Fungus gnats are small flies that infest potting mixtures, soil, and anything else that contains decaying organic matter. As adults they aren’t damaging, merely annoying, but as larvae they can sometimes feed on potted plant’s roots. In the colder months of the year many folks start to notice these insects buzzing around their home as they emerge from potted plants brought in for the winter.
How to identify
Fungus gnats are most often confused with mosquitoes and fruit flies. They are small (about 1/8th of an inch long) and are dark in coloration (Figure 1). They are weak fliers and tend to stay near the plant or pot they emerged from. If you were to closely inspect them you would notice they have segmented antennae that are longer than their heads and a distinct “Y” pattern on the two top wings. The larvae are small as well and are worm or maggot like in shape. They are a translucent white color with a dark black head (Figure 2). Symptoms of a fungus gnat infestation can include just seeing the flying adults, slime trails on the top layer of potting soil, and in high enough populations wilted plants.
Since fungus gnats are a type of fly they go through complete metamorphosis, which includes egg, larva, pupa, and adult stages. An adult female can lay up to 200 eggs which hatch into larvae that will feed on organic debris for about 2 weeks and then pupate. They will emerge from their pupae after about a week as adult flies and live for another week or so. Since their life cycle is so short and the inside of our homes are so warm, they can produce many generations of gnats quickly.
Management of fungus gnats
Fungus gnats thrive when potting soil is kept constantly wet. If you can allow at least the upper 1-2 inches dry out this will minimize the chances at an infestation. If you are already suffering from a fungus gnat problem there are several steps you can take to reduce numbers of flies. First you will want to dry out or replace the potting soil present in your containers. Secondly, you will want to purchase yellow sticky card to place next to your plants. These index card sized traps are attractive to the adult flies and when they land on them they get caught up in the glue. Finally, you can treat the potting soil for the maggots if you don’t want to replace it. You can purchase traditional insecticides such as pyrethrins to apply to the soil or you can make a mixture of 1 part bleach to 10 parts water and run that mixture through soil. This dilution should kill the maggots but be harmless to the plant.
If you have further questions about fungus gnats, you should contact your local extension office.