Fungus Gnats are driving me batty

The Gnats have returned to my houseplants. I don't know where they came from, but one day there was one gnat, the next twenty, and now they are everywhere. I truly hate these guys.

I know, I know, the nursery says that these little pesky flies don't harm any plants. This is strictly speaking true. The flies don't eat a thing. However, their immature wormthings eat the roots and stunt my seedling growth. If the infestation gets really bad, the larvae eat the center out of the seedling stem and then, before my eye, the whole plant falls over and dies.

What organic solutions do I have to rid my home of this infestation?

Things I've tried:

watering less often - I already water when the soil is dry to the touch, but now I'm waiting till the plant seriously wilts. I think this is not a long term solution as it's stressing out my plants.

Diatomaceous earth and borax - still waiting on results

Ugly yellow sticky things - only reduces the adult population

watering with used dish water: (real) soap plus grease = my great grandmother's insecticide

What else can I try? Must be petroleum and soy free solutions.

Your problem is houseplants -- plants don't belong in the house in dead dirt, they want to be outside in living soil. The fungus gnats are part of the living soil that the poor house plant is trying to re-create.

If you watered less often, then yes, that is conducive to less soil fungi and hence, less for fungus gnats to eat.

But rather than water less to have almost dead and lifeless soil, why not try to balance the soil? Get more life into the soil, not less. Add some worms and nematodes and some soil carnivores that will eat the overabundance of fungus gnats.

When you start out with a sterile potting mix and then add seeds, you are creating the perfect place for herbivores to colonize. Herbivores who will eat your seedlings, but who in turn have no one eating them. Until your potting mix has a balance of carnivores eating the herbivores, you are going to be losing a good deal of your herbage to the herbivores.

If only all questions were as easy to answer as this one:

Get a butterwort, put it on the windowsill close to the window glass. If you like the flowers, get a few more.

Good points John about the problems with house plants. It's very difficult to create a balanced system in such artificial conditions.

However, we have this thing called winter which means some of my younger, more tender plants need to come inside if they will survive to be large enough to plant out later in life. Also, it greatly increases my food production and reduce the input (aka, water, soil nutrition) I need to give it, if I can start some of the plants inside so that they can be hardy enough to plant out at the end of the rainy season. If I can plant them early enough, they grow their roots deep enough to survive the drought on their own rather than depending on irrigation for the entire summer. However the rainy season is still quite cold, so the seeds wouldn't germinate outside.

I usually use a mixture of 'wild' dirt and old (organic) potting soil I found in the back of the shed. I like giving the plants some of the microbes, fungus, &c from the live soil, to help the plants grow stronger, plus a handful of manure in the bottom of each pot so they have something to 'eat' when their roots grow. Thinking back on it, it wasn't until I started mixing in compost from my worm bin that these gnats appeared.

In the past I tried the nematodes. They caused an explosion in the gnat population and made the humans itchy if they touched the plants/soil. I don't know why this was. Perhaps it was a bad batch of nematodes or perhaps the watering cycle needed for the nematodes to thrive is also the watering cycle that the gnats thrive with. Either way, I don't have the funds to try again this year.

Are there wild sources of predatory soil dwellers? Could they survive in such small pots of soil?

Would earthworms actually eat gnats? I thought they dwelled in a darker part of the soil and ate only dead things.


What about a betta fish? I seem to remember in the past that the gnats are attracted to water, and my betta fish ate any bugs that were stupid enough to come near his home. He even use to jump slightly to catch a hovering house fly. Alas I don't have a fish anymore, but maybe someone here does and they could try an experiment? I think it would be difficult to get the fish tank near the plants without being in direct sun. Then again, that would just be control for the adult population.

Michael Radelut wrote:If only all questions were as easy to answer as this one:

Get a butterwort, put it on the windowsill close to the window glass. If you like the flowers, get a few more.

Those look cool. Funding is a bit of an issue right now, but I'll check them out next time I'm at the nursery.

I would hate to get rid of the houseplants, especially the ones that do well in a dark corner near a window. We've got a delphinium that is in a big pot, and I put small redwood bark chips over the top of the pot, and there are no fungus gnats in it. But summer and fall are big gnat times of the year here.

You might want to do a really vigilant check with a flashlight to make sure they aren't actually somewhere else that you don't expect. I've stopped storing fruit outside of the refrigerator. Just bring it back to room temp before you eat it. I don't keep kitchen scraps in an indoor composting container. I've found them in coffee grounds and tea bags that I save for composting. I've found them in potatoes stored in a cupboard. I've found them in some kinds of dog and cat food that seem to have more natural ingredients in them. I've found them in the dregs of milk or tea/milk or coffee/milk in the bottom of a cup on the counter. I even found them on the bottom of a sink stopper that had bits of food on it after doing dishes, so that gets scrubbed all the time. I use the stopper to catch the bits to keep them from going down the drain, and some slip underneath.

If something gets shoved to the back of a shelf and is rotting, something has fallen between the oven and the cupboard, or bits that have fallen off the cutting board and gone between cupboards.

And if you see a few in an area, don't touch the area, just go get the vacuum and go after them. They are easy to catch, and you might be surprised how many more come out when you only see one or two. Well, you probably know that!

I've got two kinds of gnats, the small ones that only like fruit, and bigger ones that can't resist dairy.

Gant Catcher: You can make a gnat catcher with a small plastic container, covered with plastic wrap held on by a rubber band, that has a few small slits cut in the top and about 1/2 inch of vinegar in it. They go through the slits to get the vinegar and get trapped.

The bigger ones will come for milk in the container with the plastic wrap, and if you stir in a little yogurt first, it won't smell bad, but it does need to be changed more often.

These catchers give you notice of how many are around even when you think they aren't.

Ah, good ole fungus gnats. If it weren't for their attraction to CO2, and thus there love of hovering in your face, they'd almost be bearable.

I made the mistake of top dressing some of my house plants with worm castings and got a major infestation of them.

Pour some hydrogen peroxide in the soil. It'll kill the larvae VERY effectively and won't harm the plant as it breaks down into hydrogen and water quickly. The adults have a short life span, so a couple of waterings should eliminate your problem.

I have some house plants that go outside in the good season and and brought back inside when it gets cold. This tends to bring in gnats from time to time.

Something that worked well for me is a combination of topdressing my suspected gnat-infested pots with vermiculite to prevent egg laying/keep the uppermost soil dryer and bottom watering the plants via trays.