Fungus Gnats

If you’ve ever had plants growing in your home and found that there were little flies hovering above the soil, you’ve had an encounter with fungus gnats (Bradysia species). These 1/8th inch long insects are seldom harmful and they don’t bite or carry disease; they’re more of a nuisance than anything else.

Adult Fungus Gnat

The life cycle of fungus gnats is similar to the butterfly in that both undergo a complete metamorphosis. The adult fungus gnat lives 7-10 days and during that time the female gnat will lay more than 200 eggs on the surface of moist potting soil. Within a week the eggs hatch into larvae – the caterpillar stage. The small maggot-like larvae live in the top few inches of the soil. The larvae spend 2 weeks eating their fill of algae, fungus and decaying plant matter. (If the infestation is really bad, they can start eating live plant roots but that’s unlikely to be a problem in the home.) Next, the larvae pupate in the top-part of the soil. This is their cocoon stage. In less than a week another crop of fungus gnats emerges from the pupae to start the cycle over again.

Every spring I have an infestation of fungus gnats. The female gnat is attracted to potting soil with a high peat content – that describes all of my potting soil. But they also need the soil to be moist. If the top inches of the soil dry out, the eggs don’t hatch and any larvae die and the cycle is interrupted.

During the fall and winter I tend to keep my plants on the dry side so gnats aren’t a big problem. In the spring I propagate many of my plants and also start seeds. As a result, I have a lot of pots filled with peat-based medium that can’t dry out or the cuttings will die and the seeds won’t sprout. No wonder I have fungus gnat in the spring!

There are a few ways to control gnats. If you only have a few plants, you can remove some of the soil from the top of the pot and replace it with a layer of sand or pebbles. The female gnat wants to lay her eggs on moist peat, not sand and rocks.

If you have a lot of plants, avoiding overwatering and allowing the pots to dry out before watering will limit the gnat population. There is also a systemic insecticide called imidacloprid that comes in a granular formula. When added to the top of the soil in a pot, it kills the gnat larvae. It is also taken up by the plants roots and will kill insects like aphids, mealybugs and scale. This product can only be used on ornamental plants. If you’re going to eat any of the plant that you’re growing (i.e. an herb plant), you never want use a systemic insecticide.

The other way to control fungus gnats is by using sticky cards. These bright yellow pieces of plastic are covered with a substance that makes them sticky. The cards can be attached to a popsicle stick and put in a pot or hung in the area where plants are grown. The adult gnats are attracted to the color yellow and when they land on the card, they’re stuck. While this won’t eliminate them, it will keep their population in check. I’ve found them to work especially well around trays of seedlings.

It’s spring and it’s fungus gnat time. But this year, with a little judicious use of imidacloprid and lots of sticky cards, I’ve got the gnats under control… not gone but under control.

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2 responses to “Fungus Gnats

  1. Thanks for taking the time to write this! I never knew the story behind these flies. I’m kind of solving this problem by having a few carnivorous plants around- they get a continuous supply of food.

  2. That’s a good idea too – living sticky cards!

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