WSU Extension

Pestsense

Caption: Dark winged fungus gnat
Photo by: Ken Gray
  
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Fungus gnats (houseplants)

(revision date: 7/14/2015)


Use Integrated Pest Management (IPM) for successful pest management.

Biology
Fungus gnats are tiny, dark, slender flies about 1/8” long which commonly infest the soil and roots of houseplants. Adult fungus gnats are mainly a nuisance, flying around plants and running on the soil and other nearby surfaces. The larvae of some species attack roots. With sufficient numbers of larvae present, larval damage may cause yellowing and wilting of leaves. Larvae of other species simply feed in decaying organic matter in the soil and do no harm to the plants. Mature larvae are about 1/4” long with a shiny black head and white, almost transparent body. Fungus gnats may be brought inside when plants are moved indoors from the garden, come in through screens, or be brought into the home with new plant purchases. Fungus gnats prefer moist areas and soils high in organic material.
Management Options

Non-Chemical Management
  • Do not move plants inside from outdoors. If you must move plants indoors, they should be quarantined (for example, in a spare bedroom) and kept isolated from other indoor plants for about a month. During this time, regularly inspect the quarantined plants for pest problems. Treat pest problems as necessary and continue isolation until the plants are “clean” for at least one full month before integrating them with the rest of your plant collection.
  • Quarantine and monitor newly purchased plants or plants from your collection which have become infested with pests.
  • Plants which suffer repeated heavy fungus gnat infestations should be discarded.
  • Heavily infested plants which cannot be discarded may benefit from being repotted with fresh soil to knock down the population of fungus gnat larvae. Be sure to use a potting soil with good drainage.
  • Healthy plants are able to tolerate some damage. Maintain plant health by proper nutrition and watering, taking special care not to overwater. When possible, let the soil get somewhat dry between waterings to make plants less attractive to fungus gnats. Use a houseplant soil moisture meter to help prevent overwatering.
  • Keep soil, pots, trays, and display areas clear of decaying plant matter.
  • Yellow sticky traps (plastic or laminated cards with an adhesive coating) can be used to monitor and help control adult fungus gnat populations. Light infestations may be adequately controlled with sticky traps. They should be mounted vertically (by using a stake or other means) to catch the flying adults. Sticky traps may be available through nurseries or nursery supply catalogs.
Select non-chemical management options as your first choice!

Chemical Management
IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

Chemical controls are NOT RECOMMENDED on edible plants (i.e. herbs, salad greens, etc.) grown indoors for culinary use. No pesticides are recommended for control of fungus gnats on houseplants.

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Caption: Dark winged fungus gnat
Photo by: Ken Gray