Fungus gnats (Mycetophilidae and Sciaridae families) are bothersome flying insect pests that resemble tiny mosquitoes, although these gnats do not bite or sting. They are found commonly around decaying vegetation and organic-rich potting mixes. Controlling adult gnats requires eradicating their soil-dwelling larvae. For severe infestations, you may need to apply a soil drench pesticide to kill these unseen interlopers before they damage your plants.
Even though adult gnats are aggravating, they do not damage plants. Fungus gnats are so named because their larvae, or maggots, feed on fungi from decomposing plant matter. In addition to fungi, maggots also eat leaf mold, mulch and plant roots. They attack tiny root hairs and feeder roots that supply nutrition to plants. Damage includes stunted growth, yellowing leaves and plant death. Host plants that are particularly susceptible to fungus gnat damage are African violets, geraniums, chrysanthemums and lilies.
Soil drenches are chemical pesticides that saturate soil and kill pathogens or insects. Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium that is the only microbial insecticide available in common use, according to the Colorado State University Extension. Different strains of Bt kill different insect species. Bt var. israelensis (Bti), also called H-14 strain, kills fungus gnat larvae. When a Bti soil drench saturates soil, it coats plant roots harmlessly while providing a toxic snack for fungus gnat larvae. Outdoor plants and houseplants are vulnerable to attack, and Bti formulations are available for garden soil and container potting mixes.
Other Bt strains, such as Bt vars. kurstaki and tenebrionsis, will not kill fungus gnats. Using other soil drenches with organophosphate ingredients, such as malathion and carbaryl, will kill fungus gnat larvae, but they also will kill beneficial species and also may be harmful to plants. Repeat applications of Bti, used according to label directions, often are needed to control persistent pest populations. When using soil drenches on houseplants, the University of California recommends removing plants outside your home for treatment before returning them inside the following day.
Whenever possible, the best defense against fungus gnats is prevention. Before bringing plants home from your local nursery or garden center, check to see if gnats hover over containers or fly from potting media. Even if you shoo them away, remember their eggs and larvae are hidden from view. Introducing new plants to your garden or home with these silent, immobile hitchhikers is a surefire way to pose a future infestation threat to all your healthy plants. Don’t overwater your plants and make sure your plants have adequate drainage. Overly moist soil facilitates insect population growth. Use sterile potting mix for houseplants to discourage a flurry of gnats inside your home.
- University of California IPM Online: Fungus Gnats, Shore Flies, Moth Flies, and March Flies
- University of Vermont Extension Department of Plant and Soil Science: Fungus Gnats
- Colorado State University Extension: Bacillus Thuringiensis
- University of Tennessee Extension: Fungus Gnats
- University of Florida IFAS Extension: Management of Fungus Gnats in Ornamentals
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