Detecting and combating fungus gnats

In the wild, fungus gnats (aka sciarid flies) pose no real problem. But get an infestation of these tiny black insects in your home, and you’ll know all about it. They feed on plant roots when they can’t find other sources of food and cause a genuine annoyance while you try and get to the heart of the problem. 

So, how do you identify fungus gnats, and how do you take action against the little critters?

Well, you’ve come to the right place: this article is about detecting and combating fungus gnats and preventing reinfestation once you’ve got rid! I’ll also recommend natural home remedies, practical traps, and chemical solutions. 

Ready? Let’s get started.

Fungus gnats at a glance

Fungus gnats are small flying insects. Outdoors, they play an essential role in the ecosystem, but indoors, they quickly become significant troublemakers. Their eggs get into flower pots filled with fresh soil, and as soon as they’ve hatched, the population will rapidly grow. 

They like moist and warm conditions, so they are attracted to your breath. And before you know it, you’re batting off the fungus flies from flying in your face every couple of minutes. 

Annoying! 

However, while you’ll find yourself bugged out by these tiny little critters, they’re not particularly harmful. They will gnaw at your plants’ roots, which usually only negatively affects young plants. 

Take action over fungus gnats

I recommend taking action as soon as you spot fungus flies in your home because each female lays around two-hundred eggs — and that adds up to an infestation in no time.   

Fungus gnat profile

Size	1-7mm
Life expectancy	1 week
Reproduction	200 larvae per female
Habitat	Moist environments, at least 20℃
Commonly found	Forests, wet meadows, moorlands
Food	Leaf litter, wood bark, and organic matter – absorbing only moisture
Feature	In some subspecies, females are wingless and unable to fly

Do you have a fungus gnat infestation?

If your fungus gnat isn’t making itself known by flying in your face, you’ll recognise the problem by looking at the topsoil of your house plants. 

The flies live on the soil surface or in the vicinity — you may find them on the windowsill alongside the pot if the population is overgrowing the soil. 

The larvae are thin, white grubs that can grow up to 5mm long. They live in and on the soil, feeding on the roots and other organic materials such as dead leaves. 

You probably won’t notice the infestation at first — but once yellow spots have appeared on your plant’s leaves, they may have already damaged the roots. 

Causes of fungus gnat infestation

Fungus gnats prefer moist and nutrient-rich soil, in which they lay their eggs knowing that their larvae will have an organic food source. 

Therefore:

The ideal spot for a fungus gnat infestation is in a plant pot with good-quality potting soil. It’s not necessarily a bad sign — it means that the earth is full of good-quality nutrition, which is great for your houseplants. And whether your potting compost is the cheapest in the garden centre or a selection from the premium end, larvae are likely to be found within the packaging. 

The infestation is boosted by:

• Over-fertilisation
• Excessively high temperatures
• Overwatering or excessive soil moisture

The problem is exacerbated if your pots lack drainage — making the soil more likely to remain wet for longer periods. 

However:

You can fight your fungus gnat infestation — you just need to know how. 

Fighting fungus gnats with natural home remedies

There’s a surprising amount of effective home remedies for ridding your home of fungus gnats, including: 

Spraying dish soap

Washing up liquid is a surprisingly versatile tool in the arsenal against a range of aphids and garden pests. 

Mix water with a few drops of dish soap, and spray it around affected plants to kill the larvae. Very simple and effective.

However:

This is a solution for established plants and can damage seedlings and young plants. 

Baking soda

Baking soda seems so innocent — great for delightful cakes and sponges — but it’s adept at cleaning out your drains and killing bugs. 

So, spread baking soda over the potting soil surface with a sieve, then moisten it with a water spray. 

And within a few days, the soda will penetrate the larvae and kill them. No fairy cakes for them. 

Cinnamon against fungus gnats

Again, that delightful smell of Christmas is like napalm in the morning for fungus gnats. Sprinkle a thin layer of cinnamon, and the larvae will be toast in no time. 

This is an excellent approach as it is safe for your plants and pets and might even make your room smell all cosy!

Killing fungus gnats with garlic

Not everyone loves garlic — but if you’re a fungus gnat, you have no chance against it. 

Garlic contains the chemical Allicin, which quickly kills fungus gnat larvae. Simply slice or dice a couple of garlic cloves, and spread them across the soil. 

This is a great way to kill larvae, but it depends on your relationship with the smell of garlic because your room will hum for a couple of weeks. I love it — my only concern is that it might make me hungry!

