Fungus gnats are a common and persistent pest of houseplants and greenhouse plants. They are often called “fruit flies”, which are a different species altogether.
I have written several tip sheets over the years on Fungus Gnats, and I covered them more than ten years ago in my book “Gulf Coast Gardening”. But this is a recurring subject on the Gardenline radio show, and I think this extended update on the subject will be useful.
Let’s start with some basic facts. Fungal Gnats are common pests of potted plants. They also tend to hang around trash cans, rotting fruit and vegetables and in some cases sinks. The adults are tiny, mosquito-like flies. They don’t bite but can be nuisances flying about the house.
Folks who keep potted plants near their computer or TV often notice them flying near the monitor. The larvae feed on fungi and organic matter in potting media and they sometimes damage plants by feeding on root hairs and roots. African violets and cyclamen are especially susceptible to this type of root pruning, but other potted plants can also be injured.
Gardeners who keep potted plants outside during the warm months may begin noticing these small flies after they have brought their plants in for the winter. Fungus gnat larvae live in the upper few inches of media in potted plants where they feed on fungi, decaying organic matter and root hairs.
What to Do??!!!
Spraying them in mid-air with a pyrethrin-based indoor aerosol spray insecticide. It will kill the adults, but this won’t solve the problem. Sticky Traps will work to catch those flying about as well, but again, it doesn’t solve the problem. You have to control the larvae that are developing in your potted plants.
The easiest and best way to manage fungus gnats is to allow the soil in the upper part of the pot to dry between waterings. If heavy infestations persist, drench pots with insecticides containing the biological insecticide, Bacillus thuringiensis israelensis. But for me, the smartest way to control them in potted plants is to top dress the top of the soil with about a ¼ to ½ inch layer of sharp sand – think playground sand or builder’s grade sand. You are suffocating the breeding ground and preventing the random adults from re-entering.
If you don’t have potted plants, but still see gnat-like insects flying near trash cans or the sink, you have another species altogether – either the common fruit fly or perhaps drain flies. Even though these three separate families of tiny pests look similar and are common in households, each has their own control.
If you want to prevent fungus gnats from ever appearing, and you have potted plants indoors, avoid overwatering. This is the main cause of Fungal Gnat problems. Use only well-composted organic matter in your potting mix. Incompletely composted organic matter is more favorable to fungus gnats. Keep drain saucers clean; larvae can also develop in accumulations of organic matter in dirty saucers.
Lastly, if you think you’ve done everything already mentioned in controlling these pests, and yet there are still some random Fungal Gnats flying around, you can trap and kill them with a little bit of apple cider vinegar and water. I usually recommend clear plastic (think party/cocktail glasses) filled with half and half water and apple cider vinegar, with a couple of drops of lemon-scented dish soap. The “loose” gnats will find that, dive bomb it and be trapped and killed almost instantly. Some people have been known to use the apple cider vinegar and soap drops exclusively, but I know the half and half mix works quite well. This will also trap fruit flies, which are sometimes called “vinegar flies”, and drain flies.
Good luck with any and all these control methods and if you have any problems in the future, don’t hesitate to call the GardenLine radio program so we can help hone in on the problem and advice. Or if you have any other control or baiting systems, give us a call as well.