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For the last year I've had small black flies (1-2mm) in my house. I also noticed that some of my plants were slowly dying, and when removing one from the pot I saw small silver (1mm) larvae undergound around the roots and the rest of the pot.

A friend of mine said these where Rouwvliegjes in Dutch which the Wikipedia page names Bibio lanigerus.

I have tried a trick of my grandmother with cinnamon and water which did not help at all. I also used a package which is filled with microscopic worms/eels that are said to eat the larvae and will then die off without a foodsource present. These eels are called Steinernema Feltiae on the package. All these have failed. My Ficus elastica and Bonsai are the biggest plants I have and now these are also suffering.

Any tips or tricks? I am living in the Netherlands in Noord-Brabant.

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They are known as Fungus gnats in English. There are many tips and tricks, but like you have already noticed none of them are bullet proof. Here an example of a recent post with the same question.

My own experience is that these gnats always come back, but you can keep them under control a bit. The nematode treatment, like you tried already, works very good for me. Usually that will keep them away for 6-8 weeks, but then they'll come back. You can keep the number of flies acceptable by not giving the plants too much water, if the soil in the pots will dry up once in a while it will reduce larvae growth.

But like I said, to get them away for ever is very hard. You'll probably need plastic plants for that.

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    Thanks for your answer. I had about the same idea but wanted to know for sure. The problem with some of my plants is that they need a moist soil to thrive, I might have to give those an extra dose 4 weeks later. The extra expense is worth it for me. But it basically means control is more probable than eradication?
    – Tom Sol
    Mar 3 at 20:01

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