3 Things to Know about Fungus Gnats

When you bring in those potted plants, you may not be bringing in just your photosynthetic pals! Many different kinds of arthropods may take up residence in the potting medium, one of the most annoying of which is the fungus gnat. 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.

darkwinged fungus gnat JAK114 (2)
An adult fungus gnat resting on a leaf
  1. How do you identify a fungus gnat?

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. 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. 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.

dkwinged fungus gnat larvae JAK760 (2)
Larval fungus gnats gathered in a group
  1. How does their life cycle work?

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 maggots that will feed on organic debris for about 2 weeks and then pupate. After about a week they will emerge from their pupae 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 quickly produce many generations of gnats.

dkwinged fungus gnats mating JAK112 (2)

  1. How do you get rid of them?

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 traps 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 use newer, organic options like the Bti product Gnatrol.


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