Fungus gnats or Sciarid flies

Difficult to photograph, the photo shown above was the best that I could do.

Fungus gnats or Sciarid flies live in damp compost and only really are a problem indoors, living in your houseplants and flying around your home. The one in the photo is about 2mm long. Apparently, they do no damage to the plants if they are established, but they can cause harm to cuttings, seedlings and may carry diseases such as damping off. The thing that I most dislike about them is that they seem to enjoy flying in to cups of tea!

I’ve tried a few methods of dealing with them, but none have been entirely successful. I suspect that it may be an issue that requires multi-pronged attack.

• Cups of water near the plants where the flies are usually found. This captured one or two flies who flew in to it, but didn’t really have much of an effect. Odd really, given the number of flies I have found in the sink and toilet bowl of my bathroom. After a few days, no other flies went near the trap.
• Cups of sugary water near the plants. This type of trap worked about as well as the plain water.
• Cups of tea near the plants. Now this was a trap of my own invention – they seem to enjoy flying in to hot cups of tea! It worked a bit better than the plain water, but didn’t capture more than about 5 flies in a week.
• Replacing the top part of the soil with sand. This is a recommendation from “Garden Alchemy” by Stephanie Rose. She recommends leaving the plants to dry out and then removing 2.5 cm of the soil from the top of each pot and replacing it with sand or gravel. I couldn’t remove that much soil because the plants are small, but I took as much as I could, which was about 1cm, and replaced it with either horticultural grit (works quite well) or play sand (for a sandpit. Does not work well, I do not recommend this). This has had no appreciable effect on the fungus gnats but the plants do look a bit happier. Perhaps it has helped with drainage.
• Sprinkling cinnamon around the plants. Bit of a strange idea, I saw it online and thought that I might as well try it. I used some ground cinnamon I had for baking, but it was part of a mixture of spices for gingerbread so had other spices too. I didn’t really notice any difference in the number of flies about the place. Then I tried putting the cinnamon on top of the sand-topped pots but this doesn’t seem to have done anything either.
• Spraying with neem oil. Neem oil is supposed to be a selective insecticide. I already had a small bottle of it at home, so I made up a solution in a spray bottle of water, a few drops of washing up liquid and some neem oil. Shaking all this together, I sprayed the plants and the soil around the plants, trying to get the solution down in to the soil. This also does not seem to have had a great effect on the flies, but the plants suddenly looked a lot better and started to put on growth. I’m wondering whether it has done something in the soil to allow the plants to take up nutrients better than before.
• Yellow sticky traps. I hung a sticky yellow piece of card above the plants which are on a window-sill. It has trapped quite a good number, so this is the most obviously successful method of control. The flies seem to be most active early in the morning, so closing the curtains and only opening them later in the day seems to allow the sticky trap to work better.

And yes, the photo at the top is of a fly stuck to the yellow sticky trap. The flies seem to me to fall in to 2 classes, the ones which fly slowly and in straight lines, and smaller flies which zig-zag about with no obvious directional heading, but both are difficult to photograph.

I first got the fungus gnat problem about a year ago, when I purchased a bag of multi-purpose compost and I think that there were some larvae or eggs in that bag which is where the problem came from. They have since spread and there are definitely some now living and multiplying in another half-used bag of compost I have. Apparently, if you can store any compost in an air-tight container, that would help as the insects need oxygen to survive. But since I don’t have anything that would work, for sowing seeds this year, I have been microwaving the compost before using it, taking the temperature up to 80^oC. I only did that a couple of days ago, but it looks ok so far.

If you’ve got any tips or advice on how to get rid of fungus gnats, please let me know – they’re driving me crazy!