How to get rid of fungus gnats on indoor plants

What buzzes but doesn't bite, gets right up in your face and loves indoor plants just as much as you do? Fungus gnats.

"It's OK for them to be there in small numbers but it's not OK when they become unmanageable," says indoor plant expert Jason Chongue.

The tiny flying adults are mostly just annoying, but it's the larvae under the soil that do the real damage. Baby larvae feast on fungus, algae and even the roots of your plants.

In small numbers they mostly just stick to the organic stuff in potting mix but in large numbers they can do some serious damage to a plants root system.

"Being consistent with your plant care is the key to managing them," says Jason.

Read on for his tips on dealing with them.

Overwatering can attract fungus gnats

If you remember nothing else from this article, hang onto this tip: "Let the soil dry out between watering!"

Why? Gnat larvae love warm, wet environments so your overwatered indoor plant is like a little paradise for them.

Letting a finger-length of top soil dry out before you water again will help prevent or slow an infestation.

How to tackle a minor infestation

If you already have fungus gnats at home, you'll need to take a multi-step approach to get rid of them. It'll involve targeting the flying adults and the larvae under the soil.

"Everyone wants insects to disappear instantly, but it will never be instant," warns Jason.

"It takes months to get rid of a pest once it's in your house but with the right treatment it will settle itself over time."

1. Move the plant

First things first: "Isolate the infected plant."

"Normally it's one plant that's really infested so I always try and separate it out from others by putting it in a sheltered location outside," Jason says.

It's best to keep it isolated from other plants, especially pot plants, that you might have outside.

2. Invest in sticky traps or plants

Sticky traps or sticky plants will trap (and kill) flying adults. One adult can lay up to around 200 eggs, so this can slow population growth.

You can pick up sticky traps at hardware stories and nurseries.

Or if a plant takes your fancy, sundews (Drosera sp.) are a type of carnivorous plant who lure small insects like fungus gnats into their sticky leaves.

3. Use neem oil spray

Neem oil is an organic insecticide made from the seeds of neem trees and you can pick up a bottle pretty much anywhere that sells plants.

It's sold as a concentrate (which needs to be diluted in a spray bottle) or in a pre-diluted spray bottle ready to go.

Jason recommends spraying the soil with diluted neem oil to deal with the larvae.

Soak the top 5-10cm of the soil with your neem oil spray about once a week for at least two months (or for one month after you stop seeing the flying adults). Outside of your neem oil soak, remember to let the soil dry out between watering.

Unlike many other pesticides, it won't harm your pets or kill beneficial insects like earthworms.

4. Consider creating a soil barrier

Some people have also had success with adding horticultural sand (not the beach stuff) or small pebbles on top of their potting mix. This can prevent female fungus gnats from being able to lay eggs in the soil.

Jason says it's not his preference because it can make it difficult to see what's going on in the soil but each to their own!

How to treat a major infestation

Jason notes that when things get bad, the key is to be consistent and committed to multiple treatments.

5. Try it all at once

"Check on your plants every week, regularly spray the soil with neem oil and regularly replace sticky traps when they get full," he says.

6. Repot as a last resort

If things don't start to turn around after a few weeks Jason recommends repotting with a new batch of potting mix to help reduce the egg count in the soil. Think of this as a last resort because there's no guarantee the new potting mix won't have eggs or larvae in it too.

"Even after repotting keep up the neem treatment and sticky traps."

Sound like a lot? Jason is reassuring.

"Gardening is about adapting and it does get easier," he says.

Chrissi Charles is a queer writer and plant scientist passionate about propagating plant literacy on Wurundjeri country.