Skip Richter, based in Houston, is a popular speaker for garden clubs, Master Gardener programs, and other gardening events across Texas. He has...Full Bio


Fungal Gnat Control

Let’s start with some basics. Fungal gnats are common with 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 they can be nuisances flying about the house.

Photo: Randy Lemmon

Folks who keep potted plants near their computer or TV often notice them flying near the screen. The larvae feed on fungi and organic matter in potting media, and they sometimes damage plants by feeding on roots and root hairs. African violets and cyclamen are especially susceptible to this “root pruning,” but other potted plants can also be injured.

Gardeners who keep potted plants outside during warm months may notice these small flies after bringing the plants in for the winter. The 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 with an insecticide in mid-air can kill them if it makes contact. You can use pyrethrin-based indoor aerosol sprays, but this won’t solve the problem. Sticky traps will catch them, but that won’t do the trick, either. You have to control the larvae developing in your potted plants, in the trash and in the sink. For best results with pots, allow the upper part of the soil to dry between waterings. If heavy infestations persist, drench the pots with products containing the biological insecticide Bacillus thuringiensis israelensis. I personally prefer to top dress the soil with a ¼- to ½-inch layer of sharp (playground or builder’s grade) sand. That cuts off oxygen to the breeding area and prevents random adults from re-entering.

To prevent them from appearing in the first place, avoid overwatering. And use only well-composted organic matter in your potting mix. Incompletely composted organic matter is more favorable to the pests’ development. And keep drain saucers clean - larvae can also develop in accumulations of organic matter found there.

If you don’t have potted plants, search around trash cans, the pantry, the sink or wherever there might be rotting fruit or vegetables – this is why fungal gnats are often referred to as fruit flies.

After doing all of the above there are still some random gnats flying around, you can trap and kill them with a little bit of apple cider vinegar and water. I suggest clear plastic cups filled with a half-and-half mixture plus a couple drops of lemon-scented dish soap (inset in the image above). The random pests will dive bomb it, be trapped and die almost instantly. Some people use just the apple cider vinegar and soap drops, but I know the half-and-half mix works quite well.

Good luck with these control methods, but if you continue to have problems, call the GardenLine radio program this weekend. You can also call in with other effective control or baiting systems you’ve discovered.

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