Houseplants are amazing! They brighten up your home and bring nature indoors. But sometimes, a pest makes an appearance. Enter the pesky little fungus gnat.
Fungus gnats (Orfelia and Bradysia species) can be quite a nuisance. These dark, tiny (1/16 to 1/8 inch long) almost delicate-looking flies can resemble tiny mosquitoes and be just as bothersome. I’ve swatted at many a fungus gnat while watering my houseplants, as they like to hang out on the foliage. They’ve even flitted around my head while I am sitting on my couch, which happens to be near a window, as fungus gnats are attracted to light sources.
While fungus gnats don’t damage plants or bite people, their larvae, in large numbers, can damage delicate root structures and cause your houseplants’ growth to become stunted. They can even damage the roots of young seedlings you may be germinating indoors.
Why do fungus gnats swarm around your houseplants? The females like to lay their eggs in moist organic debris or potting mix. If your houseplants are excessively moist, this is the perfect breeding ground for the fungus gnat. Not certain if they are breeding in your favorite pothos or dieffenbachia? Look carefully at the top of the soil; you may be able to see slime trails on the top of the potting soil that look similar to the trails of snails or slugs.
Managing fungus gnats is not difficult. Allowing the surface of your soil to dry between waterings, draining excess water from the saucer or trays beneath your potted plants, and using only pasteurized container or potting mix soil will most likely control the problem. If you do have an infected pot, do not move it next to any unaffected plants and containers as this can allow the fungus gnats to make that potted plant its next home.
If you do have a large population of fungus gnats, you can possibly reduce the problem by using yellow sticky traps, found at retail nurseries and garden centers. Attach them to a wooden stick, like a skewer or popsicle stick, and place them in the plant at the same height as the plant. Alternately or in conjunction with the sticky traps, place raw potato chunks in the soil next to the plant. The larvae and adult gnats are attracted to the potatoes and can not only help you to see if you indeed have fungus gnat larvae, but it can also keep the larvae away from the plant roots. Larvae will move to the potato instead of your plants roots, and the adults will hang out on them as well. Be sure to check the potato chunks in a few days and remove the infected pieces and replace them.
Insecticides are rarely needed to control fungus gnats. Read more about controlling them from UC IPM at Fungus Gnats Management Guidelines--UC IPM
Fungus gnats can be challenging, but with proper care of your houseplants and some simple preventative measures and controls, you will be enjoying your indoor garden minus the tiny flying nuisances.
References:
Fungus Gnats Management Guidelines – UCIPM Pest Notes Publication 7448 https://ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnfungusgnats.pdf
Fungus Gnats as Houseplants and Indoor Pests, Colorado State University Extension Fact Sheet No. 5.584 https://extension.colostate.edu/topic-areas/insects/fungus-gnats-as-houseplant-and-indoor-pests-5-584/
Terry Lynn Pellegrini has been a UC Master Gardener since 2020.