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How to get rid of Fungus Gnats

Updated: Sep 22, 2023

It seems this has been one of the worst summers for Fungus Gnats (Scariad flies), even if you’ve never had them before. I've had quite a few people coming into the shop asking how to get rid of Fungus Gnats the last few months, so I thought this post might be timely.

On the plus side, they’re mostly harmless – gnats cause little to no harm to plants, but they can be a nuisance. Especially as they love carbon dioxide, the only thing more appealing to them than moist soil, is the CO2 in your breath. Which is why you always know when you've got them as they tend to annoyingly fly at your face.

Several years ago I bought a bag of houseplant compost from a garden centre to repot my plants at home only to discover I was suddenly inundated with gnats. I promptly removed the new compost and took the other bags back to the garden centre.

So how do the fungus gnats get into the sealed compost bags?

I've been researching this and it seems that this most likely happened between when the compost was made or manufactured or during the shipping process on the way to store shelves.

Most bags have tiny air holes in, which are just enough for a Fungus Gnat to get in. They are attracted to decaying organic matter but also love feeding on the micronutrients added to compost. They then lay their eggs and the infestation spreads.

Because their life cycle is so short, by the time the compost reaches you, it could potentially be overrun with Fungus Gnats.

The good news is - after years of searching I have now found a peat-free compost which is guaranteed gnat free, yay! This is because they can't survive the coir compression process. Also, the coconut oil present in coco coir mix repels them and there's no air holes needed in the packaging, so they couldn't get in even if they wanted to.

The blocks are light and easy to carry but by adding water expand to 3 times the size. You can also cut a bit off if you don't want to use it all.

If you already have a gnat problem how do you get rid of them!

I tried almost every remedy out there, starting off with what I had at home, cinnamon, apple cider vinegar, potatoes. Moving on to sprinkling diatomaceous earth on the soil, you name it, I tried it.

Some things were more effective than others, and some remedies seemed to work for a while but then they'd come back. What I have learnt is that you need a two pronged attack, to get the adult flies and their larvae that live in the soil.

The larvae live in the top 5-8cm of compost, where they feed on algae, fungi and plant roots. Healthy house plants can tolerate this minor root damage, but they can harm seedlings or weak plants. Replacing the top layer of compost with fresh, uninfested compost can help.

But for a more persistent problem you'll need two things, to combat all stages of the lifecycle. Both are inexpensive and easy to get!

Firstly, use yellow sticky traps to catch the adult flies. I stock these in the shop in A5 sheets. They are double sided, so once you've used one side you can turn them over.

Cut a piece off to cover about a third of the soil surface, peel off one side of the paper and lay it flat on the soil, sticky side up.

To target the larvae and break the cycle. The most effective product I've come across is Mosquito Bits. You add the granules to your watering can and soak in water for about 30 minutes. They release Bacillus thuringiensis sp israelensis (Bti), a naturally occurring bacterium that acts as a species-specific larvicide. Although deadly to Fungus Gnat and Mosquito larvae, it's harmless to other living things. So won't harm animals, birds, bees, fish, aquatic life, vegetation or us. It can be safely used in bird baths, indoor and outdoor potted plants, rain barrels water troughs, and ponds.

I have mesh bags of Mosquito Bits of the correct quantity in the shop.

Simply put the mesh bag into 2 pints of water to soak, for about 30 minutes

Remove the bag

Give it a stir or shake

Water onto the surface of the soil, when it's time to water your plants.

You can keep the mixture for up to 21 days. You should see results within 2-3 weeks. Their life cycle can be 3-4 weeks, so for a larger infestation repeat for at least 2 lifecycles (8 weeks) to ensure they are totally eradicated.

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