Sticking matches into the soil

Don’t worry — you’re not going to set fire to your plant! This might sound unlikely, but the sulphur in the unstruck matchhead is a great fungus-gnat deterrent. 

So, stick several matches into the soil, head-end down. Then water as usual, and over time, the sulphur will leak from the match, killing the larvae. 

Replace the matches every three weeks. And bear in mind; this method is only suitable for established plants. 

Piling sand

Apply a thick layer of quartz sand to the potting soil, preventing the gnats from laying their eggs. Then, water your plant from below so you don’t disturb the sand layer. 

Coffee grounds

Don’t throw your used coffee grounds out; use them in the garden or your plant pots. They’re full of high-quality nutrition, especially nitrogen, which will help your plants thrive. 

And similar to the quartz sand approach, create a layer of dried coffee grounds across the soil surface, and the gnats won’t lay their eggs. 

Ensure the grounds are dry before applying, and water from below. 

Essential oils to fight fungus gnats

The following essential oils are effective in tackling fungus gnats (and will make your room smell delightful!):

• Lavender oil
• Tea tree oil
• Neem oil

Add the oil to the water, and irrigate your plants with it. You’ll need 20 drops for tea tree and lavender oil and just 10 for neem oil.  

Biological warfare: Spreading nematodes

Nematodes are microscopic insects that act as parasites to other creatures in the insect population. And while that sounds a little gross, they’re considered one of the best ways to treat aphids and plant pests. 

But don’t worry — you’re not replacing one insect with another, not permanently, anyway. Nematodes travel solely through moist soil. This means they won’t escape your plant pot. And once they’ve exhausted their food source (i.e., the fungus gnat infestation), they die naturally.

Find out more about these amazing critters in our expert guide to nematode use in the garden. 

Bacterial attack 

OK — this is starting to sound more and more like the latest instalment of the Alien movie franchise — but bear with me.

Another approach to killing fungus gnat populations is to introduce Bacillus thuringiensis israelensis (BTI) — a bacteria which will paralyse the larvae, preventing them from feeding. 

Sure, it sounds a bit disgusting, but BTI, when used at the recommended rates (check the packaging), is only toxic to fly larvae, such as fungus gnats, black flies, and mosquitos. 

You’re likely to need to repeat the application a few times to break the reproductive cycle for long-term control. 

How to use fungus gnat traps

Fortunately, fungus gnats aren’t the Einsteins of the insect world, so they’re highly susceptible to walking into traps. Check your traps regularly, and renew them whenever necessary. 

And remember: keep them out of the reach of children and pets. 

Sticky fly papers

Place yellow sticky fly papers near affected plants, and the flies will be attracted to the yellow colour. And once they’ve stepped foot on the sticky stuff, they’ll get stuck (and that’s the end of them!). 

Set up vinegar bowls

Place a bowl of apple cider vinegar, water, and a few drops of washing-up liquid next to the affected plant, and the gnats will swoop into the bowl (and never leave). 

The washing-up liquid affects the viscosity of the water surface, which will stick the flies to the liquid. 

Chemical solutions against fungus gnats

You can get chemical insecticides, which offer excellent protection against fungus gnat populations. But we always think that chemicals should be the last course of action. 

If you do choose the chemical route, ensure your insecticide is suitable for indoor use. Above all, always read the instructions on the packaging. 

Preventative measures to protect against fungus gnats

Now you’ve tackled the infestation, it’s time to make sure your home stays fungus-gnat-free.

The best defence against a problematic sequel is ensuring your plant pots have excellent drainage so the soil doesn’t remain soggy after watering. So, without starving your plants of essential moisture, avoid overwatering, and add a layer of sand or coffee grounds to the soil surface. 

And check the soil regularly for early signs of a new infestation. The sooner you take action, the better. 

Fungus gnats in the garden

Fungus gnats rarely cause problems in your garden. This is because insects of all types are an excellent food source for birds, frogs, and spiders — so it’s uncommon to be troubled by the flies outside. 

Fungus gnats are actually beneficial to your outdoor soil because they help break down organic material. This makes your earth full of beneficial microorganisms that boost the health of plant life. 

If you do spot an infestation, use the remedies discussed for indoor plants. Or lay birdseed near the infestation site — and the birds will tackle the problem for you! 

FAQs

Ready to get started?

I hope you’ve got all the info you need to get tackling your fungus gnat infestation. But if you have any questions, check out our comprehensive Help & Advice section, or drop us an email. 

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