I have never really had a problem with these before, occasionally I have seen them in chilli plants that are overwintered indoors, or other house plants. But mostly they tend to stick to indoor plants with old neglected soil.
For those that don’t know about them, they are a tiny black fly that hangs around the soil in a pot, you often don’t see them until you water it and they all fly up in a panic. Their even tinier grubs will be living in the soil, feeding on algae, rotting organic matter, and according to some experts but not others, the roots of your plant.
This year, however, I seem to have been inundated with them in the greenhouse. I think this is most likely due to the very mild winter. Where I live in South Devon we only had a couple of very light frosts, and the greenhouse never went below zero, hence poor sterilization of the soil. I had a couple of chilli plants out there which developed fungus gnats in their pots but I wasn’t too worried, normally they would die and be discarded before spring. I also had a bag of compost left over from the previous year which had a few in. I didn’t want to use it for potting so I dug it into the beds in the greenhouse. I foolishly thought the flies wouldn’t survive in natural soil because other bugs would kill them off. This was wrong, and before I knew it the beds were crawling, along with some other pots.
I am a big fan of biological controls, Nemaslug etc. and I urge people to use them whenever I can, but it is only recently that I have seen a control for Fungus Gnats. In my Growing Chillies book I deal with Fungus Gnats, but don’t mention a biological control for them as, at the time, there didn’t seem to be one available, hence this update. I have always used the old fashioned method of sticking a piece of potato in the soil which attracts the grubs and can later be discarded. Luckily now there is a biological control widely available, it is inexpensive, easy to use and very effective. It is microscopic nematode worms that infect the fly grubs. It comes as a light powdery substance in a small sealed tray. Dilute it in a watering can and water it into the soil of infected pots. It seems to me that the results are obvious within just a day or two. I suppose the grubs are infected almost immediately and the adults don’t live more than a day or two, so their life cycle is immediately halted.
This will be my first port of call in future, I might even order it as a matter of course every spring when I order my Nemaslug, you can get them shipped together from ‘all good purveyors biological controls’ I use www.greengardener.co.uk