package. Nematodes migrate into the sponge during shipping. Rinse the sponge and sponge package completely into one gallon of water. You now have a concentrate. Constant agitation of the solution while applying will ensure the most consistent coverage. For smaller applications; such as for houseplants and seedlings, you may use a watering can. For larger applications; such as for greenhouse plants, you can apply the nematodes using an injector system or dilute them with more water and use a pump sprayer, hose-end sprayer, watering can or pail. Do not leave the nematodes in standing water for more than 12 hours.

Important Note: The Gnat Guard must be "watered in" to the soil. A rinse of plain water after they have been applied will help ensure they have been carried into the soil. 

And so I following the instructions. I took the blue sponge out of the package. I wasn't tempted to take a magnifying glass to look at the little squirming critters as the package suggested. I figure they'd be happier if they didn't get a good look at me and vice-versa. 


I wasn't sure exactly what it meant by "rinse the sponge and sponge package", but I figured this would do it. I bend the sponge into the spout of a one-gallon cleaned-out milk container and put it under the sink's tap. 

Once the jug was almost full, I put the sponge in and gently shook it. Presumably, millions of little wormies were now swimming around in the water.


The next step was pouring it into my big plant pots. As you may recall, one of the tactics I used was to cover the top of the planter with foil like this. 

This was pretty effective as far as keeping the gnats away, but one unintended consequence was that when I took the cover off, there were at least 2-3 other kinds of creepy crawling things that decided they liked the dark, moist environment. I suppose they'd been sitting in the soil the whole time. Hopefully the little fellows can chomp up their progeny as well as the fungus gnat babies. 

Anyway, I poured the nematode concentrate into each pot I could, and then followed it up with more water. When I looked closely at the soil, it sort of "glistened", and I knew my little friends were at work. Now, I thought I might be a little grossed out by the thought of those little things in my house, but at the end of the day I've grown quite fond of them. They're like thousands of little friends who are helping clean my house. And they're safe to have around you, your kids, and your pets. 

It's been a few days now, and not one gnat has flown into my face. And so as thanks to Gardener's Supply Company I will happily endorse this product.