Pests and health

Getting rid of those annoying flies on your houseplants

Anyone who’s had houseplants for any decent period of time will probably have come across the annoyance that is fungus gnats.

How to tell if you have fungus gnats

These tiny black flies are a couple of milimetres big and scuttle about on the soil surface of your houseplants. They’ll often fly up from the pot when disturbed by you poking around the soil or watering.

They’re much smaller than fruit flies, and float about in an annoying bumbling way, often towards your face.

In terms of how they affect your plants, the adults are harmless – but the larvae munch on young roots, so can stunt plant development.

Living with plants

As an aside: I don’t think for a moment that it’s possible to live with plants and have a sterile environment – soil after all is full of living microbes and micro-organisms, which is natural, normal, and often good for your plants.

However one Sunday morning I’d made a fresh cup of tea, and curled up in my reading chair with a good book I’d been looking forward to reading. I picked up my perfectly-brewed mug of tea for my first sip…and found a drowned fungus gnat in it.

That was the straw that broke the camel’s back. 😒

How to get rid of fungus gnats

The single most effective way I found of getting rid of fungus gnats was using predatory nematodes.

One treatment and the house was clear of them for another 6 months. I didn’t realise how annoying they had been until they were gone!

How the nematode treatment works

1. Order the nematodes

I bought Nemasys Sciarid Fly nematodes. Upon arrival you have to keep the packet in the fridge and have a window of few weeks to use them – so plan accordingly.

It’d be sensible to time your watering schedule to make sure that all your plants are ready for a full drenching, and set aside a good few hours to work your way around.

2. What to expect

The packet, when it arrives, is somewhat underwhelming! It’s a matchbox-sized plastic container with a slightly lumpy white/cream powder in, a bit like cornflour. It’s hard to imagine that it’s packed full of microscopic predators waiting to be unleashed!

3. How to mix it

One packet can make up enough to do 15m². Like most people, I’ve no idea what this equates to in terms of plants – but one packet was more than enough to do all of my then 100 odd plant pots.

If you have fewer pots than that, you can just make it up as a stronger solution – the nematodes won’t damage the plant in greater concentrations.

I found it easiest to mix up the solution in a bucket, giving it a good stir so that it turns cloudy, and then decant into smaller watering cans to take round the house.

4. Watering

I watered every single pot in the house thoroughly with the solution, until it ran through the bottom of the pot – there’s no point leaving any soil untreated.

As with all watering make sure that the plant isn’t left sitting in a pool of water, and pour any excess away.

You might find it easier to pop all the plants in a bath tub or sink, and work your way through them systematically that way.

5. Follow up measures

The nematodes will get to work munching away on the larvae currently in your soil. They’ll also hang about in the soil in case new eggs are laid by adult flies and hatch into larvae.

You’ll want to also look at trapping the remaining adult flies in your home – people often recommend sticky traps, which do the job – but I also like carnivorous plants, namely pinguiculas which seem incredibly effective!

Finally, adopting a more careful watering regime, mainly watering from the bottom, keeps the top soil from getting too wet, and becoming ideal breeding conditions for them to recolonise.

What next

Though I successfully eradicated them from our house, I fully expect to see fungus gnats back at some point soon.

Plant and their pots are already living ecosystems. Combined with new plants joining your collection from often unknown sources, and windows and doors, some arriving in your house to enjoy your jungle with you is probably inevitable.

The best you can do is keep it manageable, through vigilance, quarantining, careful watering, and treatment where needed.

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