What I Learned From a Surprise Fungus Gnat Infestation in My Houseplants

When a friend gifted me some houseplants, I had no idea there were stowaways in the soil until I ended up with a full-blown fungus gnat infestation. Here’s everything I learned about how to get rid of fungus gnats in houseplants!

They came out of nowhere!

When a friend moved to the city, she arrived with a large collection of Pelargoniums – so large, in fact, that they would not all fit into her new apartment. As my place is bigger, she gave over the care of her flowers to me which, as it turns out, came with quite a large population of fungus gnats.

I was completely unaware of the imposters until about a week later when my house was suddenly filled with tiny flies.

I tracked the source of the bugs to my windowsill and – looking closer still – saw many more resting on the leaves of the Pelargoniums.

How had this happened?

A quick Google search later and I had identified the insects as fungus gnats, a harmless but irritating species of houseplant pest. The mystery behind their sudden appearance was also revealed: fungus gnats spend most of their lives in their egg, larval and pupal forms, all of which take place in the soil. When I brought the houseplants home, there hadn’t been any adults on them but there must have been larvae lurking in the soil.

Over the next week, my well-watered plants were to host the final stage of the fungus-gnat life cycle, when dozens of adult bugs emerged to mate and create a new generation of nuisance flies.

How common are fungus gnats, and what do they do to houseplants?

Fungus gnats are a common houseplant and greenhouse pest, and they thrive in warm, moist conditions. The tiny, winged adults can be very annoying, but they don’t bite people or damage plants.

Fungus gnat larvae, on the other hand, can cause serious damage to plant roots if they are present in large numbers. They are especially harmful to young plants, as they can easily chew through the tender roots, causing stunted plant growth and even early death.

If I was going to save my Pelargoniums, it was clear I had to act quickly to exterminate each and every stage of the fungus gnat life cycle.

How I dealt with my fungus gnat infestation

Step 1: Research and identification

Step 1 to tackling any infestation is to find out what it is you’re facing!

A quick Google search gave me all the information I needed to identify and exterminate my enemy. With the guidance of the internet, I learned to spot the following telltale signs of fungus gnats:

  • Small flies: Fungus gnats look like small flies. They are about 1/16 to 1/8 inches long and have a distinctive, Y-shaped vein on their wings. I found them all over my house, though most of them were buzzing around the windowsill where I keep my Pelargoniums.

picture of fungus gnats stuck to a trap close-up

  • Slime trails: Fungus gnat larvae can leave slime trails on the soil around houseplants if they are present in large numbers.
  • Fungus gnat larvae: These tiny wormlike bugs have a black, shiny head and long, whiteish bodies, and can be found wriggling around in the soil of your houseplants.

Step 2: Killing the adult gnats

Now I knew what I was up against, I could begin my war on the flies. I returned to my laptop to find out the fundamentals of fungus gnat control.

It turned out that all I needed to eradicate the bugs was a simple, twofold plan.

First, I had to kill the adult flies, a task easily accomplished with the help of a few yellow sticky traps.

picture of fungus gnat traps

These inexpensive flycatchers lure in the adult fungus gnats (who are supposedly fond of the color yellow) and trap them before they can lay more eggs. This breaks the lifecycle and stops the infestation from growing, making it a lot easier to quickly kill the remaining larvae.

Step 3: Soil maintenance

With my house a fly-free zone once more, I moved on to implement the final phase of my plan: soil maintenance. Another thing I’ve learned about fungus gnats is that they love excessively moist conditions and, by enthusiastically over-watering my new houseplants, I’d unwittingly created the perfect breeding ground.

I began to water to Pelargoniums much less frequently, allowing the top two inches of the soil to dry out. This not only kills the larvae lurking in the top layer of the soil but also makes it far less appealing for adult gnats looking for somewhere to lay their eggs.

Step 4: The waiting game

With my anti-fungus gnat measures in place, all I could do was sit back and wait for the larvae to die and the adult flies to fall victim to the sticky paper.

Stopping my fungus gnats from returning

Despite being quite large, my fungus gnat infestation was surprisingly easy to get rid of. It didn’t take long for the flies to disappear. In fact, most of them seemed to get stuck on the sticky traps within the first few days.

sticky trap with fungus gnats on it

It’s been a while since I last saw the bugs, but I have my preventative control plan to thank for that!

Here’s how I’m making sure the fungus gnats never return to my Pelargoniums:

Monitoring

The sticky traps are still in place and are my first line of defense against fungus gnats and other flying houseplant pests. I’ve also learned a valuable lesson about not bringing new plants indoors straight away. All new additions to my indoor garden are now subject to a one-week quarantine on the balcony, during which I inspect them carefully for any signs of passengers.

Maintaining the right soil conditions

I’m always careful not to overwater my houseplants, as excessively moist soil can attract all kinds of bugs. Fungus gnat larvae also feed on decaying organic matter, so I make sure to remove all dead leaves and other debris quickly, so they have nothing to eat.

In case of emergencies: break out the insecticides

It was pretty easy for me to get rid of my fungus gnats, and I didn’t have to use any insecticidal sprays and solutions. However, in cases of very severe infestation, there are insecticides available that can be used to treat the soil of houseplants and will get rid of the larvae much more quickly.

3 Comments

Mark

Hi IC, Great blog. I suddenly have infestation of an extremely small flying bug (Under 1 mm). It stings my feet, both hard and in a softer way. It loves going into my short hair and moving around in it. This going on for 5 weeks. (Yes, amy losing mind!) I remembered my garbage disposal was not working when I rented house 5 years ago. So I never used it. I just removed it and discovered absolute putrid green and black globs in its pipe to sink drain pipe, then lots more ilk in the drain pipe from main sink. So removed all pipes and cleaned out very well. Phewie. Almost immediately problem seems to be subsiding. My Question is, what kind or what is that bug? It stings, only sometimes leaving a small blood mark, but mostly not. It goes through head hair. I live in Northen New Mexico, 1 hour north of Santa Fe. Thank you for any thoughts, in advance. Best regards, Mark.

    InsectCop

    Sounds like you might be dealing with biting gnats. But it also sounds like you have dealt with the problem already.

CH Matthews

Reside in SoCal with a large atrium with screened top, lots of heat in the summer, LOTS of potted plants and a fungus gnat problem. Tried yellow cards and various chemicals with no success. GardenTech/Sevin 5 dust on the soil surface worked. HOWEVER the leaves of a a few plants caught some of the dust and died. I think the heat may have been a contributing factor.

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