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.